Chromium-activated phosphors: from theory to applications

Abstract

The development of chromium-activated phosphors is essential for applications such as near-infrared (NIR) lighting and in vivo bioimaging. The intra-configurational transition within the 3d orbital of chromium ions, which is susceptible to the surrounding crystal environment, leads to remarkable luminescence tunability from the NIR-I to NIR-II regions. The striking NIR luminescence property makes chromium-activated phosphors a versatile material platform for fundamental optical studies and practical device applications. This review comprehensively summarizes the recent developments in chromium-activated phosphors, encompassing the phenomenological crystal field theories, design principles, materials preparation methodologies, and emerging applications. We delve into the correlation between the local coordination environment (e.g., the geometrical structure of polyhedra, nephelauxetic effect, and interaction coupling) and optical properties (e.g., spectral profile, quantum efficiency, luminescence thermal stability, as well as persistent luminescence and mechanoluminescence), aiming to provide a theoretical basis and experimental guidance for further refinement of chromium-activated phosphors. We also discuss future opportunities and challenges of chromium-activated phosphors, such as extended optical tuning by controlling crystal size and energy level coupling.

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Shengqiang Liu

Shengqiang Liu received his BS (2018) and PhD (2023, with Professor Quanlin Liu) from the University of Science and Technology Beijing. He carried out postdoctoral research at the City University of Hong Kong from 2023 to 2025 with Prof. Feng Wang. He recently joined Shenzhen University as an assistant professor. His research interests focus on the design and preparation of inorganic near-infrared luminescent materials and devices.

Leipeng Li

Leipeng Li obtained his PhD degree in Physics from the Harbin Institute of Technology in 2021. He is an Associate Professor and doctoral supervisor at the College of Physics Science and Technology, Hebei University. Concurrently, he is conducting postdoctoral research at the City University of Hong Kong, where he is a recipient of a fellowship from the prestigious Hong Kong Scholars Program. His research focuses on rare-earth and transition element functional materials for applications in visible-to-UVC upconversion, mechanoluminescence and persistent luminescence.

Bing Chen

Bing Chen received his BE (2012) and PhD (2017) degrees in Materials Science and Engineering from Zhejiang University. After carrying out postdoctoral research work in Prof. Feng Wang's group at the City University of Hong Kong, he joined the Nanjing University of Posts and Telecommunications at the College of Electronic and Optical Engineering & College of Flexible Electronics (Future Technology) as a professor in 2022. His research interests focus on the development of luminescent micro/nanomaterials for sensing, lighting and imaging.

Quanlin Liu

Quanlin Liu completed his PhD degree in Condensed Matter Physics in 1998 at the Institute of Physics, Chinese Academy of Science (IOP CAS). From 1998 to 2005, he served as an assistant and associate professor at IOP CAS, during which he also held a JSPS fellowship at the National Institute for Materials Science in Japan (2001–2003). Since 2025, he has been a full professor in Materials Science at the University of Science and Technology Beijing (USTB). His research interests mainly concern inorganic luminescent materials.

Feng Wang

Feng Wang is a professor in the Department of Materials Science and Engineering at the City University of Hong Kong. He obtained his BE and PhD degrees in Materials Science and Engineering from Zhejiang University, China. He then carried out postdoctoral research at the National University of Singapore and the Institute of Materials Research and Engineering of A*STAR. His current research focuses on micro- and nanostructured luminescent materials incorporating lanthanide and transition metal ions.

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1. Introduction

Near-infrared (NIR) light encompassing NIR-I (750–950 nm) and NIR-II (1000–1800 nm) provides promising opportunities for non-destructive analysis, night vision, anti-counterfeiting, and biological imaging.^1–4 In response to the growing demands in these applications, extensive efforts have been devoted to the study of impurity-activated NIR luminescence materials in recent years. Lanthanide (Ln³⁺, [Xe]4fⁿ) ions are an important class of NIR activators. However, the partially filled 4fⁿ shell of the Ln³⁺ ion is substantially shielded by the lower-energetic 5s² and 5p⁶ shells, resulting in sharp multiplet luminescence essentially independent of the coordination environment.^5,6 Transition metal (TM) ions, characterized by a partially filled d shell, exhibit pronounced sensitivity to their local surroundings.⁷ Accordingly, the associated intra-configurational d–d transitions manifest considerably broader absorption and emission spectra. For example, Mn²⁺ ions, with an [Ar]3d⁵ electronic configuration, present tunable emissions from green emission in tetrahedral coordination to red/NIR emission in octahedral coordination.^8–10 Similarly, Fe³⁺ ions, also with an [Ar]3d⁵ electronic configuration, demonstrate tunable NIR-I emission spanning 650 and 1000 nm.^11,12 However, the ⁴T₁ → ⁶A₁ transition typically suffers from low radiative decay rates, ascribed to the intrinsic spin- and parity-forbidden nature. Ni²⁺ ions, with an [Ar]3d⁸ electronic configuration, typically emit broadband NIR-II emission ranging from 1000 to 1800 nm in weak octahedral environments, ascribed to the ³T₂ → ³A₂ transition.⁷ Nevertheless, the practical applications are also hampered by the low luminescence efficiency resulting from substantial Stokes shift and severe thermal quenching. In contrast, chromium ions, typically existing in the trivalent Cr³⁺ ([Ar]3d³) and quadrivalent Cr⁴⁺ ([Ar]3d²) states, have attracted the most attention in NIR technology, ascribed to their high absorption cross-section and tunable luminescence from NIR-I to NIR-II.^13–15

The element chromium (No. 24) was first discovered by Vauquelin through chemical etching of the mineral crocoite PbCrO₄ with hydrochloric acid in 1797.¹⁶ Chromium, which is primarily found within the spinel-structured chromite (Fe,Mg)Cr₂O₄, as well as in crocoite PbCrO₄ and (Al,Cr)₂O₃ at lower contents, constitutes a total 0.01 wt% of the earth's crust. The luminescence of Cr³⁺ as an impurity center in ruby Al₂O₃:Cr³⁺ was first identified by Becquerel in 1867.¹⁷ After that, the Al₂O₃:Cr³⁺ crystal as the first solid-state laser was demonstrated by Theodore Maiman at Hughes Lad in 1960.¹⁸ Following this breakthrough, Cr³⁺ ions in octahedral environments have become a subject of intensive research, especially in ionic crystals and coordination complexes. Representative examples include emerald Be₃Al₂Si₆O₁₈:Cr³⁺,¹⁹ alexandrite BeAl₂O₄:Cr³⁺,²⁰ MgO:Cr³⁺,²¹ Cr(urea)₆³⁺,²² and Cr(acac)₃.²³ Along with the advances in photophysics and photochemistry, luminescence of Cr³⁺ has also been explored in diverse crystal structures for improving luminescence performance, including perovskite, pyroxene, garnet, fluorite, and olivine structures. Apart from modulation of the host crystal structure, ionic substitutions by various cations (Al³⁺, Ga³⁺, Sc³⁺, In³⁺, etc.) and anions (N³⁻, O²⁻, F⁻, Cl⁻, etc.) have been reported for local structural engineering, giving rise to precise spectrum tuning from 700 to 1200 nm.¹⁴ In parallel investigations, Cr⁴⁺ ions were also explored as activators for laser crystals featuring tetrahedral coordination, such as garnet-structured (Y,Lu)₃Al₅O₁₂:Cr⁴⁺,^25,26 zircon-structured ZrSiO₄:Cr⁴⁺,²⁷ and olivine-type Mg₂(Si,Ge)O₄:Cr⁴⁺.²⁸

Within the crystal field theory, the energy levels and radiative transitions of chromium ions in O_h symmetry can be depicted using the Tanabe–Sugano diagram, which is expressed in terms of the crystal field parameter Dq, and Racah parameters B and C.^29,30 However, actual crystals often exhibited reduced symmetries (e.g., pseudo-octahedral, trigonal, and quadrate symmetries), causing secondary splitting of the degenerate multiplets. Furthermore, the ²E state of Cr³⁺ and the ¹E state of Cr⁴⁺ ions, although insensitive to crystal field strength, are influenced by the electron nephelauxetic effect. The nephelauxetic effect also impacts Racah parameters B and C, which dictate the Dq/B boundary value of ²E and ⁴T₂ excited states and thus determine the radiative transition pathway.^31,32 Due to these complications, the profound role of the host crystal in influencing the intra-configurational d–d transition of chromium ions remains incompletely understood. For example, the long luminescence lifetime for the high-rate ⁴T₂ → ⁴A₂ transition, the exact coupling of the Cr³⁺–Cr³⁺ pair, and the luminescence of tetrahedrally coordinated Cr³⁺ ions are still open for further investigations.

Inspired by the fundamental importance and technological significance of chromium ions, extensive efforts have been devoted to experimental and computational investigations of chromium-activated NIR phosphors in recent years. Meanwhile, several publications have reviewed the NIR luminescence characteristics and specialized applications of these phosphors.^14,15,24 Nevertheless, a comprehensive review encompassing energy level determinations, spectrum design, and materials preparations is lacking. This review aims to fill the gap and provide the theoretical basis and experimental guidance for developing chromium-activated NIR phosphors with desired properties (Fig. 1). We begin with the basic crystal field theory for 3d orbitals and delve into the effects of polyhedral structure/distortions, nephelauxetic effect, and ionic coupling interactions on the energy levels of Cr³⁺/Cr⁴⁺ ions. We also discuss the method for determining the position of the chromium level within the host bandgap, similar to constructing the host-referred binding energy (HRBE) of Ln³⁺ ions within various matrices. Following this, we outline the design principles to realize precise control of chromium luminescence, encompassing emission wavelength/bandwidth, efficiency, thermal quenching resistance, persistent luminescence (PersL), and mechanoluminescence (ML). Next, we highlight the recent advances in the chemical synthesis of bulk and nanoscale chromium-activated NIR phosphors. We continue with an overview of the recent progress in utilizing these phosphors for photonic and biomedical applications. Finally, we conclude by discussing future opportunities and challenges for advancing chromium-activated NIR phosphors.

Fig. 1 Advances in theories and practical applications of chromium-activated phosphors. (a) Crystal field splitting of 3d orbitals within regular octahedral and tetrahedral coordination. (b) Tetrahedral coordination of Cr³⁺ ions and the corresponding non-radiative relaxation process. (c) Schematic representation of Cr³⁺–Cr³⁺ pairs and energy level of magnetic coupling. (d) Tanabe–Sugano diagram of octahedral Cr³⁺ ions emphasizing the critical boundary between the ²E and ⁴T₂ states. (e) Schematic HRBE diagram for TM ions. (f) Inhibited energy quenching in concentrated chromium systems. (g) Secondary energy level splitting of Cr³⁺ ions in disordered lattices (D_3d symmetry). (h) Emerging applications of chromium-activated phosphors in non-destructive analysis, (i) plant lighting, (j) night vision, (k) information security, and (i) bioimaging. Reproduced with permission.²⁴ Copyright 2023, John Wiley and Sons.

2. Crystal field theory and energy level splitting

TM ions are characterized by their partially filled d orbitals, which are highly susceptible to the coordination environments. In the 1940s, Finkelstein initially computed the energy levels of the 3d³ configuration in a cubic symmetry potential within potassium chrome alum KCr(SO₄)₂ by starting with the empirical spectroscopic data of Cr³⁺ ions.³³ Subsequently, the crystal field theory was progressively developed based on the fundamental principles of molecular orbital theory and ligand field theory. In this section, we begin with the energy level splitting of chromium ions in crystals, followed by the interionic energy coupling and energy level position within the host bandgap.

2.1. 3d orbital splitting in regular polyhedra

The 3d orbital with an angular quantum number of 3 comprises 5-degenerate sub-orbitals (d_xy, d_yz, d_xz, d_x²−y², d_z²) with quantum numbers (m_l) ranging from −2 to 2, characterizing the magnetic momentum. The effect of crystal field splitting on the 3d orbital (single d-electron) in octahedral O_h symmetry has been investigated, in which the 5-degenerate orbitals split into three low-lying t_2g orbitals (d_xy, d_yz, d_xz) and two high-lying e_g orbitals (d_x²−y², d_z²), with a total splitting energy of 10 Dq (Fig. 2a).³⁴ Besides the octahedron, there are some other regular polyhedra, such as a 4-coordinated tetrahedron (T_d symmetry), 6-coordinated cube (O_h symmetry), and 12-coordinated cuboctahedron (O_h symmetry). The crystal field splitting magnitudes among these regular polyhedra are significantly different.

Fig. 2 Splitting of the 3d orbital in polyhedral coordination. (a) Visualization of d orbitals in octahedral and tetrahedral sites. (b) Free 3d orbital with a 5-degenerate state and associated crystal field splitting in a regular polyhedral potential.

The crystal field splitting can be approximately evaluated by the first-order perturbation theory, and the matrix elements can be expressed as^35,36

where V represents the crystal field potential and Ψ_nlm denotes the single d-electron wavefunction,

Ψ_nml = R_nl(r)Y^l_θ(θ, l) (2)

where R_nl and Y^l_θ represent the radial and angular parts of the spherical harmonics, respectively. Considering the 5-degenerate orbital with 5 magnetic quantum numbers, the perturbation matrix features an order of 5, containing a total of 25 elements. After matrix diagonalization, the crystal field splitting can be obtained from the eigenvalues and eigenfunctions, as shown in Fig. 2b. The splitting values differ in magnitude depending on the exact coordination environment, even for the identical point symmetry of O_h. The total crystal-field splitting energy decreases with increasing coordination number, except for tetrahedral coordination. Specifically, the 4-coordination tetrahedral field is demonstrated with the lowest splitting energy of 40/9Dq, whereas the 6-coordination octahedral field has the highest splitting energy of 10Dq. This difference arises from the spatial orientation of the d orbitals relative to the ligand. In a tetrahedral coordination, the t_2g orbitals point more directly toward the ligands, resulting in significant electrostatic interaction with higher splitting energy, while the e_g orbitals are directed toward the inter-ligand gaps (Fig. 2a). Therefore, the t_2g orbitals of the tetrahedron lie above the e_g orbitals in energy. Conversely, in an octahedral coordination, the t_2g orbitals point directly toward the gap, while the e_g orbitals are oriented toward the ligands. This results in the t_2g orbitals lying below the e_g orbitals. Finally, all the polyhedra share identical median energy as the free 3d electron.

2.2. Chromium ions in octahedral sites

TMs typically contain multiple electrons, which can form diverse distributions over the 3d orbital. Due to distinct repulsion interactions, each electron configuration corresponds to a particular energy state denoted as spectral term ^2S+1L, in which the 2S + 1 represents the spin multiplets and L is the algebraic sum of magnetic quantum numbers. The detailed terms of various electron configurations are listed in Table 1. Due to the complementary nature of electron numbers in dⁿ and hole numbers in d¹⁰⁻ⁿ configurations, they share identical spectral terms. Besides, according to Hundt's rule, the spectral term with the largest spin multiplicity and magnetic number is identified as the ground state with the lowest energy.

Table 1 Spectral terms of the dⁿ configuration. Note that the Arabic numbers in the brackets denote the occurrence frequency of the associated terms

d^n electron	Spectral term ^2S+1L
d^1, d^9	^2D
d^2, d^8	^3F, ^3P, ^1G, ^1D, ^1S
d^3, d^7	^4F, ^4P, ^2H, ^3F, ^2D(2), ^2P, ^2G
d^4, d^6	^5D, ^3H, ^3G, ^3F(2), ^3D, ^3P(2), ^1I, ^1G(2), 1F, ^1D(2), ^1S(2)
d^5	^6S, ^4G, ^4F, ^4D, ^4P, ^2I, ^2H, ^2G(2), ^2F(2), ^2D(3), ^2P, ^2S
d^10	^1S

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Owing to the splitting of the 3d orbital in a crystal field as described above, the degenerate spectral terms (^2S+1L) correspondingly split into a series of spin multiplets. Specifically, Cr³⁺, with three 3d electrons gives rise to states with total spin S = 3/2 (quartets) and S = 1/2 (doublets). In contrast, Cr⁴⁺, with two 3d electrons, forms states with S = 1 (triplets) and S = 0 (singlets). The resulting energy levels in octahedral O_h symmetry can be elucidated using the Tanabe–Sugano diagram, which is expressed in terms of the crystal field strength parameter Dq and Racah parameters B and C.^29,30 For Cr³⁺ ions (Fig. 3a), the ground spectral term of ⁴F further splits into three spin-quartet levels ⁴A_2g, ⁴T_2g, and ⁴T_1g, while the excited term of ²G splits into four spin-doublet levels ²E_g, ²A_1g, ²T_1g, and ²T_2g. Among these states, the ⁴A_2g state is recognized as the ground state with an assigned energy of zero. According to the configuration coordinate model, the ²E_g, ²T_1g, and ²T_2g states develop parallel potential energy curves relative to the ground state ⁴A_2g due to their same electronic configuration of t_2g³. In contrast, the ⁴T_2g and ⁴T_1g states, with a different electronic configuration (t_2g²e_g) than the ⁴A_2g state, exhibit highly variable energy depending on the local coordination environments. These characteristics of Cr³⁺ ions are favorable for spectral tunability. Typically, the ²E_g state is the predominant first excited state in a strong crystal field environment, resulting in a sharp R-line emission at ∼700 nm due to the spin-forbidden ²E_g → ⁴A_2g transition. Conversely, the ⁴T_2g state becomes the first excited state in a weak crystal field environment, giving rise to a broadband NIR emission with tunable wavelengths from 700 to 1100 nm through the spin-allowed ⁴T_2g → ⁴A_2g transition.

Fig. 3 Energy level of Cr³⁺ ions in octahedral sites. (a) Tanabe–Sugano diagram of Cr³⁺ ions in octahedral O_h symmetry. (b) Linear dependence of boundary Dq/B values on C/B values. Reproduced with permission.³² Copyright 2024, American Chemical Society. (c) Branching rules of the 32-point symmetries. Reproduced with permission.⁴⁷ Copyright 2013, Royal Society of Chemistry. (d) Crystal field splitting and spin–orbital coupling of Cr³⁺ ions in D_3d symmetry.

The crystal field strength in octahedral Cr³⁺ sites can be estimated by Dq/B, corresponding to the ratio of the crystal field parameter to the Racah parameter (SI).³⁷ The critical value of (Dq/B)_b, defined as the boundary condition between weak and strong crystal fields, is identified by the crosspoint between the ⁴T_2g and ²E_g states in the Tanabe–Sugano diagram (Fig. 3a). Notably, there are different values reported for this critical (Dq/B)_b, varying between 2.1 and 2.3.^38–41 For example, the original Tanabe–Sugano diagram is generally established with a fixed C/B value of 4.5, resulting in a boundary (Dq/B)_b value of 2.1.^42,43 In contrast, Henderson and Imbusch's work cites a value of 2.3 for this boundary.⁴⁴ Besides, discrepancies sometimes arise between the calculated Dq/B value and experimental observation: the emission spectrum presents a broadband profile related to a weak crystal field, whereas the calculated Dq/B value indicates a strong crystal field environment.^45,46 Recently, Song et al. reported that the boundary (Dq/B)_b value is closely linked to the Racah parameters, which describe the strength of electron–electron repulsion within the d orbital, as well as the covalency between Cr³⁺ ions and p orbitals of ligands.³² Consequently, a linear relationship between the boundary (Dq/B)_b and the C/B value can be derived (Fig. 3b), as depicted in eqn (3):

This relationship is essential for evaluating the crystal field environment and predicting the radiative transition of Cr³⁺ ions. Typically, a larger C/B results in a greater boundary (Dq/B)_b value.

The Tanabe–Sugano diagram applies exclusively to Cr³⁺ ions at an octahedral O_h symmetrical site, where the intra-configurational d–d transition is parity-forbidden. However, the perfect octahedral symmetry is rarely encountered in actual crystals, and common phosphor materials generally crystallize to lower symmetries, such as trigonal symmetric D_3d, tetragonal symmetric C_4v, and full rotational symmetric C_n (Fig. 3c). The reduction in the site symmetry can cause secondary splitting of the spectral terms into more complex components (the twofold Kramers’ degeneracy is preserved). Taking the trigonal D_3d symmetry in weak crystal fields as an example (Fig. 3d), the term T splits into A and E states. Specifically, the spin-quartet ⁴T_2g splits into ⁴A_1g and ⁴E_g levels, and the spin-doublet ²T_1g splits into ²A_2g and ²E_g levels. Moreover, due to spin–orbital coupling, the degenerate energy levels undergo additional splitting, albeit with a lower magnitude of splitting energy. Therefore, the high sensitivity to the coordination environment enables substantially tunable absorption and NIR luminescence in Cr³⁺ ions. Notably, in practical settings with non-centrosymmetric octahedra, the parity designation (i.e., the even irreducible representation g) has to be disregarded due to the departure from inversion symmetry.⁴⁴

In contrast to Cr³⁺ ions, the ground state of Cr⁴⁺ ions ([Ar]3d²) at octahedral sites is represented by the ³T₁ term, and their excited states are characterized as ¹T₂ in strong crystal fields and ³T₂ in weak crystal fields, as depicted by the Tanabe–Sugano diagram. However, there are scarce reports of Cr⁴⁺ ions at octahedral sites, ascribed to elevated formation energy. Alternatively, Cr⁴⁺ ions are reduced to the +3 valence.

2.3. Chromium ions in tetrahedral sites

Although Cr³⁺ ions prefer the octahedral site, recent research has claimed Cr³⁺ NIR luminescence within the tetrahedral lattice, such as SrGa₂O₄:Cr³⁺,⁴⁸ SrGa₄O₇:Cr³⁺,^49,50 and Mg₂Al₄Si₅O₁₈:Cr³⁺.⁵¹ As discussed before, Cr³⁺ ions at tetrahedral sites experience significantly weaker crystal fields compared to those at octahedral sites, thus allowing the emission to shift to a longer wavelength. The crystal field splitting of Cr³⁺ ions in tetrahedral sites is similar to that of ions with an [Ar]3d⁷ electronic configuration in octahedral sites, in which both the ground and lowest excited states are spin-quartets, that is, (⁴T₁, t₂e²) and (⁴T₂, t₂²e). Notably, Cr³⁺ ions at tetrahedral sites generally experience relatively high formation energy, resulting in low occupation rates, as shown in Fig. 4a. Besides, the energy levels of Cr³⁺ at tetrahedral sites (e.g., Al–O₄ tetrahedron within Mg₂Al₄Si₅O₁₈ matrix) undergo substantial secondary splitting due to the distorted Cr–M bond lengths and Jahn–Teller distortions (Fig. 4b). As a result, the radiative transition of ⁴T₂ → ⁴T₁ could be completely quenched through multi-phonon relaxation mediated by a series of sub-levels (Fig. 4c and d). Therefore, experimental observation of Cr³⁺ luminescence in tetrahedral symmetry remains difficult due to high defect formation energy and significant non-radiative processes. The reported NIR luminescence of Cr³⁺ ions in tetrahedral lattices may originate from the local pseudo-octahedra, which can be ascribed to the structural distortion following the incorporation of Cr³⁺ ions.

Fig. 4 Energy level and quenching process of Cr³⁺ ions at tetrahedral sites. (a) Energy diagram of defect formation for Mg₂Al₄Si₅O₁₈:Cr³⁺ under a moderately reducing atmosphere. Dashed lines indicate the Fermi level determined by the intersection of the lowest intrinsic defects, thin solid lines represent the primary intrinsic defects, and thick solid lines denote the Cr-related defects. (b) Kohn–Sham 3d energy levels of Cr³⁺ at the [Mg–O₆] octahedral site and the [Al–O₄] tetrahedral site. In the [Mg–O₆] octahedral site, Cr³⁺ experiences minor energy level splitting, whereas in the Al(A)–O₄ tetrahedral site, Cr³⁺ undergoes substantial secondary splitting due to intrinsic and Jahn–Teller distortions. (c) Three-dimensional configuration coordinate diagram for the ⁴T₁ ground state and ⁴T₂ lowest excited state of Cr³⁺ ions in Al(A)–O₄ tetrahedral sites. (d) Schematic diagram of the excitation and relaxation pathways. Reproduced with permission.⁵² Copyright 2022, American Chemical Society.

Compared to Cr³⁺ ions, Cr⁴⁺ ions are preferentially accommodated in the tetrahedral site. As the electron configuration of 3dⁿ is complementary to the hole configuration of 3d¹⁰⁻ⁿ and the 3d orbital manifests an inverse splitting pattern at tetrahedral versus octahedral sites (Fig. 2b), the Tanabe–Sugano diagram of Cr⁴⁺ ions ([Ar]3d²) in tetrahedra can be extrapolated from that of [Ar]3d⁸ configuration in octahedra (Fig. 5). The rationality was validated by Song et al. employing an irreducible tensor operator calculation.⁵³

Fig. 5 Energy level of Cr⁴⁺ ions. Tanabe–Sugano diagram of (a) 3d⁸ electronic configuration in octahedral and (b) 3d² electronic configuration in tetrahedral sites. Reproduced with permission.⁵³ Copyright 2022, John Wiley and Sons.

However, the boundary value of (Dq/B)_b of the 3d² configuration at tetrahedra is around 3.7, much larger than that of 3d⁸ at octahedra (1.65). The possible explanation can be elucidated through crystal field theory. The ¹E excited state at the tetrahedral site manifests an electron configuration (e²) identical to the ³A₂ ground state, and the associated ³A₂ → ¹E transition characterizes spin inversion that counteracts the Coulomb interaction. In contrast, the ³T₂ excited state involves an electron transfer from e to t₂ orbital that requires overcoming the crystal field effect. As discussed in Section 2.1, tetrahedral coordination undergoes significantly reduced crystal splitting relative to octahedral coordination. Therefore, the boundary value in the tetrahedral site is much larger, beyond which the ³T₂ state is higher-energetic than the ¹E state due to the pronounced crystal field effect rather than Coulomb interaction. In actual lattices, the Dq/B values typically range from 1.3 to 1.8 (SI),⁵³ and the ³T₂ state is the lowest excited state, which renders tunable NIR-II emission through the spin-allowed ³T₂ → ³A₂ transition. Only in limited lattices with strong crystal fields, such as Li(Al,Ga)O₂:Cr⁴⁺ and Li₂MgSiO₄:Cr⁴⁺,^54,55 the ¹E state becomes the predominant first excited state and produces sharp-line emission through the ¹E → ³A₂ transition.

2.4. Interaction coupling of chromium pair

In addition to the crystal field effect, unshielded 3d orbitals of neighboring chromium ions may engage in substantial interaction.⁵⁶ Given that the interaction strength typically scales inversely with the spatial separation distance, the coupling of chromium pairs is generally observed in concentrated systems that feature densely compacted octahedral/tetrahedral configurations and short Cr–Cr interionic distances. These interaction couplings predominantly manifest as magnetic coupling and intervalence charge transfer (IVCT).

Magnetic coupling, also known as spin–spin interaction, generally manifests as ferromagnetic coupling (FMC) or antiferromagnetic coupling (AFMC), as schematically illustrated in Fig. 6a. When the neighboring octahedral Cr³⁺ ions exhibit the parallel spin-up alignment (i.e., ferromagnetic order), FMC manifests through a lower-energy radiative transition. Conversely, if the two Cr³⁺ ions are in antiferromagnetic order with up-spin and down-spin 3d electrons, AFMC occurs with one fully-filled bonding state and one empty anti-bonding state, resulting in a higher-energy radiative transition. The anomalous luminescence due to FMC has been documented in inorganic matrices heavily doped with Mn²⁺ or Cr³⁺, whereas the AFMC has only been experimentally observed in limited cases involving Mn²⁺ activated sulfide.^8,57–59

Fig. 6 Energy level coupling of the Cr³⁺–Cr³⁺ pair. (a) Schematic diagram of FMC and AFMC with the Cr³⁺–Cr³⁺ pair. The energy level and optical transitions of the Cr³⁺–Cr³⁺ pair in (b) strong and (c) weak crystal fields.

Fig. 6b and c display the schematic presentations of the energy level splitting of the Cr³⁺–Cr³⁺ pair due to magnetic coupling. Under strong crystal fields, the ground state of the Cr³⁺ ion is the spin-quartet ⁴A₂ (S = 3/2), and the lowest excited state is the spin-doublet ²E (S = 1/2). Consequently, the ground and lowest excited states of the Cr³⁺–Cr³⁺ pair can be denoted as [⁴A₂, ⁴A₂] and [⁴A₂, ²E], with total spin numbers S in the range of 0–3 and 1–2 through (S_A + S_B), …, (S_A–S_B) interactions. Accordingly, the emission originates from the lowest excited spin state to various spin components of the ground state, especially those with identical spins. Due to the partial relaxation of the spin-forbidden nature, the luminescence lifetime of the Cr³⁺–Cr³⁺ pair is shorter than the spin-forbidden ²E → ⁴A₂ transition of isolated Cr³⁺. Additionally, the sharp-line emission for the Cr³⁺–Cr³⁺ pair is also demonstrated at longer wavelengths in contrast to isolated Cr³⁺, and the energy value E(S) associated with the spin interaction can be estimated as follows:

E(S) = −J[S(S + 1) − S_A(S_A + 1) + S_B(S_B + 1)] (4)

where S_A and S_B denote the spin quantum number of activators A and B, respectively, S represents the total spins, and J is the spin–spin coupling strength. The value of J significantly depends on the distance between ions in the pair, typically falling within tens of wavenumbers, e.g., –66.6 cm⁻¹ for LaAlO₃:Cr³⁺.^56,60 Consequently, the spin interaction energy E(S) is approximately 100 cm⁻¹. Similarly, the ground and lowest excited states of Cr³⁺–Cr³⁺ pairs in weak crystal fields can be denoted as [⁴A₂, ⁴A₂] and [⁴A₂, ⁴T₂], with total spin numbers S ranging from 0 to 3. Accordingly, the maximum value of E(S) reaches approximately 450 cm⁻¹. As a result, the magnetic coupling energy is comparatively minimal. Additionally, as the concentration of Cr³⁺ ions increases, energy transfer and reabsorption among Cr³⁺ enhanced, which may also result in spectral red-shift.

Compared with magnetic coupling, IVCT transition occurs with configurational electron transfer between neighboring dopants, resulting in significantly increased Stokes shift and bandwidth. The IVCT luminescence was previously observed in mixed-valence Ce³⁺/Ce⁴⁺, Yb²⁺/Yb³⁺, and Bi²⁺/Bi³⁺ compounds.^61–64 Actually, as Cr³⁺ ions concentration increases, the electron transfer from one Cr³⁺ ion to the neighboring Cr³⁺ ion becomes feasible, forming Cr²⁺–Cr⁴⁺ pair, i.e., Cr³⁺–Cr³⁺ → Cr²⁺–Cr⁴⁺. The anomalous NIR-II luminescence attributed to IVCT of the Cr³⁺–Cr³⁺ pair has recently been observed in heavily doped LaMgGa₁₁O₁₉ and LiGa₅O₈.^65,66 Nevertheless, further experimental spectra and theoretical support are imperative for understanding the IVCT luminescence caused by dopant aggregation

2.5. Position of chromium levels within the bandgap

Based on the preceding discussions, the luminescence spectra of Cr³⁺ and Cr⁴⁺ ions can be feasibly tailored by engineering the crystal fields or the local surroundings. Additionally, the position of ionic levels relative to the host band is also imperative for the luminescence properties. For example, substantial thermal quenching occurs if the lowest excited state integrates into the conduction band (CB) or the ground state is situated within the valence band (VB).

Dorenbos has constructed the HRBE and vacuum-referred binding energy (VRBE) diagrams of Ln^3+/2+, which delineate the relative position of the Ln^3+/2+ ground state with respect to the host bandgap.^69–71 Specifically, the zig-zag curve patterns can be readily reconstructed by referring to the charge transfer (CT) energy of Eu³⁺ or Yb³⁺ ions.^72,73 Compared to the well-shielded 4f orbitals of Ln³⁺, 3d orbitals of TM ions exhibit significant sensitivity to their surroundings as discussed before. Templating a universal framework to construct the HRBE and VRBE diagrams of 3d orbitals for TM ions remains challenging. Notably, Cr³⁺ ions demonstrate pronounced cationic selectivity (e.g., Al³⁺, Ga³⁺, Sc³⁺, and In³⁺) and preferential occupancy in octahedral sites. Therefore, the prediction concerning energy levels of Cr³⁺ relative to the host band can typically be confined to a lattice with octahedral symmetry. In this regard, Qu et al. developed a zig-zag VRBE diagram of TM ions in octahedrally-coordinated aluminates by employing the experimental CT energies.⁶⁷

Fig. 7a displays the zig-zag patterns of VRBE for TM ions in Al₂O₃ and YAG with an octahedral coordination. The Mⁿ⁺/⁽ⁿ⁻¹⁾⁺ denotes the CT band that elucidates the energies for electron transition from valence band maximum (VBM) to Mⁿ⁺. Except Cr^3+/2+ and Mn^4+/3+, the energy levels undergo a significant decrease with the atom evolving from Sc to Zn, aligning with the ionization potentials of TM ions. The phenomenon can be explained by the fact that Cr³⁺ and Mn⁴⁺ ions both exhibit a fully occupied t₂ orbital (t₂³ configuration) with aligned spin in octahedral coordination. The CT process allows an electron to populate from VBM to the higher-energy e orbital with aligned spin (denoted as a high-spin state) or to the t₂ orbital with opposite spin (denoted as a low-spin state). These two processes need to overcome the perturbations induced by the crystal field or the electron–electron Coulomb interactions, thereby resulting in an anomalous increase in VRBE. Notably, a linear dependence between VRBE and VBM has also been observed, exhibiting a slope of 0.13 for Cr^3+/2+, as shown in Fig. 7b. Therefore, the VRBE diagram in other octahedral-coordinated aluminates can be readily reconstructed by referring to the relative energies (columns 2 and 3 in Table 2) and the linear dependence behavior. However, as for Ga/In/Sc-based compounds, more experimental data are needed to verify this universality.

Fig. 7 Positioning of chromium ions within the bandgap. (a) The zig-zag VRBE patterns of 3d orbitals for TM ions at octahedral aluminum sites. The horizontal lines represent the VBM and conduction band minimum of Al₂O₃ (red) and YAG (blue). (b) The VRBE of Cr^3+/2+, Mn^4+/3+, and Fe^3+/2+ at octahedral sites as a function of VBM in aluminates. Reproduced with permission.⁶⁷ Copyright 2019, Royal Society of Chemistry. (c) The zig-zag VRBE patterns of 3d orbital for TM ions at tetrahedral sites. (d) The VRBE at tetrahedral sites as a function of VBM in different host lattices. Reproduced with permission.⁶⁸ Copyright 2022, Elsevier.

Table 2 The predicted relative VRBE of TM ions in octahedral and tetrahedral sites. All energies are in eV^67,68

Atom	Octahedra		Tetrahedra
	TM^3+/2+	TM^4+/3+	TM^3+/2+	TM^4+/3+
Sc	–1.679	—	–1.758	—
Ti	–2.626	–3.792	–2.765	–4.117
V	–3.709	–5.221	–2.815	–5.046
Cr	–2.468	–6.235	–3.771	–4.802
Mn	–4.360	–4.820	–4.874	–5.940
Fe	–4.580	–6.464	–4.321	–6.815
Co	–5.904	–6.962	–5.282	–6.999
Ni	–5.853	–6.214	–5.466	–7.889
Cu	–5.924	–7.494	–6.420	–7.986
Zn	–7.566	–7.885	–7.201	–8.522

---

Unlike Cr³⁺ ions, Cr⁴⁺ ions exhibit a preferential occupation within the tetrahedral sites. Fig. 7c illustrates the VRBE diagram of TM ions in tetrahedral coordination. As the atomic number increases, the energy levels also undergo a significant decrease, with an anomalous increase observed for Cr^4+/3+ and V^3+/2+, which can be ascribed to the fully-filled e orbital (e² configuration) with aligned electron spins of Cr⁴⁺ and V³⁺ in tetrahedral coordination. Additionally, the VRBE also demonstrated a linear dependence on VBM with a slope of 0.25 (Fig. 7d). Therefore, it is also straightforward to reconstruct the VRBE diagram of TM ions in tetrahedral coordination by referring to the relative energies (columns 4 and 5 in Table 2) and linear dependence behavior.

3. Rational design of chromium-activated phosphors

As discussed above, the luminescence of chromium ions resulting from the intra-configurational d–d transitions is substantially influenced by the local surroundings. Subsequently, we will present the principles of material design for deliberate control of chromium luminescence, including spectral profiles, luminescence efficiency, and thermal quenching resistance. Besides common photoluminescence (PL), we also discuss the design principles for realizing PersL after the cessation of excitation, as well as light emission by mechanical action, i.e., ML.

3.1. Spectral profile

The luminescence spectral profile is predominantly characterized by two parameters, namely emission peak position and bandwidth. In general, Cr³⁺ ions in strong crystal fields present distinct R-line emissions accompanied by multiple vibrational phonon bands, attributed to the spin-forbidden ²E → ⁴A₂ transition.^74,75 In this case, significant splitting of the ²T₁ state may occur in quadrate symmetric environments, leading to possible line emission from the lowest ²T₁ sub-level (i.e., ²E^Q; Q represents the quadrate fields).⁷⁴ With the decrease of crystal field strength, dual emission appears due to the thermally activated ²E → ⁴T₂ population, enabled by reduced energy of the ⁴T₂ state.^76,77 In weak crystal fields, the ⁴T₂ → ⁴A₂ transition that is highly sensitive to the Dq/B value becomes dominant, resulting in tunable luminescence ranging from 700 to 1000 nm with a significant Stokes shift. Therefore, manipulating the local coordination surroundings around Cr³⁺ ions effectively regulates the emission profile.

Based on the Cartesian coordinate, in which the doped luminescence center is set as the coordinate origin, the crystal field strength parameter Dq characterizes a reciprocal relationship with the 5th-power of coordination distance to the anion:⁷⁸

where Ze signifies the ligand charge, R represents the coordination bond length, and r denotes the polar coordinates of the electron which is generally treated as a constant. This association highlights that an increase in the bond length R inversely correlates with the Dq values, providing fundamental guidance for regulating the spectra of chromium ions.

Due to high coordination energy and charge balance, Cr³⁺ ions preferentially occupy equivalent cation sites with octahedral coordination, such as Al³⁺ (0.535 Å), Ga³⁺ (0.62 Å), In³⁺ (0.8 Å), Sc³⁺ (0.745 Å), and Y³⁺ (0.9 Å), as well as partial heterovalent cation sites including Ge⁴⁺ (0.39 Å), Sn⁴⁺ (0.69 Å), Mg²⁺ (0.72 Å), Zr⁴⁺ (0.72 Å), and Zn²⁺ (0.74 Å), as shown in Fig. 8a.^45,79–84 Given a radius of 0.615 Å at octahedral symmetry, the incorporation of Cr³⁺ ions into small-radius Al³⁺ crystallographic sites typically presents significantly short bond lengths and encounters a strong crystal field environment. Due to the identical electron configuration (t₂³) between ²E and ⁴A₂ states, the ²E → ⁴A₂ transition exhibits a pronounced R-line emission at ∼700 nm with negligible Stokes shift, as depicted in Fig. 8b and c. In contrast, Cr³⁺ ions in large-radius Ga³⁺, Sc³⁺, In³⁺, and Y³⁺ crystallographic sites undergo a relatively weak crystal field environment and manifest broadband emissions ranging from 700 to 1000 nm (Fig. 8d). Additionally, with atom evolution from Ga³⁺ to Y³⁺, the crystal field undergoes a continuous decline. For example, emission peaks of perovskite-type Sr₂MSbO₆:Cr³⁺ (M = Ga, Sc, In, and Y) phosphors presented continuous red-shift from 825 to 995 nm with increasing cation radius from Ga³⁺ to Y³⁺, ascribed to reduced crystal fields.⁸⁵ Phosphate KMP₂O₇:Cr³⁺ (M = Ga, Sc, In, and Lu) also demonstrated octahedron-dependent broadband NIR luminescence, showing a progressive reduction in crystal field strength and spectral red-shift from 815 to 900 nm with increasing cationic radius.⁸⁶

Fig. 8 Occupation and configuration coordinate diagram of chromium ions. (a) Selective cationic sites for the incorporation of Cr³⁺ ions. (b) Schematic luminescence spectrum of Cr³⁺ in octahedral sites. Configuration coordination diagram of Cr³⁺ ions in (c) strong and (d) weak crystal field surroundings. (e) Selective cationic sites for the incorporation of Cr⁴⁺ ions. (f) Schematic luminescence spectra of Cr⁴⁺ in tetrahedral sites. Configuration coordination diagram of Cr⁴⁺ ions in (g) strong and (h) weak crystal field surroundings. ΔE₁ denotes the energy gap between the ²E and ⁴T₂ states for Cr³⁺ (¹E and ³T₂ states for Cr⁴⁺), and ΔE₂ represents the ionization energy from the ⁴T₂ or ³T₂ state to the intersection position. The red arrows denote the radiative process and the purple arrows indicate the quenching pathway.

Unlike Cr³⁺, Cr⁴⁺ ions (0.41 Å, CN = 4) preferentially occupy equivalent cation sites with tetrahedral coordination, for example, Si⁴⁺ (0.26 Å), Ge⁴⁺ (0.39 Å), Ti⁴⁺ (0.42 Å), Sn⁴⁺ (0.55 Å), and Zr⁴⁺ (0.59 Å), as well as partial heterovalent cation sites including Al³⁺ (0.39 Å) and Ga³⁺ (0.47 Å), as shown in Fig. 8e.^87–90 The radiative transition of Cr⁴⁺ ions predominantly arises from the spin-allowed ³T₂ → ³A₂ transition with tunable emission spanning 1100 and 1400 nm, since the tetrahedral coordination produces significantly weaker crystal field strength compared to octahedral coordination (Fig. 8f, g, and h). Additionally, the emission peaks experience a continuous red shift as the cationic radius increases. For example, the olivine-type Mg₂SiO₄:Cr³⁺, as a NIR-II solid-state laser material, has been demonstrated with a reduced crystal field and red-shift emissions from 1150 to 1200 nm after substitution of larger Ge⁴⁺ (0.39 Å) for smaller Si⁴⁺ (0.26 Å).^87,91–95

As discussed in Section 2.4, an increase in the dopant concentration leads to strong interactions between the unshielded 3d electrons of neighboring chromium ions, including magnetic coupling and IVCT. This coupling further results in the reconfiguration of energy levels and a consequent modification of the emission peak. For example, LaAlO₃:Cr³⁺ phosphor exhibited R-line emission at 735 nm accompanied by multiple vibronic lines (745.2, 755.4, 770.3, and 776.8 nm) at a low Cr³⁺ concentration, while new emission lines at 740.5, 747.3, and 758.2 nm appeared with increasing concentration due to magnetic couplings of the Cr³⁺–Cr³⁺ pair.⁶⁰ The anomalous broadband emission of Cr³⁺ ions has also been observed in magnetoplumbite-type Sr(Al,Ga)₁₂O₁₉, spinel-type (Mg/Zn)Ga₂O₄, and Na-β-Al₂O₃.^96,97 However, the magnetic coupling energy is relatively low, thereby limiting the spectral tunability in a narrow range. In contrast, the IVCT presents significantly increased Stokes shift and red-shifted emission. Recently, LaMgGa₁₁O₁₉:Cr³⁺ and LiGa₅O₈:Cr³⁺ have been documented to exhibit anomalous NIR-II luminescence at high-level doping concentrations of Cr³⁺ ions, which was ascribed to the IVCT of the Cr³⁺–Cr³⁺ pair.^65,66 Since the interactions occur among neighboring Cr³⁺ ions, these lattices generally feature a short cationic separation. For example, LaMgGa₁₁O₁₉:Cr³⁺ presents face-sharing octahedra with a Ga–Ga distance of 2.80 Å. (Mg/Zn)Ga₂O₄:Cr³⁺ also demonstrates a short Ga–Ga distance of 2.93 Å.

Besides the peak position, the emission bandwidth is another essential parameter that characterizes the spectral profile. The emission bandwidth can be quantified by the value of full width at half maximum (FWHM), which is elucidated by eqn (6):⁴⁴

where S denotes the Huang–Rhys factor which is indicative of the electron–phonon coupling degree, ℏω represents the phonon energy of the host lattice, and k is the Boltzmann constant. Therefore, strong electron–phonon coupling and large phonon energy are favorable for broadening the bandwidth. Cr³⁺ ions experiencing weak crystal fields are advantageous for producing broad emissions with strong electron–phonon coupling. Additionally, since the spin-allowed ⁴T₂ → ⁴A₂ transition is highly susceptible to local coordination, multi-site occupation of Cr³⁺ ions in different surroundings is favorable for further increasing the emission bandwidth by inhomogeneous broadening. For example, β-Ga₂O₃, containing only one Cr³⁺ crystallographic site, manifested an NIR emission with an FWHM of approximately 100 nm.⁹⁸ In contrast, dual-site occupation of Cr³⁺ ions in perovskite-structured La₂MgZrO₆ lattice resulted in an FWHM of 210 nm,⁸⁴ and Ca₃Ga₂Ge₃O₁₂:Cr³⁺ displayed an FWHM of approximately 260 nm due to the triple-site occupation of Cr³⁺ ions.⁹⁹ Nevertheless, despite the structural feasibility of incorporating Cr³⁺ ions into different surroundings to expand the emission, it is challenging to precisely regulate the concentration fraction of Cr³⁺ ions at these octahedral sites within a single phase, thereby posing certain limitations in the spectral regulation.

In addition to multi-site occupation, manipulating the oxidation states of chromium ions within a single phase is also useful for broadening the emission bandwidth. Due to distinct luminescence natures, the simultaneous presence of Cr³⁺ and Cr⁴⁺ ions enables the extension of the luminescence spectrum from the NIR-I to NIR-II region. For example, the Cr³⁺/Cr⁴⁺ co-doped CaGa₄O₇ demonstrated a dual-emission at 740 and 1290 nm, in which the Cr³⁺ and Cr⁴⁺ selectively occupied the [Ga–O₆] octahedral and [Ga–O₄] tetrahedral sites.¹⁰⁰ Similarly, olivine-type Mg₂SiO₄:Cr³⁺/Cr⁴⁺ phosphor demonstrated a dual-emission at 760 and 970 nm with an FWHM of 419 nm.¹⁰¹ Despite the encouraging progress, achieving precise regulation of chromium valence within a single matrix remains challenging. The incorporation of charge compensators, such as Li⁺ and Na⁺, proves advantageous in mitigating the local charge imbalance, thereby regulating the valence states of chromium ions. For example, Cai et al. realized a single emission at 935 nm from Cr³⁺ by incorporating 1 wt% Li₂CO₃ into an olivine-type Mg₂GeO₄ compound.⁸⁷ Moreover, the specialized oxidation/reduction conditions could effectively stabilize the Cr⁴⁺/Cr³⁺ valence states during synthesis.⁸⁰ Accordingly, they also achieved sole Cr⁴⁺ luminescence at 1190 nm under a high-purity oxygen atmosphere and observed comparable dual emissions from Cr³⁺ and Cr⁴⁺ by using 0.5 wt% Li₂CO₃ flux under air conditions.

Co-doping the auxiliary activator is also viable for modulating the spectral profile by introducing additional emission peaks. Several activator ions, including Nd³⁺, Yb³⁺, Er³⁺, and Ni²⁺ can accept the excitation energy of Cr³⁺ and produce secondary emission in the longer-wavelength range.^102–110 For example, the LiScP₂O₇:Cr³⁺ phosphor presented a broadband NIR emission at 880 nm due to weak crystal fields. After incorporating Yb³⁺ ions as a co-dopant, 1000 nm emission through the ²F_5/2 → ²F_7/2 transition of Yb³⁺ ions emerged due to the sensitization of Cr³⁺ ions. Notably, the lanthanide luminescence originating from the intra-configurational f–f transitions was less susceptible to the surrounding environment than d–d transitions of Cr³⁺,^6,111,112 contributing to improved luminescence efficiency and thermal stability (vide infra).

3.2. Luminescence efficiency

High luminescence efficiency is the key determinant for the integration of high-performance devices, characterized by superior light output and photoelectric efficiency. The absorption efficiency (AE) of intra-configurational d–d transition remains notably lower compared to parity-allowed f–d or s–p transitions in Ce³⁺, Eu²⁺, and Bi³⁺ ions. For example, the major absorption of Cr³⁺ due to electron transition from ⁴A₂ to ⁴T₂ and ⁴T₁ states in the visible region typically shows a low absorption cross-section in the range from 10⁻¹⁹ to 10⁻²⁰ cm², while the f–d transition manifests a cross-section of 10⁻¹⁶–10⁻¹⁷ cm². The ⁴A₂ → ⁴T₁(P) transition in the UV region and occasionally observed ⁴A₂ → ²E transition at ∼700 nm are even weaker (10⁻²¹ to 10⁻²² cm²), making the photo-excitation process inefficient. Regarding this issue, considerable efforts have been devoted to improving the luminescence efficiency of chromium-activated phosphors by leveraging crystal structure engineering and heavy doping.

3.2.1. Polyhedral distortion. The luminescence characteristics of chromium ions exhibit high sensitivity to the local octahedral/tetrahedral coordination geometry. The intra-configuration d–d transition is parity-forbidden within centrosymmetric O_h sites, resulting in significantly low AE. Structural distortion facilitates the relaxation of the parity-forbidden rule by introducing odd-parity components, thereby enhancing the AE. Although this distortion simultaneously generates additional lattice vibrations that promote partial non-radiative relaxation and consequently reduce the internal quantum efficiency (IQE), the external quantum efficiency (EQE), produced by AE × IQE, typically exhibits an overall enhancement due to the prominent effect of the increased AE. In this regard, the incorporation of chromium ions into disordered polyhedral sites is advantageous for improving the luminescence efficiency. Taking the octahedra as an example, the distortions characterized by bond lengths (D) and angles (σ²) can be quantified using the Baur distortion index (BDI):^113,114where l_i and l_av denote the i^th bond length and corresponding average value, and φ_i refers to the i^th angle. The regular octahedron with O_h symmetry is characterized by uniform bond lengths and angles, resulting in distortion indices of zero. Any infinitesimal perturbation of a single vertex could result in the disruption of local symmetry, thereby promoting luminescence efficiency by breaking the parity-forbidden rules.¹¹⁵ The NaSbF₄:Cr³⁺ phosphor exhibited highly distorted [Sb–F₆] octahedra with a D value of 0.1478, resulting in substantially higher AE (56.2%) compared to previously reported fluorides, e.g., 44.6% for Na₃ScF₆:Cr³⁺ and <30% for K₂NaInF₆:Cr³⁺.^81,116,117 In another example, a gradual substitution of larger Sc³⁺ for smaller Al³⁺ ions in the Ca₃Al₂Ge₃O₁₂:Cr³⁺ matrix was found to enhance the angle distortion, with the σ² index increasing from 2.69 to 5.12. In line with the observation, a high EQE of 33.5% was recorded for the intermediate composition of Ca₃Al_1.8Sc_0.2Ge₃O₁₂:Cr³⁺, significantly surpassing that of the Ca₃Al₂Ge₃O₁₂:Cr³⁺ counterpart (25.7%).¹¹⁸

Besides octahedra, other regular polyhedra, including tetrahedra, hexahedra, and cuboctahedra, also present equal bond lengths and angles. The BDI method that separately quantifies the distortion of bond length and angle may not appropriately reflect the degree of polyhedron distortion. To provide a unified quantification of distortion in different polyhedra, Song et al. proposed a polyhedral distortion index M to characterize the difference between the real and idealized polyhedra, denoted as the best-fitted idealized polyhedron (BFIP) method:¹¹⁹

where N denotes the coordination numbers, (xⁱ, yⁱ, zⁱ) and (x^R, y^R, z^R) represent the ligand coordinates in idealized (i) and real (R) polyhedra. The calculation of M can be accessed online at https://bfip.crystalstructure.cn, and a large M implies a more significant polyhedral disorder.

The BFIP fitting has been proven versatile in quantifying polyhedral distortion. In the phosphate series KAP₂O₇:Cr³⁺ (A = Ga, Sc, In, and Lu) phosphors, for example, the distortion index D exhibited a discontinuity when using Baur distortion fitting, while BFIP fitting revealed a progressively strengthened distortion of [A–O₆] octahedra as the A cationic size increased.¹¹⁹ In another study, the substitution of smaller Zn²⁺ for larger Ca²⁺ in the Ca₄HfGe₃O₁₂:Cr³⁺ matrix was found to increase the index M from 2.39 to 2.40, resulting in enhanced EQE from 20% to 29% due to the relaxation of the parity-forbidden rule.¹²³ Therefore, lattice distortion, which can be readily induced by the cation substitution, proves beneficial for improving the luminescence efficiency of Cr-activated phosphors.

3.2.2. Heavy doping. Besides the polyhedral distortion, an alternative approach to promote AE involves high-level doping. However, elevating the dopant concentration generally results in pronounced luminescence quenching, arising from extensive interactions among closely situated activators via Coulomb coupling or direct orbital overlap.^124,129 The rate of energy transfer typically scales inversely with the spatial separation of neighboring dopants.¹²⁸ As the doping concentration of Cr³⁺ ions increases, rapid energy transfer among Cr³⁺ ions occurs due to a reduction in inter-ionic distance, resulting in the dissipation of excitation energy to lattice defects or impurities, an effect known as concentration quenching (Fig. 9a). Although the magnetic coupling within Cr³⁺–Cr³⁺ pairs is theoretically expected to improve luminescence efficiency by relaxing the spin-forbidden rule (as discussed in Section 2.4), direct experimental confirmation of this mechanism remains limited.

Fig. 9 Energy transfer regulation in concentrated systems. Schematic depiction of typical (a) concentration quenching, (b) structural confinement effect, and (c) energy extraction by Ln³⁺ emitters within a concentrated Cr³⁺ system. Crystal structure of (d) Sr₉Cr(PO₄)₇ and (e) Sr₇NaGa(PO₄)₆ and coordination surroundings of [Cr–O₆] octahedra. Reproduced with permission.^120,121 Copyright 2021, American Chemical Society, and Copyright 2022, Royal Society of Chemistry. The cationic distances are labeled. (f) Crystal structure and (g) Ga–Ga inter-cationic distance of the Li₂Sr₂Al(PO₄)₃ lattice. Reproduced with permission.¹²² Copyright 2022, American Chemical Society.

To address the issue of concentration quenching, host lattices that can provide structural confinement of the dopant ions have been developed to inhibit inter-ionic interactions (Fig. 9b). These lattices generally characterize markedly prolonged inter-cationic distances. This behavior was early documented for the manganese-based β-calcium phosphate Sr₉MnLi(PO₄)₇ phosphor, in which the Mn²⁺ sites are separated by a considerably long distance of over 8 Å. Subsequently, similar behavior was noted in isomorphic Sr₉M(PO₄)₇:Cr³⁺ (M = Ga, Sc, In, and Lu) phosphors.^{120,129–131} As displayed in Fig. 9d, the [M–O₆] octahedra are well separated by infinite chains of [P/Sr–O_n] polyhedra with an M–M distance of over 8.9 Å, which significantly exceeds the critical energy transfer distance of 5 Å.¹³² As a result, the AE and EQE of Cr³⁺ ions in [M–O₆] octahedra progressively increased as Cr³⁺ concentration increased from 0 to 80%, without the issue of concentration quenching. Even though all the M sites were occupied by Cr³⁺, the luminescence intensity of the Sr₉M(PO₄)₇ phosphor remained 84.23% that of the Sr₉Ga_0.2(PO₄)₇:0.8Cr³⁺ phosphor. The Sr₇NaGa(PO₄)₆:Cr³⁺ phosphor also exhibited a structural confinement effect characterized by Ga–Ga distances ranging from 8.98 to 17.97 Å (Fig. 9e).¹²¹ Consequently, the interionic interactions of Cr³⁺ ions were significantly inhibited, allowing a high Cr³⁺ doping level of 15% without concentration quenching. Similar suppression of concentration quenching has been observed in the Li₂Sr₂Al(PO₄)₃:Cr³⁺ phosphor, in which luminescence quenching occurred at a high concentration threshold of 21% due to an extended Ga–Ga distance of 11.045 Å (Fig. 9f and g).¹²²

Due to the preferential occupancy of chromium ions at the octahedral/tetrahedral sites, the structural confinement effect of chromium activators is only achieved in selective materials systems, despite its popularity in non-chromium-activated phosphors, such as NaYW₂O₈:Tb³⁺,¹³³ CeMgAl₁₁O₁₉:Tb³⁺,^134,135 Rb₃Y(PO₄)₂:Eu²⁺,¹³⁶ and GdAl₃(BO₃)₄:Eu³⁺.¹³⁷ Therefore, developing new matrices exhibiting the structural confinement effect is crucial for suppressing concentration quenching and improving luminescence efficiencies of chromium ions.

A more general approach to suppressing energy quenching at a high Cr³⁺ concentration is co-doping Ln³⁺ ions to capture the excitation energy. In this way, the long-distance energy hopping in the Cr³⁺ sublattice can be disrupted to avoid energy dissipation (Fig. 9c). Instead, the excitation energy is converted into light emission of the Ln³⁺ emitter, leading to a delayed concentration quenching of the overall luminescence, as illustrated in Fig. 10a.¹²⁵ Specifically, the Yb³⁺, Nd³⁺, and Er³⁺ ions are commonly utilized as acceptors to receive energy transferred from Cr³⁺ ions ascribed to their lower-energy emission, as schematically shown in Fig. 10b. In a representative example, Xiao et al. ingeniously leveraged the competition between emitting centers and quenchers to extract excitation energies from Cr³⁺ sensitizers, achieving a high-brightness NIR emission in Cr³⁺–Yb³⁺ (15/3.3%) co-doped Gd₃Sc_1.5Al_0.5Ga₃O₁₂.¹²⁵ The phosphor presented Yb³⁺ emission at ∼1000 nm (²F_5/2 → ²F_7/2) with an EQE value of 51%, compared to 37% for the Yb³⁺-free counterpart (Fig. 10c).¹²⁵ The luminescence quenching inhibition has been documented in full-chromium-based Na₃CrF₆:Ln³⁺ (Er³⁺, Tm³⁺, Yb³⁺, and Nd³⁺) compounds. In contrast to the bare Cr³⁺ host, the Ln-doped samples manifested enhanced multiplet emissions of Ln³⁺ within the NIR-II region, ascribed to the energy transfer from Cr³⁺ to Ln³⁺ ions (Fig. 10d).¹²⁶ Specifically, the luminescence intensities of Ln-doped samples manifested 11-, 6.5-, 280-, and 4.7-fold enhancement compared to representative downshifting nanoparticles of NaYbF₄:Er³⁺, NaYbF₄:Tm³⁺, NaYF₄:Yb³⁺, and NaYF₄:Nd³⁺, respectively. Similar quenching inhibition has also been observed in other chromium-concentrated systems, such as Cr(PO₃)₃:Yb³⁺,¹³⁸ Y₃Ga₅O₁₂:Cr³⁺/Yb³⁺,¹³⁹ Y₃Mg_xGa_5−2xGe_xO₁₂:Cr³⁺/Yb³⁺/Er³⁺,¹⁴⁰ and Ca₂LuMgScGe₃O₁₂:Cr³⁺/Yb³⁺,¹⁴¹ which all exhibited highly efficient short-wavelength infrared emissions.

Fig. 10 Luminescence enhancement in the concentrated-Cr³⁺-sensitized system. (a) Theoretically predicted luminescence IQE/AE/EQE as a function of dopant concentration for typical and suppressed quenching scenarios. The blue curve represents the normal quenching process, whereas the orange curve denotes the suppressed quenching process after incorporating Ln³⁺ ions. (b) Schematic representation of energy transfer from Cr³⁺ to Ln³⁺ ions. (c) Comparative analysis of IQE for Gd₃Sc_1.5Al_0.5Ga₃O₁₂:Cr³⁺/Ln³⁺versus other phosphors. Reproduced with permission.¹²⁵ Copyright 2023, John Wiley and Sons. (d) PL spectra of Na₃CrF₆:Er³⁺/Tm³⁺/Yb³⁺/Nd³⁺ phosphors. Reproduced with permission.¹²⁶ Copyright 2024, Springer Nature. (e) Luminescence peaks and EQE values of LiGa₅O₈:Cr³⁺/Ni²⁺ in comparison to some reported downshifting nanoparticles. Reproduced with permission.¹²⁷ Copyright 2024, John Wiley and Sons.

In addition to Ln³⁺ ions, Ni²⁺ ions are also effective in preventing the dissipation of Cr³⁺ energy to quenching centers, simultaneously rendering efficient NIR-II emission. By utilizing controllable energy transfer from concentrated Cr³⁺ to Ni²⁺ ions, Liu et al. recently demonstrated broadband NIR-II luminescence at 1232 nm with a recorded EQE of 76% in LiGa₅O₈ with an inverse spinel structure (Fig. 10e).¹²⁷ Similarly, by leveraging energy transfer from Cr³⁺ clusters to Ni²⁺ ions, NIR-II luminescence of MgGa₂O₄:Cr³⁺/Ni²⁺ was significantly enhanced compared to the Ni²⁺-free counterpart, resulting in an EQE of 29.4%.^102,105 Co-doping of 1% Ni²⁺ in garnet-structured Ca₃Al₂Ge₃O₁₂:Cr³⁺ (15%) also led to efficient NIR-II luminescence with an enhanced EQE of 45.7%.¹⁴² These studies demonstrate that energy extraction from concentrated Cr³⁺ is advantageous for constructing efficient phosphors.

3.3. Thermal stability

Thermal-induced electron–phonon interactions diminish emission intensity with temperature elevation due to the augmented non-radiative relaxation rate.⁴⁴ Thermal quenching of phosphor is generally governed by three primary mechanisms: thermal ionization, multiphonon relaxation, and crossover. Thermal ionization involves the thermally assisted promotion of an electron from the excited state to the conduction band. For Cr³⁺ ions, this process is unfavorable because the ²E and ⁴T₂ excited states generally lie far below the conduction band edge. Therefore, thermal quenching of Cr³⁺ luminescence is predominantly attributed to multiphonon relaxation and crossover. In particular, for Cr³⁺ ions situated in strong crystal fields, the luminescence of Cr³⁺ is dominated by a pronounced R-line emission with negligible Stokes shift through the spin-forbidden ²E → ⁴A₂ transition. In this case, the thermal quenching process is dominated by the multiphonon relaxation because the identical electron configuration (t_2g³) of the ²E and ⁴A₂ states results in non-crossing energy profiles (Fig. 8c). Owing to the moderate influence of temperature on multiphonon relaxation,¹⁴³ the R-line emission typically manifests decent luminescence thermal stability.

As the crystal field strength gradually reduces, the ²E and ⁴T₂ states become thermally coupled, and the spin-allowed ⁴T₂ → ⁴A₂ transition starts to dominate the emission (Fig. 8d). Consequently, the thermal quenching mechanism is elucidated by crossover relaxation to the ⁴A₂ ground state on account of intersected potential energy profiles. The general Arrhenius expression is instrumental in quantifying this thermal quenching behavior:¹⁴⁴

in which I₀ denotes the luminescence intensity at a cryogenic temperature where the non-radiative processes are negligible; I_T represents the intensity at temperature T; k is the Boltzmann constant; c is a temperature-independent constant; and ΔE signifies the activation energy associated with the relaxation from the ⁴T₂ to ⁴A₂ state. Therefore, a larger ΔE is advantageous for improving the luminescence thermal stability. In general, emissions with longer wavelengths (or larger Stokes shifts) are accompanied by smaller ΔE values, resulting in diminished luminescence thermal stability. For instance, the perovskite-structured Sr₂MSbO₆:Cr³⁺ (M = Ga, Sc, In, and Y) phosphors demonstrated a progressive spectral red-shift from 825 to 1000 nm as the cationic radius increases from Ga³⁺ to Y³⁺, while the luminescence intensity at 410 K experienced a decline from 71% to 10% relative to their respective cryogenic intensity.^85,145 Similarly, isomorphic Cs₂(Ag,Na)InCl₆:Cr³⁺ phosphors presented emissions at 1010 nm, but suffered from severe luminescence thermal quenching, with the emission intensity reduced by approximately 50% at 320 K relative to cryogenic temperature.^146,147

Rigid structures, favorable for restricting the expansion of activators upon excitation, are advantageous for constructing phosphors with small Stokes shifts and high thermal stability.¹⁴⁸ Such structures typically exhibit highly symmetric cubic and hexagonal lattices interconnected by robust coordination bonds, exemplified by UCr₄C₄-type,^149–151 garnet,^152–155 magnetoplumbite,^96,156,157 and spinel structures.^105,158 For example, hexagonal magnetoplumbite-structured LaMgGa₁₁O₁₉:Cr³⁺ phosphor demonstrated an 87% NIR luminescence intensity at 410 K relative to that at room temperature. Likewise, garnet-structured (Lu,Y,Gd)₃Sc₂Ga₃O₁₂:Cr³⁺ and Ca₃Sc₂Si₃O₁₂:Cr³⁺ phosphors presented negligible reduction in luminescence intensity as temperature increases from 77 to 410 K due to the structural rigidity.^80,159 Besides, the cation substitution of Al³⁺ for Sc³⁺ in monoclinic-symmetrical CaScAlSiO₆:Cr³⁺ phosphor improved the structural rigidity, thereby suppressing the nonradiative relaxation and boosting the luminescence thermal stability.¹⁶⁰

Co-doping also proves beneficial in enhancing the resistance to thermal quenching. In contrast to Cr³⁺ ions, the non-radiative transition of Ln³⁺ ions dominantly occurs through multiphonon relaxation, which generally leads to weaker thermal quenching effects. Therefore, by transferring the excitation energy of Cr³⁺ to Ln³⁺ co-dopants as emitters, red-shifted luminescence with improved thermal stability can be obtained. For instance, the LiScP₂O₇ phosphor co-doped with Cr³⁺ and Yb³⁺ ions demonstrated improved thermal quenching resistance compared to the LiScP₂O₇:Cr³⁺ counterpart, originating from energy transfer from Cr³⁺ to thermally-stable Yb³⁺ activators.¹⁶¹ Additionally, zero-thermal quenching in Cr³⁺ and Yb³⁺ co-doped garnet-structured Gd₃Sc_1.5Al_0.5Ga₃O₁₂ has also been accomplished via energy transfer from Cr³⁺ to Yb³⁺ ions.¹⁶²

3.4. Persistent luminescence

PersL is characterized by continuous light emission after a brief storage period of light excitation, lasting from seconds to hours.^163–168 PersL processes utilizing the CB as the carrier transfer medium have gained widespread acceptance, as schematically illustrated in Fig. 11a.^70,169–172 During the charging process, the charge carriers can be partially photo-pumped to the high-energy ionic level or directly to the CB (steps 1 and 2) under X-rays, UV, visible, and NIR irradiation.¹⁶⁴ These photo-generated carriers subsequently diffuse into the electron (steps 3 and 4) and hole (steps 5 and 6) traps. After termination of the light excitation, the trapped carriers are gradually released under thermal stimulation (steps 7 and 8) and eventually captured by the luminescence centers to produce the PersL (step 9). The long-lived carriers in deep traps can also be de-trapped through quantum tunneling (step 10).

Fig. 11 PersL and trap states. (a) PersL charging and de-trapping mechanism: excitation process (steps 1 and 2); thermal-assisted electron (steps 3 and 4) and hole (steps 5 and 6) trapping; electron (step 7) and hole (step 8) de-trapping; PersL emission through activator (step 9); electron release from deep traps due to the quantum tunneling effect (step 10). (b) HRBE zig-zag curve of Ln³⁺ within the YPO₄ host. Reproduced with permission.¹⁷⁶ Copyright 2024, Springer Nature.

In 2011, Bessière et al. reported the Cr³⁺-activated PersL material consisting of ZnGa₂O₄, with an emission peak at 695 nm due to strong crystal fields.^173,174 In a subsequent report in 2012, Pan et al. realized an unprecedented NIR PersL (696 nm) duration of 360 h under sunlight excitation in Zn₃Ga₂Ge₂O₁₀:Cr³⁺.¹⁷⁵ Benefiting from the exceptional tolerance of octahedral sites to Cr³⁺ ions, this groundbreaking result firmly established gallate spinel as the preferred system for fabricating Cr³⁺-activated NIR PersL phosphors and further stimulated research interests in developing associated applications. Later on, Cr³⁺-activated NIR PersL behavior was also identified in other gallium germanate compounds, including Y₃Ga₅O₁₂:Cr³⁺,¹⁷⁷ La₃Ga₅GeO₁₄:Cr³⁺,¹⁷⁸ and LiGa₅O₈:Cr³⁺.¹⁷⁹ In addition, some non-gallate matrices doped with Cr³⁺ ions have recently been documented with PersL behaviors, as detailed in Table 3. For example, spinel-structured Mg₂SnO₄:Cr³⁺ and Zn₂SnO₄:Cr³⁺ were reported to exhibit an NIR PersL duration of 50 h.^180,181 Compared with Cr³⁺ ions, Cr⁴⁺ ions typically exhibit NIR-II emission with a significantly larger Stokes shift, which results in pronounced non-radiative transitions and low luminescence efficiency. Therefore, the reports of PersL behavior in Cr⁴⁺-activated systems are considerably scarce.

Table 3 NIR PersL properties of partial Cr³⁺-activated phosphors

Hosts	PersL excitation	PersL emission (nm)	Excited state	Duration (h)	Ref.
a The threshold of PersL duration is dictated by the observation constraints inherent to night-vision monocular systems. b The threshold of PersL duration is quantified at 0.067 mW Sr^−1 m^−2. c The threshold of PersL duration is quantified at 0.016 mW Sr^−1 m^−2.
β-Ga2O3	UV	720	^2E/^4T2	4	188
ZnGa2O4:Cr^3+	UV-red	688/695	^2E	—	173, 174 and 189
MgGa2O4:Cr^3+	UV	707/715	^2E	—	190
Mg2SnO4:Cr^3+	UV	800	^4T2	50	180 and 181
Zn2SnO4:Cr^3+	UV	800	^2E/^4T2	—	191
Zn3Ga2GeO8:Cr^3+	UV-red	696	^2E	150^a	189 and 192
Zn3Ga2Ge2O10:Cr^3+	UV-red	696	^2E	360^a	175
Zn3Ga2SnO8:Cr^3+	UV-red	696	^2E	300^a	193
LiGa5O8:Cr^3+	UV	716	^2E	1000^a	179
Li2ZnGe3O8:Cr^3+	UV	688	^2E	50	194
Y3Al2Ga3O12:Cr^3+	UV	690	^2E	0.5^b	177
GdY2Al3Ga2O12:Cr^3+	UV	691	^2E	—	195
Ca3Ga2Ge3O12:Cr^3+	UV	749/803/907	^4T2	∼2	196
Gd3Mg0.5Ge0.5Ga4O12:Cr^3+	UV	729	^4T2	—	197
Na2CaSn2Ge3O12:Cr^3+	UV	810	^4T2	—	198
Na2CaTi2Ge3O12:Cr^3+	UV	780	^4T2	—	198
LaGaO3:Cr^3+	UV	740	^2E	—	199
LaAlO3:Cr^3+,Sm^3+	UV	735	^2E	1.5^c	200 and 201
Na0.5Gd0.5TiO3:Cr^3+	UV-red	760	^4T2	—	182
La2MgGeO6:Cr^3+	UV	710	^2E	—	202
Na2CaGe5SiO14:Cr^3+	UV	694/733/815	^2E/^4T2	10^a	203
La3Ga5GeO14:Cr^3+	UV-blue	785/960	^4T2	—	178
Mg4Ga8Ge2O20:Cr^3+	UV	700	^2E/^4T2	25	204
Li5Zn8Ga5Ge9O36:Cr^3+	UV-red	700	^2E	∼11	205
Mg4Ga4Ge3O16:Cr^3+	UV	693	^2E/^4T2	10	206
La3GaGe5O16:Cr^3+	UV-red	700	^2E	—	207
LaZnGa11O19:Cr^3+,Yb^3+	UV	710	^2E/^4T2	1000^a	73
SrGa12O19:Cr^3+,Sm^3+	UV-red	760	^4T2	360^a	183 and 184

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Most Cr³⁺-activated PersL phosphors require charging by high-energy excitation sources (e.g., UV and X-rays), posing certain constraints in utilizing the PersL process. To address this issue, Huang et al. engineered a perovskite-structured Na_0.5Gd_0.5TiO₃:Cr³⁺ PersL phosphor featuring a narrow bandgap of 3.59 eV, enabling charging by low-irradiance red photons (∼2.5 µW mm⁻²).¹⁸² Recently, Liu et al. further demonstrated a broadband NIR PersL phosphor consisting of gallate magnetoplumbite-structured SrGa₁₂O₁₉:Cr³⁺/Sm³⁺, which can be charged by UV to red light via electron tunneling charging, achieving a PersL duration of 360 h.^183,184 In another development, Pan et al. devised an up-converted PersL system, achieved by integrating an up-converting ion pair of Yb³⁺ and Er³⁺ ions into the Zn₃Ga₂GeO₈:Cr³⁺ matrix, demonstrating an NIR PersL duration of more than 24 h by NIR charging at 980 nm.^185–187

PersL behavior is strongly dictated by the attributes of the defect states, including the type, depth, and density.¹⁷⁶ Initially, the PersL traps were typically identified as intrinsic lattice defects, such as oxygen vacancies, cation vacancies, and anti-site occupations. Therefore, the substitution of chromium ions for heterovalent lattice cations is favorable for generating positively and negatively charged defects, which function as electron and hole traps for PersL behavior. For example, Na₂Ca(Sn,Ti)₂Ge₃O₁₂ displays notable NIR PersL behavior following the incorporation of Cr³⁺ ions into distorted Sn⁴⁺–O₆ and Ti⁴⁺–O₆ octahedra to produce oxygen vacancies and substitutional defects (Sn_Cr/Ti_Cr).¹⁹⁸ Similarly, the Na₂CaGe₅SiO₁₄:Cr³⁺ phosphor demonstrates exceptional NIR PersL performance with an afterglow duration of 10 h, ascribed to the oxygen vacancies induced by heterovalent substitution of Cr³⁺ for Ge⁴⁺ ions.²⁰³ Additionally, non-stoichiometric synthesis also introduces significant defects for improving the PersL performances. For example, non-stoichiometry in spinel-structured Zn_2.94Ga_1.96Ge₂O₁₀:Cr³⁺/Pr³⁺ phosphors, characterized by substantial Zn²⁺ vacancies, was demonstrated to enhance PersL intensity with a prolonged afterglow duration of 15 days.²⁰⁸ Through adjusting the Li⁺/Zn²⁺ ratio, the NIR PersL duration of Li_1.97Zn_1.0292Ge₃O₈:Cr³⁺ extends beyond 50 h, which is 2.5 times longer than that of stoichiometric Li₂Zn_0.9992Ge₃O₈:Cr³⁺ phosphors.¹⁹⁴

In addition to intrinsic lattice defects, Aitasalo et al. suggested that co-doping Ln³⁺ ions is also favorable for creating effective traps for PersL, with simultaneous control over trap depth.²⁰⁹ The HRBE and VRBE diagrams of Ln³⁺/Ln²⁺, which illustrate the relative positions of the Ln³⁺/Ln²⁺ ground state with respect to the host bandgap (Fig. 11b),^69–71 are useful for understanding lanthanide-related traps. Specifically, the depths of electron traps can be estimated from the energy difference between the ground state of Ln²⁺ [E_4f(Ln²⁺)] and CB minimum, whereas the depths of hole traps can be determined by the energy difference between the ground state of Ln³⁺ [E_4f(Ln³⁺)] and VBM. Due to the well-shielded 4f orbital by 5s and 5p orbitals, the 4f energy levels of lanthanide ions exhibit a similar Zig-Zag pattern across different matrices, and this pattern can be easily constructed by using Eu³⁺ as a reference, as listed in Table 4, in which the energies of Eu^3+/2+ are set to zero. Based on the electronic energy levels of Ln³⁺ ions in the HRBE diagram, the NIR PersL of garnet-structured Gd₃Sc₂Ga₃O₁₂:Cr³⁺ phosphor was effectively improved by rational co-doping of selected Ln³⁺ ions as electron traps, including Sm³⁺, Eu³⁺, Tm³⁺, and Yb³⁺.²¹⁰ Similarly, Liu et al. demonstrated an exceptionally prolonged NIR PersL duration of 1000 h in Cr³⁺ and Yb³⁺ co-doped LaZn(Al,Ga)₁₁O₁₉ matrix by exploiting the E_4f(Yb²⁺) energy level as an electron trap.⁷³

Table 4 The relative relationship of energy values for constructing the HRBE and VRBE diagram. The ΔE_vf(n + 1, 2+) and ΔE_vf(n, 3+) denote the energy gaps between Ln²⁺/Ln³⁺ and Eu²⁺/Eu³⁺ 4f states. ΔE_vd(n + 1, 2+) and ΔE_vd(n, 3+) represent the energy gaps between Ln²⁺/Ln³⁺ and Eu²⁺/Eu³⁺ 5d states

Ln^3+	n	ΔEvf(n + 1, 2+)	ΔEvf(n, 3+)	ΔEvd(n + 1, 2+)	ΔEvd(n, 3+)
La	0	5.61		0.45
Ce	1	4.13	5.24	0.29	0.86
Pr	2	2.87	3.39	0.24	0.52
Nd	3	2.52	1.9	0.17	0.32
Pm	4	2.34	1.46	0.08	0.2
Sm	5	1.25	1.27	0.03	0.11
Eu	6	0	0	0	0
Gd	7	4.56	–1.34	0.14	–0.04
Tb	8	3.21	3.57	0.18	0.85
Dy	9	2.27	2.15	0.22	0.9
Ho	10	2	1.05	0.03	0.65
Er	11	2.58	1.12	0.48	0.48
Tm	12	1.72	1.28	0.45	0.53
Yb	13	0.43	0.236	0.43	0.596
Lu	14		–1.01		0.75

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Previous research reveals that room-temperature PersL generally relies on trap depths within 0.6–0.8 eV in thermoluminescence analysis (SI).²¹¹ Lanthanide doping typically creates discrete trap states and may not achieve precise modulation of trap depth in this range. Therefore, cationic substitution from the same principal group has been proposed to continuously modulate the trap depths by engineering the host bandgap. For example, the trap depths of garnet Y₃Al₅O₁₂:Cr³⁺ phosphor can be optimized via partial substitution of Ga³⁺ for Al³⁺ to reduce the bandgap. The PersL intensity of the intermediate Y₃Al₂Ga₃O₁₂:Cr³⁺ phosphor was approximately 5-fold greater than that of ZnGa₂O₄:Cr³⁺.¹⁷⁷ The NIR PersL properties of the magnetoplumbite-structured LaZnGa₁₁O₁₉:Cr³⁺/Yb³⁺ phosphor were improved in a similar way. After partial substitution of Al³⁺ ions (1.5%) for Ga³⁺, the PersL duration of Cr³⁺ emission extended beyond 1000 h.⁷³

3.5. Mechanoluminescence

ML is characterized by light emission under mechanical action, such as compressing, bending, friction, and impact.^212–218 ML generally includes fracto-ML, plastico-ML, elastico-ML, and triboluminescence.²¹⁹ Fracto-ML and plastico-ML are associated with permanent deformation of the materials, resulting in irreproducible luminescence with minimal utility. Therefore, the mainstream research of ML is primarily focused on elastico-ML and triboluminescence, which leverage mechanically induced electrical potential due to piezoelectric and triboelectric effects. To date, ML materials in the visible light spectrum have been extensively documented and refined, achieving exceptional repeatability under continuous mechanical load. For instance, Jeong et al. engineered a composite film comprising ZnS:Mn²⁺ and polydimethylsiloxane (PDMS), which maintained an ML brightness of 120 cd m⁻² even after over 10⁵ continuous loading cycles.²²⁴ Nevertheless, NIR ML materials with high brightness and repeatability are awaiting further advancements, which have stimulated the exploration of chromium-activated phosphors for long-wavelength ML.

Chromium-activated PersL materials characterized by extensive lattice defects often display ML under mechanical stimulation, which promotes the release of charge carriers by inducing piezoelectric potentials. Notably, most host materials for chromium activators comprise centrosymmetric lattices (Table 5). The piezoelectric properties in these ML phosphors generally result from a local polyhedral distortion due to intrinsic lattice defects. A representative example is ZnGa₂O₄:Cr³⁺, which exhibits far-red ML resembling the PersL spectrum due to the ²E → ⁴A₂ transition. The ML property is largely owing to zinc vacancies and cation antisite defects , which on one hand contributed to charge storage, and on the other hand triggered a reduction of site symmetry from O_h to rotational C_s, resulting in local piezoelectricity (Fig. 12a).²²⁰ In another example, Liu et al. demonstrated a trap-controlled broadband NIR ML in magnetoplumbite-structured SrGa₁₂O₁₉:Cr³⁺.²²⁰ Due to the anti-site cation defects and cation vacancies, the phosphor exhibited NIR ML with an output of 4 × 10⁻⁴ nW Pa⁻¹ m⁻² under a relatively low mechanical load. These studies suggest a straightforward approach to discovering ML by screening chromium-activated PersL phosphors with judiciously engineered defects.

Table 5 Partial Cr³⁺-activated ML phosphors and associated properties

Mineral prototype	Host	Space group	UV pre-charging	Emission (nm)	Ref.
Garnet	Lu3Al2Ga3O12	Ia3̄d	Yes	706	225 and 226
	Y3Al4GaO12	Ia3̄d	No	689	227
	Ca2YGa3Ge2O12	Ia3̄d	No	715	228
	Y3Ga3MgSiO12	Ia3̄d	No	761	229
Spinel	LiGa5O8	P4332	No	716	230
	LiAl5O8	P4332	No	711	231
	ZnGa2O4	Fd3̄m	Yes	700	220 and 232
	MgAl2O4	Fd3̄m	Yes	693	233
	MgGa2O4	Fd3̄m	Yes	710	234 and 235
	MgAlGaO4	Fd3̄m	Yes	709	236
	Zn3Ga2GeO8	Fd3̄m	Yes	700	220
	Mg3Ga2GeO8	Imma	Yes	787	237
Magnetoplumbite	SrGa12O19	P63/mmc	Yes	750	220, 238 and 239
	BaGa12O19	P63/mmc	No	735	240
	LaMgAl11O19	P63/mmc	No	700	241
	LaMgGa11O19	P63/mmc	No	768	241
	LaGa12O19	P63/mmc	No	718	241
Perovskite	LaAlO3	R3̄c	No	734	242
	Sr2GaSbO6	I4/m	No	830	243
	Sr2ScSbO6	P21/n	No	890	243
	Ba2ScSbO6	Fm3̄m	No	1010	243
Others	α-Al2O3	R3̄c	—	696	233
	β-Ga2O3	C2c	No	715	221
	MgO	Fm3̄m	No	805	244
	MgF2	P42/mnm	No	722	245
	ScBO3	R3̄c	No	812	246
	KGa11O17	P63/mmc	Yes	710	247
	Mg13.7Ga4.6Ge1.7O24	Cmmm	Yes	815	248

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Fig. 12 Typical ML processes in chromium-activated phosphors. (a) [Cr–O₆] octahedra with ordered neighboring cation, cation antisite defect , and Zn vacancy in the ZnGa₂O₄:Cr³⁺ phosphor. Due to the lattice defects, the O_h point symmetry is reduced to C_s rotation symmetry that manifests a piezoelectric nature. Reproduced with permission.²²⁰ Copyright 2022, John Wiley and Sons. (b) Piezoelectric force microscopy amplitude of pristine Ga₂O₃ and Ga₂O₃:Cr³⁺ compounds. The typical amplitude butterfly loop indicates the piezoelectric nature of the Ga₂O₃:Cr³⁺ phosphor. (c) ML stability against UV irradiation and thermal de-trapping. Reproduced with permission.²²¹ Copyright 2023, Elsevier. (d) Schematic presentation of absorptions and emissions of Cr³⁺, Yb³⁺, and Er³⁺ ions. (e) ML spectra of Cr³⁺ singly-doped and Cr³⁺/Yb³⁺ (or Er³⁺) co-doped Ga₂O₃. Reproduced with permission.²²² Copyright 2025, John Wiley and Sons. (f) Schematic structure of phosphor-substrate composite ML materials, and interfacial movement between inorganic phosphor and organic substrate under mechanical actions. (g) Variations in triboelectric charge and voltage between Ba₅(PO₄)₃Cl:Eu^3+/2+ and PDMS in the pressing-releasing cycles. Reproduced with permission.²²³ Copyright 2024, Springer Nature.

The trap-mediated ML process typically requires repeated irradiation by high-energy X-ray or UV light to maintain a stable emission intensity, posing certain limitations in practical applications. Fortunately, selective Cr³⁺-activated phosphors can be directly excited by mechanical action due to the local piezoelectric effect, giving rise to self-recoverable ML without the assistance of traps. For example, NIR ML was discovered in perovskite-structured Sr₂ScSbO₆:Cr³⁺ with excellent repeatability, arising from local piezoelectric properties due to Cr³⁺ doping.²⁴³ This phosphor demonstrated a 6-fold stronger integral intensity than the well-known CaZnOS:Nd³⁺. Besides, β-Ga₂O₃ with Cr³⁺-doping-induced local piezoelectricity, as evidenced by piezoelectric force microscopy (Fig. 12b), was also reported to show NIR ML under sole mechanical excitation.²²¹ This phosphor delivered self-recoverable ML with no noticeable decline in emission intensity over 10 consecutive loading cycles free of additional energy input (Fig. 12c). Furthermore, the piezoelectric β-Ga₂O₃ is also a favorable host for sensitizing Ln³⁺ ML by concentrated Cr³⁺ ions (Fig. 12d). Owing to preferential ET from Cr³⁺ to Ln³⁺ rather than defect states, Liu et al. demonstrated 5.9- and 3.0-fold enhancements of Yb³⁺ and Er³⁺ ML within a Cr³⁺-concentrated Ga₂O₃ system compared to the well-established CaZnOS:Yb³⁺ and CaZnOS:Er³⁺, respectively (Fig. 12e).²²² Despite these encouraging achievements, the observation of ML behavior in Cr⁴⁺-activated phosphors remains scarce, primarily due to their low luminescence efficiency. On a separate note, the current discovery of self-recoverable ML is largely reliant on the trial-and-error method. Innovative theories and approaches are desired to speed up this process.

Triboluminescence results from triboelectric fields at phosphor–organic interfaces due to the dynamic contact under mechanical action (Fig. 12f). In this case, the phosphor materials are typically integrated into an elastomer substrate, such as epoxy resin (ER), polyurethane (PU), polydimethylsiloxane (PDMS), and polyethylene terephthalate (PET). Interfacial triboelectrification occurs when the two materials are in contact, leading to a positive charge accumulation on the organic substrate and a resultant charge flow towards the inorganic phosphor, as depicted by the contact charge and voltage curves in Fig. 12g. Triboelectric fields can directly trigger electron–hole separation within the phosphor, thereby enabling self-recoverable triboluminescence without the need for pre-excitation.^223,249 Pan et al. systematically quantified the triboelectric series, which delineated the proclivity of different phases to gain or lose electrons, and revealed a positive correlation between triboelectricity and ML intensity.²²³ Despite these achievements, the ML of chromium-activated phosphors utilizing triboelectric potential remains undocumented. The development of heterostructured host materials,^250–252 coupled with innovation in organic substrates, might fill this research gap.

4. Synthetic strategies for chromium-activated phosphors

Besides electronic structure, the luminescence characteristics of chromium ions, including emission efficiency, decay lifetimes, and thermal stability, are further influenced by factors such as morphology and crystallinity, which are highly associated with synthesis approaches. Apart from the classic solid-state reaction, chromium-activated phosphors with specific size and morphology can now be prepared through a variety of advanced synthetic strategies, such as molten salt shielded synthesis, hydrothermal processing, coprecipitation and thermal decomposition, sol–gel method, template method, combustion technique, and so forth. In this section, we present a comprehensive overview of the synthetic methodologies for chromium-activated phosphors, including both micro-sized and nano-sized morphologies.

4.1. Solid-state reaction

For an extended period, the solid-state reaction has been regarded as a robust and straightforward method for synthesizing chromium-activated phosphors, in which the powder precursors typically undergo annealing at substantially high temperatures. This approach introduces several variable conditions that can influence experimental outcomes. (i) Addition of fluxing agents: In addition to the primary constituents forming the target phosphors, minor amounts of fluxing agents (∼1–5 wt%), such as LiF, NaF, LiCl, and H₃BO₃, are often incorporated. These agents facilitate nucleation and crystal growth, thereby improving the overall quality of the phosphors. (ii) Pre-annealing step: A pre-annealing process at moderate temperatures (typically 600–800 °C) is sometimes employed before the final high-temperature reaction. This step helps eliminate carbon residues and promotes initial nucleation, laying a better foundation for the subsequent synthesis. (iii) Atmosphere control: maintaining special atmosphere conditions is critical to preserving a desired valence state of chromium ions. For example, a reductive atmosphere (using CO) is essential to avert the oxidation of Cr³⁺ ions in Ca₃Sc₂Si₃O₁₂,⁸⁰ whereas an oxidizing atmosphere is necessary to prevent the reduction of Cr⁴⁺ ions in Mg₂GeO₄.⁸⁷ (iv) Reaction temperature: the ultimate synthesis temperature is predominantly dependent on the chemical reactivity of the precursor materials. For example, aluminates necessitate high enough synthesis temperatures (>1500 °C) due to the chemical inertness of Al₂O₃,^77,255 and the synthesis of gallate varies from 1200 to 1500 °C,^{65,79,156,256} while compounds such as boron phosphate and halogen compounds can be synthesized at markedly low temperatures (<1000 °C) owing to their low melting points.^146,257

Powder samples synthesized by solid-state reaction are characterized by loose morphology and inter-particle voids, presenting pronounced light reflection and scattering. In contrast, glasses/ceramics, characterized by high optical transparency, generally exhibit more uniform and continuous morphologies, which favor high light output by reducing light scattering. Conventional glasses/ceramic composites are obtained by encapsulating phosphors in organic components, followed by solid-state sintering under high-pressure and vacuum conditions. Recently, an alternative approach was developed by directly consolidating mixtures of target phosphors and inorganic glass powders into bulk composites, known as bulk sintering. Due to the low thermal expansion coefficient (0.55 × 10⁻⁶ K⁻¹) and high optical transparency, silica (SiO₂) is an excellent glass substrate for bulk sintering. However, the fabrication of such phosphor-glasses composites necessitates specialized techniques, e.g., spark plasma sintering (SPS) method, ascribed to the significantly high softening temperature (>1700 °C). To address this issue, amorphous SiO₂ nanoparticles and siloxane-based polymers have recently been proposed to enable glass formation at relatively low sintering temperatures (<1250 °C).^{253,254,258–260} A general schematic presentation is displayed in Fig. 13a. Typically, the composite slurries consisting of SiO₂ nanoparticles and target phosphors are polymerized into a flat plate, followed by solidification under UV irradiation. Subsequently, the mixture undergoes high-temperature solid-state sintering and thermal debinding. In contrast to powders, the resultant composite glass presents ultrahigh density, as shown in Fig. 13b.²⁵³ Accordingly, the resultant NIR luminescence EQE reaches as high as 59.5%, while the EQE of the powder-type counterpart is only 21.3%. In addition to silica, other glasses such as tellurite and phosphate, along with ceramics including alumina, fluorite, and hydroxyapatite, can function as substrate matrices for composite glass ceramics.^261–264 Compared to the low thermal conductivity of silica glass (∼1.45 W m⁻¹ K⁻¹), Cr³⁺-activated MgO translucent ceramics have demonstrated an enhanced thermal conductivity of 52 W m⁻¹ K⁻¹ and a record-breaking EQE of 81%. Compared to MgO powders, these ceramics also exhibit a compact microstructure, as shown in Fig. 13c. Consequently, this ceramic reaches a 6 W NIR output under blue-laser excitation, with a photoelectric conversion efficiency of 29%.

Fig. 13 Ceramics and glasses by high-temperature sintering. (a) Schematic presentation of phosphors–silica composite glass preparation using amorphous silica nanoparticles. Reproduced with permission.²⁵³ Copyright 2022, John Wiley and Sons. (b) Micro-scale morphology of Y₂CaAl₄SiO₁₂:Cr³⁺ glass ceramics. Reproduced with permission.²⁵⁴ Copyright 2022, John Wiley and Sons. (c) SEM images of MgO:Cr³⁺ powders and resultant composite ceramics. Reproduced with permission.¹ Copyright 2024, Springer Nature.

Chromium-activated phosphors synthesized via high-temperature solid-state reactions generally exhibit well-defined crystallinity, which significantly contributes to their high luminescence efficiency. Besides, the elevated sintering temperature also induces the formation of thermal defects (e.g., oxygen vacancies and interstitials), which facilitate electron trapping and de-trapping processes essential for PersL emission and ML. Nevertheless, this conventional method is constrained by intrinsic limitations, such as large grain size (>1 µm, Table 6), irregular morphology, and high energy consumption. Thereafter, more advanced solid–liquid hybrid synthesis methods (vide infra) have been developed to address these issues.

Table 6 Applicability of various synthetic routes in the preparation of chromium-activated phosphors, including annealing conditions and luminescence properties of the product

Synthetic strategy	Compound	Annealing condition	Particle size	Emission (nm)	Ref.
Solid-state reaction	BaMgAl10O17	1575 °C for 4 h	∼4 µm	692, 727	77
	Mg4Ta2O9	1450 °C for 6 h; H3BO3 flux	1–7 µm	842	79
	(Lu,Sc)BO3	1150–1300 °C for 5 h; H3BO3 flux	2–5 µm	830	45
		1300 °C for 10 h; H3BO3 flux	1–3 µm	800	321 and 322
	LaMgGa11O19	1450 °C for 6 h; H3BO3 flux	∼3 µm	890, 1200	65
	MgAlGa0.7B0.3O4	1300 °C for 4 h;	∼4–10 µm	713, 748, 895	323
	LiGaP2O7	750 °C for 6 h; Li2CO3 flux	5–15 µm	846	257
	Cs2AgInCl6	400 °C for 4 days	∼15 µm	1010	146
MSS synthesis	ZnGa2O4	650 °C for 6 h; (K,Na)NO3 as salts	4–5 nm	696	277
	Zn3Ga2Ge2O10	1000 °C for 1 h; NaCl as a salt	114.5 nm	695	275
	InBO3	1100 °C for 4 h; H3BO3 as a salt	500–1000 nm	820	276
Hydrothermal synthesis	ZnAl2O4	200 °C for 24 h	500 nm	689	279
	ZnGa2O4	180 °C for 12 h; 500–1000 °C for 3 h	400 nm	—	324
		120 °C for 24 h; 750 °C for 5 h	400 nm	695	325
	Ga2O3	180 °C for 10 h; 1000 °C for 2 h	8.5 µm × 65 nm	740	221
	LiGa5O8	180 °C for 24 h; 1100 °C for 3 h	66–243 nm	716	305
	Na3GaF6	200 °C for 10 h; HNO3 solution	1.6–4.3 µm	756	278
	Na3Al2Li3F12	200 °C for 10 h; HF solution	7.9 µm	752	280
	Na3Ga2Li3F12	200 °C for 10 h; HF solution	11.9 µm	764	280
	K2NaInF6	220 °C for 12 h; HF solution	∼0.5–3 µm	774	117
Coprecipitation and thermal decomposition	Al2O3	1250 °C for 4 h; NaOH as precipitator	220 nm	694	294
	Ga2O3	800 °C for 3 h; NaOH as precipitator	1–5 µm	—	288
	ZnGa2O4	900 °C for 36 h; (NH4)2CO3 as precipitator	43 nm	700	292
	Lu3Ga5O12	1000 °C for 3 h; NH3·H2O as precipitator	20–35 nm	703	291
	Na3Al2Li3F12	300 °C for 5 h; HF as precipitator	1–5 µm	750	40
Sol–gel method	Zn3Ga2Ge2O10	1000 °C for 3 h	∼50 nm	695	208
	ZnGa2O4	700 °C for 2 h	∼20 nm	695	326
		500 °C for 1 h; 1000 °C for 1 h	∼60 nm	694	299
	LiGa5O8	1100 °C for 3 h	81–480 nm	716	305
Template method	ZnGa2O4 & MSN	600 °C for 2 h	150 nm	696	307
	ZnGa2O4 & carbon	800 °C for 2 h	50–500 nm	696	308
	β-Ga2O3 & carbon	500 °C for 2 h	80–200 nm	—	327
	MgGa2O4 & MSN	1200 °C for 5 h	79 nm	710	158
Combustion technique	Al2O3	900 °C for 3 h	<10 nm	694	315

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4.2. Molten salt shielding synthesis

Compared to traditional solid-state reactions, the molten salt shielded (MSS) method enables crystal growth at considerably reduced sintering temperatures by incorporating massive metal salts (>50 wt%).^265–268 These salts generally present critical parameters: (i) low melting temperature (T_m), (ii) excellent compatibility with reactants, (iii) high thermal and chemical stability, (iv) low vapor pressure to prevent evaporation loss during sintering, and (v) high solubility for easy cleaning. The commonly used metal salts include halides (e.g., KCl, NaCl, LiCl, and KBr) and oxosalts (e.g., hydroxides and nitrates), as well as their mixtures. It is worth noting that the MSS reaction generally operates above T_m, which provides a flowing reaction environment to significantly facilitate the elemental diffusion and nucleation. Additionally, benefiting from the remarkable chemical and thermal stability at significantly high temperatures, molten salt flux offers an exceptionally inert environment for air-sensitive reactions.^269–272 For example, CaZnOS and ZnGa₂S₄ phosphors were frequently fabricated under N₂/Ar flow protection. Recently, these phosphors have been successfully prepared by the MSS method employing NaCl as molten salt, free from specialized atmospheric conditions.^273,274

In contrast to solid-state reactions that typically yield substantially aggregated particles, the molten salt acting as a solvent facilitates the rapid transport of reactant species via convection and diffusion, thereby offering a viable approach for synthesizing nanoscale and microscopic crystals with well-defined particle morphology (Table 6).^275–277 For example, Wei et al. have recently refined the MSS method by introducing citric acid into NaCl salt.²⁷⁵Fig. 14a illustrates the schematic synthesis route of the Zn₃Ga₂Ge₂O₁₀:Cr³⁺ phosphor employing this improved method. The incorporation of citric acid induced a network structure due to the chelating effect between carboxylic acids and cations, while NaCl, as a molten salt, was anticipated to enhance the dissolution of solid reactants and solvating ions through robust polarization forces. Upon annealing at 1000 °C, the system underwent hydrolysis and dehydrogenation, yielding a phosphor-containing mixture. Subsequent washing resulted in well-defined particles with a size distribution of 114.5 ± 31.5 nm, as shown in Fig. 14b. In contrast, the NaCl-free synthesis route led to a bulk compound characterized by some amorphous aggregates. Similarly, the InBO₃:Cr³⁺ phosphor synthesized by the MSS method exhibited well-dispersed and spherelike shapes ranging from 500 to 1000 nm in diameter. These resultant particles also demonstrate a uniform elemental distribution (Fig. 14c).

Fig. 14 Molten salt shielded synthesis. (a) Schematic presentation of the molten salt shielded route for preparing Zn₃Ga₂Ge₂O₁₀:Cr³⁺ phosphor. (b) The TEM images of Zn₃Ga₂Ge₂O₁₀:Cr³⁺ phosphor synthesized with (up) and without (down) NaCl. Reproduced with permission.²⁷⁵ Copyright 2023, American Chemical Society. (c) SEM and high-resolution TEM images of InBO₃:Cr³⁺ synthesized by the molten salt shielded method. Reproduced with permission.²⁷⁶ Copyright 2022, American Chemical Society.

The MSS method is favorable for preparing the chromium-activated phosphor with well-dispersed particles. And the resultant phosphors generally manifest high luminescence efficiency ascribed to their high crystallinity. Nevertheless, some target compounds may react with the inorganic molten salt, resulting in the formation of a secondary phase by-product. For example, the gallate compounds, which frequently function as matrices for the incorporation of Cr³⁺ ions, present significant byproducts when reacted with alkali metals, e.g., (Li,Na)GaO₂ and (Li,Na)Ga₅O₈. Consequently, this technique exhibits high compositional selectivity toward the desired phosphors.

4.3. Hydrothermal synthesis

Hydrothermal synthesis represents a typical wet-chemistry approach that utilizes water and chelating ligands to modulate reaction kinetics. This process utilizes an aqueous solution within a closed reaction vessel to establish a high-pressure reaction environment by heating at elevated temperatures. Typically, the high temperature and vapor pressure significantly accelerate the reaction by increasing the chance of collision among water molecules. The reaction system typically contains the precursors, mineralizers, and chelating ligands. The precursor for hydrothermal reactions usually includes nitrates, halogens, and organic salts, which exhibit considerable solubility in tailored liquid media. The mineralizer (e.g., HCl, HNO₃, and HCOOH) is favorable for increasing the solubility and accelerating the crystal nucleation rate. The chelating ligand is advantageous for regulating the reaction kinetics and modulating the grain growth by forming organic–metal complex intermediates. Besides, additional synthetic conditions, including reaction duration, temperature, pH value, and the use of templates, significantly influence the diffusion behaviors among the precursors as well as the particle dispersion.^281–283

The hydrothermal synthesis technique has been frequently employed for the preparation of chromium-activated phosphors (Table 6). Fig. 15a illustrates the schematic synthesis route for the Na₃GaF₆:Cr³⁺ phosphor.²⁷⁸ Initially, the Ga(NO₃)₃, NaHF₂, and Cr(NO₃)₃ precursors were dissolved in deionized water. Subsequently, the selected chelating agents, such as hexadecyltrimethylammonium bromide (CTAB), sodium citrate (Na-Cit), polyvinyl pyrrolidone (PVP), and polyethylene glycol (PEG), were further dissolved. Thereafter, the nitric solution, acting as a mineralizer, was introduced to adjust the pH value to 3. The resultant mixture was then transferred to a Teflon autoclave and subjected to high temperatures for the reaction. As the reaction temperature increased from 140 to 200 °C, the average size of the sphere-like particles increased from 1.6 to 4.3 µm (Fig. 15b). Furthermore, the particles adopted a well-defined cuboid structure with a substantially larger particle size of 20 µm when PEG was used, whereas partial particles crystallized into a hexagonal configuration when PVP was used.

Fig. 15 Hydrothermal synthesis. (a) Schematic representation of the hydrothermal synthesis route for the Na₃GaF₆:Cr³⁺ phosphor. (b) Corresponding SEM images synthesized at 140 °C with CTAB (top left), 200 °C without ligand (top right), 200 °C with PEG (bottom left), and 200 °C with PVP (bottom right). Reproduced with permission.²⁷⁸ Copyright 2025, Elsevier. (c) Schematic synthesis route and growth mechanisms for the ZnAl₂O₄:Cr³⁺ phosphor with PEG ligands and varying concentrations of NaAC. (d) The corresponding TEM images. Reproduced with permission.²⁷⁹ Copyright 2017, Elsevier. (e) SEM images of Na₃Al₂Li₃F₁₂:Cr³⁺, Na₃Ga₂Li₃F₁₂:Cr³⁺, and Na₃In₂Li₃F₁₂:Cr³⁺. Reproduced with permission.²⁸⁰ Copyright 2021, Royal Society of Chemistry. (f) SEM image and EDS mappings of Ga₂O₃:Cr³⁺ synthesized by the hydrothermal method. Reproduced with permission.²²¹ Copyright 2023, Elsevier.

Other than microcrystals, hydrothermal synthesis is also favorable for fabricating nanoparticles. For example, nanoscale ZnAl₂O₄:Cr³⁺ was readily synthesized by this method with PEG ligand, as schematically depicted in Fig. 15c.²⁷⁹ The resultant phosphor exhibited a sphere morphology with a particle size of 500 nm (Fig. 15d). After incorporating sodium acetate (NaAC), crystals with a regular octahedron morphology were obtained, and the particle size underwent a progressive increase with the rising mass of NaAC. Hydrothermal synthesis has also achieved morphologies and particle sizes in other chromium-activated phosphors, for example, tetragonal-shaped Na₃Al₂Li₃F₁₂:Cr³⁺, cubic-shaped Na₃Ga₂Li₃F₁₂:Cr³⁺, dodecahedral-shaped Na₃In₂Li₃F₁₂:Cr³⁺ (Fig. 15e), and sheet-like β-Ga₂O₃:Cr³⁺ (Fig. 15f).^221,280

Besides intrinsic factors, external reaction conditions, such as microwave heating and magnetic fields, also significantly influence the properties of the synthesized phosphors.^283,284 It was reported that microwaves operating at frequencies of 900 MHz and 2.45 GHz presented substantial interactions with reactants, enabling pronounced thermal release. In addition, microwave presents a high penetration depth, thereby facilitating uniform heating of the entire reaction and mitigating thermal gradients. Compared to the traditional hydrothermal method, the microwave-assisted approach offers a faster heating rate and a more uniform temperature distribution. Therefore, this approach is favorable for preparing phosphors with narrow size distributions and uniform morphologies. For example, Zhu et al. employed a microwave-assisted hydrothermal route at 433 K to prepare γ-Al₂O₃:Cr³⁺ microspheres, achieving a notably uniform particle size of 1.9 µm.²⁸⁵

Despite these advantages, the hydrothermal process often suffers from incomplete grain nucleation and growth, leading to final products with lower crystallinity and reduced luminescence efficiency compared to those prepared by solid-state reactions. For example, Na₃GaF₆:Cr³⁺ phosphors synthesized hydrothermally exhibited IQE/EQE values of 56.1/14.2%,²⁷⁸ and the values for Na₃Ga₂Li₃F₁₂:Cr³⁺ were only 48.7/7.79%.²⁸⁰ Therefore, a subsequent thermal treatment at a moderate temperature (500–1000 °C) is adopted to remove residual organic compounds and facilitate further crystallization. The final particle size can range from tens of nanometers to several micrometers, depending on the annealing temperature and duration.

4.4. Coprecipitation and thermal decomposition

Analogous to hydrothermal synthesis, the coprecipitation combined with the thermal decomposition technique is also a straightforward method for controllable fabrication of chromium-activated phosphors within a wet-chemistry environment. In a typical synthetic route, metal precursors, including nitrates, acetates, and halogen compounds, are prepared by dissolving corresponding metal salts or metal oxides in an acidic medium. Subsequently, various treatments such as continuous stirring, ultrasonic dispersion, thermal treatment, and pH regulation are conducted to achieve thorough homogenization. Following this, the introduction of a precipitating agent induces localized supersaturation, thereby initiating nucleation. Due to the low solubility of metal hydroxides, alkaline solutions such as NaOH, KOH, and ammonia are frequently utilized as precipitants. Once the particles reach a critical size, the seed crystals are segregated from the solutions by centrifugation. Finally, the precipitation undergoes further thermal decomposition at elevated temperatures to trigger dehydration and crystal growth, yielding the desired phosphors.

A crucial aspect of this method is the markedly low solubility of the target phosphors or intermediates in the chosen solvents, which facilitates centrifugal precipitation. Therefore, the precipitating agents are critical for the synthesis of desired phosphors. For example, β-Ga₂O₃:Cr³⁺ phosphor, an ideal matrix for high-performance NIR luminescence, was frequently synthesized using the coprecipitation method, followed by thermal decomposition of the amphoteric GaOOH in mildly alkaline solutions.^286–289 After titration with NaOH precipitating agent, the Ga³⁺ and Cr³⁺ cations in nitric acid solution precipitate into an intermediate (Ga,Cr)OOH seed crystal. Generally, insufficient NaOH results in no precipitation. With an incremental addition of NaOH reaching a pH value of 4.5, the (Ga,Cr)OOH crystallizes into spindle-shaped crystals with a particle size of 125.70 ± 35.70 nm. Further increasing NaOH to a pH value of 7.0 induces the formation of nanorods with a particle size of 313.90 ± 116.20 nm (Fig. 16a and b). The final β-Ga₂O₃:Cr³⁺ phosphor can be produced by the subsequent thermal decomposition to dehydrate the (Ga,Cr)OOH at elevated temperatures, with a morphology closely resembling the parent (Ga,Cr)OOH crystals.²⁸⁶ Besides the quantity, the type of precipitating agent is also significant for preparing the β-Ga₂O₃:Cr³⁺ phosphor. For example, when using urea as a precipitating agent, spheroidal particles were formed with an average length/width of 600/200 nm, while particle sizes decreased to 300 nm in diameter with spindle-shaped crystals when using ammonia as the precipitating agent (Fig. 16c).²⁸⁷

Fig. 16 Coprecipitation and thermal decomposition synthesis. (a) TEM images of Ga₂O₃:Cr³⁺ and GaOOH:Cr³⁺ synthesized at different pH values (G1: pH = 4.5, G2: pH = 5.5, G3: pH = 7.0). The GaOOH:Cr³⁺ was produced through the coprecipitation method by employing NaOH as a precipitating agent, which was subsequently converted into the Ga₂O₃:Cr³⁺ phosphor by thermal decomposition. (b) The associated particle size distribution histograms. Reproduced with permission.²⁸⁶ Copyright 2015, Elsevier. (c) SEM images of Ga₂O₃ prepared by employing urea, ammonia, and NaOH as precipitating agents. Reproduced with permission.²⁸⁷ Copyright 2016, John Wiley and Sons. (d) SEM image of the Na₃Al₂Li₃F₁₂:Cr³⁺ phosphor fabricated by coprecipitation and subsequent thermal decomposition. (e) Corresponding elemental distribution maps. Reproduced with permission.⁴⁰ Copyright 2021, Elsevier.

To date, this method has succeeded in synthesizing a variety of chromium-activated phosphors, including LiLaP₄O₁₂:Cr³⁺,²⁹⁰ Lu₃Ga₅O₁₂:Cr³⁺,²⁹¹ ZnGa₂O₄:Cr³⁺,^292,293 and Al₂O₃:Cr³⁺.²⁹⁴ Beyond oxides, fluoride phosphors such as Na₃Al₂Li₃F₁₂:Cr³⁺ have also been synthesized by the coprecipitation method.⁴⁰ Specifically, using HF as the precipitating agent, the resultant phosphor crystallizes into a uniform hexagonal structure with a particle size ranging from 1 to 5 µm (Fig. 16d and e). The Cr³⁺ luminescence demonstrates an enhanced resistance to thermal quenching, maintaining 99% of the intensity as the temperature increases from 300 to 423 K, which can be ascribed to the well-crystallized particles and reduced surface defects. In comparison, the luminescence intensity of the Na₃Al₂Li₃F₁₂:Cr³⁺ phosphor synthesized by the hydrothermal route decreases to 82%. Therefore, coprecipitation combined with thermal decomposition method proves advantageous for fabricating highly-crystalline phosphors of tunable sizes in the nano/micrometer scale for high-performance NIR luminescence.

4.5. Sol–gel method

Sol–gel processing typically involves two distinct phases: solution (sol) and gelation (gel). Specifically, sols generally present as dispersions of colloidal particles spanning 1 and 100 nm within a special liquid medium, whereas gels are characterized by polymeric chains forming a rigid network with sub-micrometer dimensions, exemplified by substances including citric acid and polyethylene glycol.²⁹⁵ Thereafter, sol–gel processing involves a phase evolution in which the colloidal particles within the polymeric chains agglomerate into a three-dimensional network by polycondensation reactions at elevated temperatures.

In a typical sol–gel synthesis procedure, the precursor solution is first prepared utilizing associated metal salts (e.g., nitrate, acetate, and chloride) and metal alkoxides (e.g., tetraethyl orthosilicate, TEOS).^296,297 An organic ligand is secondly incorporated into the mixture solution, which is then subjected to subsequent agitation and evaporation at an elevated temperature (<100 °C). Following a long reaction duration, a gel composed of an organic molecular substance is formed. Finally, the resultant xerogel is further annealed at an elevated temperature to eliminate the organic gel precursors and facilitate the crystallization of the desired phosphors. The specific annealing temperatures are mainly determined by the nature of the initial precursors and inorganic phosphor compositions. For example, the Zn₃Ga₂Ge₂O₁₀:Cr³⁺ phosphor, recognized for its superior NIR PersL property, has been frequently prepared by the sol–gel method. However, at low annealing temperatures (<600 °C), this phosphor presents host-related emissions ascribed to its low crystallinity and deficiency of lattice Cr³⁺. Conversely, as the temperature increases to 1000 °C, it demonstrates a pronounced NIR emission from the ²E → ⁴T₂ transition of Cr³⁺ ions, with particle sizes of 8.7 ± 15.3 nm.²⁰⁸ In comparison, isomorphic ZnGa₂O₄:Cr³⁺ nanocrystals can be effectively synthesized with exceptional NIR PersL properties at significantly lower temperatures (<700 °C) through citrate sol–gel processing, with particle sizes of 27.5 ± 1.11 nm.^298,299 Similarly, a strong temperature effect has also been observed for BaMgAl₁₀O₁₇:Cr³⁺, which manifested a pronounced R-line emission with particle sizes spanning 0.1 to 0.5 µm when subjected to sol–gel annealing temperatures between 1000 and 1500 °C. Upon raising the temperature to 1575 °C, this phosphor undergoes a substantial crystal growth, and the resultant emission presents a spectral superposition of R-line and broadband emissions.^77,300

The sol–gel route is adaptable for preparing diverse chromium-activated phosphors, for example, CaAl₁₂O₁₉:Cr³⁺,³⁰¹ ZnAl₂O₄:Cr³⁺,³⁰² Lu₃Al₅O₁₂:Cr³⁺,³⁰³ Y₃Al₅O₁₂:Cr³⁺,³⁰⁴ and LiGa₅O₈:Cr³⁺.³⁰⁵ Nevertheless, the thermal decarbonization of amorphous organic matrices at high temperatures complicates the nucleation of inorganic metal salts, thereby making it challenging to precisely regulate the resulting morphology, and generally leading to particle agglomeration. Consequently, the resulting phosphors typically exhibit substantially low luminescence efficiency ascribed to their poor crystallinity and irregular morphology. To address this limitation, the molten salt-assisted sol–gel technique has been developed to refine the synthesis.²⁷⁵ This approach promotes the coordination of cations with citrate, which effectively suppresses particle agglomeration and promotes the crystallization of precursor hydroxides. In the future, further advancements in the sol–gel synthesis route by integrating other supplementary methodologies are necessary for preparing high-quality nano-/micro-scale chromium-activated phosphors.

4.6. Template methods

Template-assisted methods offer a versatile strategy for achieving phosphors with well-defined nanoscale shapes. Two main categories exist: soft and hard templates. The soft-template processing involves the self-assembly of small molecules (e.g., peptides) to regulate crystal growth, the driving forces of which are generally characterized by hydrogen bonding and van der Waals forces.³⁰⁶ In contrast, hard-template processing presents crystal growth of inorganic membranes in well-confined voids, such as channels and holes found in mesoporous silicon nanoparticles (MSNs) or porous carbon.^307,308

For inorganic phosphors, hard-template approaches are more commonly employed, in which the particle sizes and morphologies are primarily dictated by the dimensions of the selected templates. The general synthesis route can be distinctly delineated by the independent process of MSNs preparation and phosphor fabrication. MSNs have been extensively employed for fabricating inorganic phosphors with precise morphological characteristics. For the preparation of the template, a surfactant (e.g., CTAB) and an alkali catalyst (e.g., diethanolamine and NaOH) are dispersed in an aqueous medium. Subsequently, under continuous stirring, the silica precursor [e.g., tetraethyl orthosilicate (TEOS) or sodium metasilicate (Na₂SiO₃)] is introduced for nucleation and precipitation. Finally, these organic compounds are eliminated after thermal annealing at elevated temperatures (400–600 °C), culminating in the formation of the desired MSNs. The resultant particle sizes and morphologies can be modulated into spherical, rod-like, or worm-like structures by tailoring the synthesis parameters, including the molar ratio of silica precursors to surfactants and pH value. For instance, employing Na₂SiO₃ as the precursor, the synthesized MSNs presented a larger pore size (3.3 nm) and higher specific surface area (1379 m² g⁻¹) than those synthesized using TEOS as the precursor (2.8 nm and 848 m² g⁻¹, respectively).³⁰⁹ For the preparation of template-phosphor composites, the precursor solution of the cation component is mixed with templates, followed by thermal treatment. Specifically, the confined voids within the templates necessitate annealing at considerably low temperatures (600–1200 °C) to prevent structural collapse.

The template-assisted synthesis method has been widely applied to the fabrication of chromium-activated phosphors. Satpathy et al. illustrated the synthesis of the spinel-structured ZnGa₂O₄:Cr³⁺/Ni²⁺@MSNs phosphor, as depicted in Fig. 17a. They engineered MSNs employing CTAB as a surfactant, ammonium hydroxide as a catalyst, and TEOS as a silicon source. The synthesized MSNs presented an amorphous spherical morphology with an average particle diameter of 60 nm. Subsequently, precursor solution comprising Zn²⁺, Ga³⁺, Cr³⁺, and Ni²⁺ ions was combined with the MSNs templates, followed by sequential annealing at 600 and 1000 °C. The resultant composite phosphor exhibited a morphology analogous to the MSNs templates, with nanoscale ZnGa₂O₄:Cr³⁺/Ni²⁺ particles (5 nm) embedded within the pores (Fig. 17b). In another work, MSNs template was prepared by employing hexadecyltrimethylammonium chloride (CTAC) and triethanolamine (TEOA), which manifested mesoporous nanospheres with significantly larger particle and pore diameters of 98 and 6.0 nm, respectively (Fig. 17c and d). These mesoporous MSNs, serving as hard templates, precisely controlled the size and morphology of the resultant β-Ga₂O₃:Cr³⁺@MSNs phosphor, which exhibited a comparable diameter of 93 nm. MSNs as templates have also been extended to produce other chromium-activated phosphors, such as MgGa₂O₄:Cr³⁺@MSNs,¹⁵⁸ γ-Ga₂O₃:Cr³⁺@MSNs,³¹² and ZnAl₂O₄:Cr³⁺@MSNs.³¹³

Fig. 17 Template synthesis. (a) Schematic synthesis route of ZnGa₂O₄:Cr³⁺@MSNs phosphor. (b) TEM images of MSNs (left) and nanophosphor (right). Reproduced with permission.¹⁰⁴ Copyright 2023, John Wiley and Sons. (c) Schematic synthesis route of Ga₂O₃:Cr³⁺@MSNs phosphor. (d) The corresponding TEM images. Reproduced with permission.³¹⁰ Copyright 2021, American Chemical Society. (e) TEM images of mesoporous carbon structures. Reproduced with permission.³¹¹ Copyright 2008, John Wiley and Sons. (f) Schematic synthesis route of ZnGa₂O₄:Cr³⁺@carbon phosphor. (g) The TEM images and (h) size distribution histograms of original carbon spheres and ZnGa₂O₄:Cr³⁺@carbons. Reproduced with permission.³⁰⁸ Copyright 2018, American Chemical Society.

Besides MSNs, mesoporous carbon, characterized by its uniform pore structure (Fig. 17e), also serves as an excellent hard template for preparing chromium-activated phosphors. For example, Wang et al. synthesized ZnGa₂O₄:Cr³⁺@carbon nanophosphor through a template-assisted method, as shown in Fig. 17f.³⁰⁸ The carbon template could be readily fabricated by a hydrothermal reaction within a glucose monohydrate solution. Subsequently, Zn²⁺, Ga³⁺, and Cr³⁺ ions from the precursor were effectively absorbed onto the surface of the carbon template, forming M(OH)CO₃ shells due to the chelate effect. These shells underwent decomposition upon high-temperature annealing at 800 °C, yielding the desired nanophosphors. Unlike in the case of MSNs, the resultant phosphors presented a reduced particle size of 50 nm compared to 150 nm of the pristine carbon template, which was ascribed to the dehydration and contraction effects (Fig. 17g and h). Additionally, these composite phosphors manifested monodisperse spherical morphology and retained excellent NIR PersL properties.

4.7. Combustion synthesis

The combustion synthesis technique is characterized by rapid thermal treatment and a fuel matrix that drives thermodynamically controlled crystal nucleation, thereby accelerating the transition from solution to solid for nanosized phosphors. This reaction benefits from the self-sustained exothermic nature of the starting reactants at an elevated temperature (500–700 °C). Typically, the precursor solutions are prepared from metal salts and fuels. The metal salts predominantly include cationic nitrates, which constitute the compositions of desired phosphors. Upon thermal decomposition, they generate substantial NO₂ combustion gas and release significant heat. Fuels like urea, citric acid, and leucine can be included to further facilitate the combustion process.^314–316 Following thorough homogenization, the mixture undergoes subsequent combustion, resulting in ash containing the desired crystalline products.

This method has been applied in the preparation of several chromium-activated oxide phosphors, such as BaMgAl₁₀O₁₇:Cr³⁺,³¹⁷ Al₂O₃:Cr³⁺,³¹⁵ BaAl₂O₄:Cr³⁺,³¹⁸ MgAl₂O₄:Cr³⁺,³¹⁹ and MgO:Cr³⁺.³²⁰ In a representative example of synthesizing Al₂O₃:Cr³⁺ phosphor, precursors were first prepared by dissolving stoichiometric amounts of hydrated nitrates of aluminum and chromium. Subsequently, urea serving as fuel was added to form a thick paste. After that, the paste was burned at 500 °C in a furnace, producing a flame that lasted for several seconds. The combustion gases generated during thermal decomposition facilitated interparticle separation, thereby resulting in extremely small particle sizes (<10 nm). Note that the CO in the combustion gas also favored the reduction of Cr⁴⁺ to Cr³⁺. The resultant phosphors presented sharp R-line emission ascribed to the ²E → ⁴A₂ transition of Cr³⁺.

It is worth noting that the brief duration of the exothermic combustion often results in suboptimal crystallinity. Moreover, phosphors synthesized via this route typically present a high specific surface area and defects, which act as non-radiative recombination centers and quench luminescence. Therefore, a post-synthesis annealing treatment is typically required to improve crystallinity and enhance luminescence properties. On another note, the gas parameters, including gas flow rate and quantity, are challenging to regulate manually, leading to random particle morphology. Therefore, further refinement of the combustion technique, including precursor preparation, combustion temperature, duration, fuel fraction, and annealing temperature, is needed before it can be widely used in preparing chromium-activated phosphors.

4.8. Others

As a powerful addition to the aforementioned synthetic routes, laser ablation offers a rapid method for achieving localized crystallization of target phosphors. Ablation utilizes highly energetic laser beams to induce plasma formation, thereby concentrating significant thermal energy into local confined regions of bulk materials or solution directionally. Key parameters, such as laser wavelength, power, repetition rate, and ablation pore size, can be deliberately tuned for precise control over the size and morphology of the resulting chromium-activated phosphors.^328,329 Nevertheless, the laser ablation technique tends to introduce substantial surface defects due to laser irradiation, necessitating an additional thermal treatment to mitigate surface defects and stabilize the crystals.

Besides, the mechanochemical method is also favorable for preparing chromium-activated phosphors, in which the starting materials are activated by external mechanical energy. Such activation processes encompass dehydration,³³⁰ cation exchange,³³¹ and so forth. For example, chromium-doped χ-Al₂O₃ was produced by mechanical milling of corresponding pseudoboehmite (Al,Cr)O(OH)·nH₂O for a duration of 30 h.³³⁰ Specifically, the precursor undergoes hydration during continuous ball-milling, leading to the crystallization of χ-Al₂O₃:Cr³⁺ phosphor. However, this process results in particles with low crystallinity and significant bulk defects due to the prolonged grinding.

In summary, the synthesis of chromium-activated phosphors spans both solid-state and solution-phase methodologies. Variables, including the precursor compositions, mixing protocol, precipitation technique, thermal treatment approach, and ultimate synthesis temperature, can be harnessed to control the phase purity, particle size distribution, and morphology of the resultant products. In general, chromium-activated phosphors subjected to high-temperature annealing generally exhibit well-defined crystallinity, which contributes to the high luminescence efficiency. For instance, Sr₂ScSbO₆:Cr³⁺, Mg₄Ta₂O₉:Cr³⁺, and LaMgGa₁₁O₁₉:Cr³⁺ phosphors prepared by solid-state reaction often achieve luminescence EQE exceeding 40%.^79,85,156 However, high-temperature annealing often leads to significant particle growth and irregular morphology. In contrast, wet-chemical synthesis enables better control over particle size and morphology, yielding uniform and well-shaped particles. Nevertheless, these materials frequently suffer from incomplete crystallization and low luminescence efficiency. Future development of chromium-activated phosphors may strategically integrate the advantages of both approaches to tailor material properties for specific applications.

5. Applications

The established synthesis and optical tunability of chromium-activated NIR phosphors provide great opportunities for a variety of applications encompassing NIR lighting, information security, in vivo bioimaging, and optical sensing, in light of the intrinsic attributes of NIR radiation such as high penetration depth into living tissue, characteristic absorption by organics, and high-efficiency photoelectric conversion. In this section, we will further elaborate on the emerging applications of chromium-activated NIR phosphors and associated devices.

5.1. NIR lighting

Compared with conventional NIR light-emitting sources, for example, incandescent lamps, tungsten–halogen lamps, and laser diodes, NIR phosphor-converted light-emitting diodes (pc-LEDs) provide higher photoelectric conversion efficiency, compact device size, longer service lifetime, and broader emission spectra. Accordingly, this device has emerged as the next-generation NIR light-emitting source.

5.1.1. Non-destructive analysis. The NIR spectroscopy provides comprehensive insights into molecular vibrational structures and the chemical composition of organics. Fig. 18a displays the NIR absorption bands, which contain the overtones and combination of chemical functional groups, e.g., C–H, C–O, and H–O bonds. This nature makes NIR spectra highly beneficial for the non-destructive analysis of agricultural products. For example, the absorption spectra of most fruits (e.g., apple, grapes, and watermelon) are dominated by the overtone bands of the H–O bond at 750, 960, and 1440 nm due to high water content. In contrast, for crops, e.g., rice, sorghum, and tea, the vibration absorptions associated with the H–O bond are significantly weaker and less detectable, while the absorptions of the C–H bond are dominant.^332–335

Fig. 18 Non-destructive analysis of organic compounds. (a) NIR absorption bands of diverse functional groups. Reproduced with permission.³³⁸ Copyright 2023, MDPI. (b) Schematic presentation of pc-LED fabricated by Ga_1.2Sc_0.8O₃:Cr³⁺ ceramic for non-destructive analysis. (c) Photographs of fresh and damaged oranges captured by visible (left) and NIR (right) cameras, respectively. Reproduced with permission.³³⁹ Copyright 2024, John Wiley and Sons. (d) NIR luminescence spectra of GdAl₃(BO₃)₄:Cr³⁺ phosphor in the presence and absence of tomato and grape. The insets display the corresponding photographs of fresh and rotten fruits. Reproduced with permission.³⁴⁰ Copyright 2021, Royal Society of Chemistry. (e) Schematic illustration of the experimental setup for ML-powered non-destructive analysis. (f) ML spectra along with the calculated ML differences of β-Ga₂O₃:Cr³⁺ before and after passing through ethanol, cyclohexane, and deionized water. Reproduced with permission.²²¹ Copyright 2023, Elsevier.

Chromium ions are especially valuable in this context because they offer tunable broadband NIR emissions across 700–1600 nm, effectively overlapping with the overtone absorptions of the chemical bonds. For example, NIR pc-LED, fabricated using ZnTa₂O₆:Cr³⁺ phosphor in conjunction with a 460 nm blue chip, produced a broadband emission centered at 935 nm (FWHM = 185 nm) with an output power of 39.8 mW at 300 mA.³³⁶ This device was utilized for identification among deionized water, absolute ethanol, glucose solution, hexamethylene, edible oil, and methyl alcohol. Specifically, deionized water demonstrated an absorption near 975 nm, ascribed to the second overtones of the O–H stretching bands. Hexamethylene exhibited an absorption around 950 nm due to the vibration of the C–H bond, whereas ethanol manifested an absorption at approximately 900 nm. In another study, Cai et al. developed a single-phase Mg₂GeO₄ phosphor with valence-controllable chromium that exhibited a dual-emission spanning NIR-I and NIR-II.⁸⁷ This advancement facilitated the successful detection of NIR-II absorption signals of the overtones of C–H and O–H bonds in organic compounds. These findings elucidate the significant potential of chromium-activated phosphors in non-destructive component analysis.

These NIR phosphors also demonstrate substantial utility for quality control in raw and final products.³³⁷ For example, Xiong et al. constructed a reflective NIR pc-LED fabricated by (Ga,Sc)₂O₃:Cr³⁺ ceramic and multiple lenses, as schematically depicted in Fig. 18b.³³⁹ Under 450 nm laser excitation, the enhanced broadband NIR emission, achieved through multiple focusing and reflection, enabled the photographic differences between the fresh (left) and partially-damaged (right) oranges (Fig. 18c). The variations in freshness could be further visualized by spectral analysis. In parallel to the above developments, Huang et al. conducted a spectral comparison of fresh and rotten tomato and grape, employing GdAl₃(BO₃)₄:Cr³⁺ phosphor as the NIR light source.³⁴⁰ Specifically, the rotten specimens presented markedly increased absorption at the overtones of C–H and O–H bonds, ascribed to the decomposition of the fiber and saccharides that resulted in elevated alcohol and water content (Fig. 18d).

In addition to NIR pc-LED, Suo et al. innovatively devised a class of Ga₂O₃:Cr³⁺/Yb³⁺ nanophosphor powered by mechanical stress as the light source for non-destructive analysis.²²¹Fig. 18e schematically shows the experimental setup. An optical fiber coupled with a spectrometer was inserted into a glass tube containing 2.5 mL of ethanol, cyclohexane, or deionized water. The NIR ML signals, generated by sliding the tube on an ML film comprising the Ga₂O₃:Cr³⁺/Yb³⁺ nanophosphor, were collected after passing through the solutions. The results indicated that the ML signals were sufficiently strong for distinguishing the characteristic absorptions of ethanol, cyclohexane, and deionized water (Fig. 18f). Therefore, the ML-based system provided a self-powered solution for non-destructive analysis.²²¹

Overall, chromium-activated phosphors exhibit considerable advantages for the non-destructive analysis of organic components, and the advancements of high-performance phosphors and novel excitation methods are therefore pivotal for the advancement of this analytical method.

5.1.2. Plant growth lighting. The rhythmic patterns of plant growth are driven by light through phototropism and photosynthesis reactions. The biological pigments, such as chlorophyll A, chlorophyll B, red/phytochrome (P_R), and far-red/phytochrome (P_FR), are crucial to accelerating these interactions.^342,343 As shown in Fig. 19a, chlorophyll A and B exhibit distinct absorption within the blue and red regions to drive the photosynthesis reaction, which facilitates the carbon dioxide fixation and the adenosine triphosphate production.^343,344 In contrast, P_R presents robust absorption in the red-light region at approximately 660 nm, whereas P_FR demonstrates absorption in the far-red/NIR region at 730 nm. The absorption of these pigments partially overlaps with the NIR emission of chromium-activated phosphors. Therefore, recent studies have explored the potential of chromium-activated phosphors to enhance plant growth.

Fig. 19 Plant growth lighting through NIR pc-LED. (a) Absorption spectra of chlorophyll A, chlorophyll B, P_R, and P_FR, along with a scheme of P_R and P_FR interactions. Reproduced with permission.⁹⁸ Copyright 2020, Royal Society of Chemistry. (b) EL spectrum of the NIR pc-LED fabricated using a 450 nm chip and K(Al,Ga)₁₁O₁₇:Cr³⁺ phosphor. (c) Schematic diagram illustrating plant growth scenarios with and without NIR pc-LED illumination. (d) Soybean germination experiment. Reproduced with permission.⁸³ Copyright 2023, John Wiley and Sons. (e) EL spectrum of the pc-LED fabricated using a 460 nm chip and Zn₃(Al,Ga)₂GeO₈:Cr³⁺ phosphor. (f) Morphological comparison of Begonia semperflorens subjected to 30 days of growth without (left) and with (right) NIR pc-LED lighting. The pigment analyses of (g) leave and (h) stem without NIR illumination. The pigment content of (i) leaves and (j) stems following NIR illumination. Reproduced with permission.³⁴¹ Copyright 2021, John Wiley and Sons.

In a representative example, Dou et al. conducted a comparative investigation on Soybean germination to identify the advantages of NIR light in promoting plant growth, employing K(Al,Ga)₁₁O₁₇:Cr³⁺ phosphor as the light converter in conjunction with a 450 nm chip.⁸³Fig. 19b displays the electroluminescence (EL) spectrum of the pc-LED device, which effectively covers the absorption regions of P_R and P_FR. The experimental Soybean samples were subjected to NIR light (No. 1) and no NIR light (No. 2) for 12 hours daily (Fig. 19c). The findings revealed that significantly more Soybeans in the No. 2 sample had germinated and broken ground within the first 5 days, compared to the No. 1 sample (Fig. 19d). After a growth of 7 days, the Soybean sprouts in the No. 2 sample demonstrated notably longer stem lengths, indicating that NIR light is advantageous for Soybean sprouting. A study by Fang et al. corroborated the positive effects of NIR radiation on promoting plant growth, using a high-efficiency Ga₂O₃:Cr³⁺ phosphor.⁹⁸ Their study showed that, after NIR illumination, Aglaonema and Plectranthus specimens experienced growth enhancements by 5% and 8%, respectively. In another work, Lv et al. quantitatively assessed the effects of NIR light on leaves and stems of Begonia semperflorens employing Zn₃(Al,Ga)₂GeO₈:Cr³⁺ phosphor as NIR light converter.³⁴¹Fig. 19e displays the EL spectrum of the corresponding pc-LED device fabricated with a 460 nm chip. The broadband NIR emission spanning 600–800 nm overlaps with the absorption spectra of P_R, and P_FR. Following 30 days of NIR pc-LED illumination, the Begonia semperflorens underwent a color change from green to red (Fig. 19f) and a substantial increase in the average number of flowers. Specifically, chlorophyll A, chlorophyll B, and total chlorophyll contents increased by 16.37%, 20.35%, and 36.72% (Fig. 19g and i) in the leaves and by 21.86%, 27.75%, and 49.60% (Fig. 19h and j) in the stems due to NIR illumination. These outcomes substantiate that the NIR lighting is favorable for promoting the production of biological pigments and photosynthesis, thereby facilitating the growth of the Begonia semperflorens.

5.1.3. Night-vision and tracing. Fig. 20a displays the functional visual acuity of the human eyes, revealing a significantly reduced sensitivity to NIR light compared to visible light.³⁴⁵ This makes NIR-emitting materials ideal for covert illumination in night-vision and tracking applications when used with auxiliary equipment such as NIR cameras or night-vision monoculars. The chromium-activated NIR phosphors, for example, Ca₃Sc₂Si₃O₁₂:Cr³⁺, LiIn₂SbO₆:Cr³⁺, and K₃ScF₆:Cr³⁺, have demonstrated significant potential in night-time object detection.^346–348 Nevertheless, the power output of the fabricated NIR pc-LEDs is generally limited within tens of milliwatts, and transmission distance is limited within a few meters by substantial attenuation over longer distances. To address these bottlenecks, Liu et al. recently demonstrated a watt-level NIR output from MgO:Cr³⁺ ceramic with a 29% photoelectric conversion efficiency through blue-laser pumping. By employing multiple lens reflections and focusing techniques (Fig. 20b), they further assembled a light-emitting device capable of night-vision observation over a distance of up to 45 meters (Fig. 20c). In a subsequent work, the team successfully engineered an NIR pc-LED employing a blue laser and MgAl₂O₄:Cr³⁺ ceramic, achieving a peak output power of 2.13 W that enabled a detection distance of over 100 meters.³⁴⁹

Fig. 20 NIR night-vision and tracing. (a) Functional visual acuity of the human eye under photopic and scotopic conditions. (b) Schematic presentation of the NIR pc-LED fabricated using a blue laser and MgO:Cr³⁺ ceramic. (c) Light source and night-vision images captured at a distance of 45 meters. Reproduced with permission.¹ Copyright 2024, Springer Nature. (d) NIR PersL tag produced by LaZn(Al,Ga)₁₁O₁₉:Cr³⁺/Yb³⁺ phosphor affixed to the shoulder of a human subject and a camouflage uniform coated with NIR PersL paint. Reproduced with permission.⁷³ Copyright 2022, Elsevier. (e) Schematic text information encryption. The red and blue codes were printed with bare ZnAl_1.4Ge_0.3O_3.7 and ZnAl_1.4Ge_0.3O_3.7:Cr³⁺ phosphor, respectively. Reproduced with permission.³⁵² Copyright 2020, John Wiley and Sons. (f) Fingerprint photographs under natural lighting. (g) Magnified fingerprint patterns under NIR light, showing details such as short ridge, delta, lake, bifurcation, whorl, termination, island, and hook. (h) Enlarged patterns revealing details such as sweat pores and scars. Reproduced with permission.³⁵⁴ Copyright 2024, Elsevier.

Compared to PL, PersL exhibits self-sustained luminescence that lasts from seconds to hours following a brief storage period of energy excitation. Employing NIR PersL materials as tracking markers, their inherently passive nature allows for covert and long-term tracking without the need for constant excitation. For instance, the LaZn(Al,Ga)₁₁O₁₉:Cr³⁺/Yb³⁺ phosphor exhibited exceptional NIR PersL properties with a duration of 1000 hours due to the efficient E_4f(Yb²⁺) electron traps.⁷³ When integrated with acrylic acid to produce composite PersL coatings, the target tag and pattern with coatings after charging by 254 nm UV light could be identified under nocturnal conditions without external excitation (Fig. 20d). These objects maintain visibility for over 10 hours. Similarly, the LaMgGa₁₁O₁₉:Cr³⁺/Sm³⁺ phosphor as PersL tag has also been validated for night vision applications at a detectable distance of 4.5 meters.³⁵⁰ Therefore, chromium-activated PersL phosphor effectively serves as a secret light source for overnight tracing and positioning. Despite these innovations, higher-brightness NIR PersL materials and optimized packaging processes are urgently required for long-distance night-vision and tracing applications.

5.2. Information security

The low sensitivity of human eyes to NIR radiation is also favorable for information encryption and anti-counterfeiting applications. Chromium-activated phosphors are frequently incorporated into printing inks along with visible phosphors to fabricate encrypted labels with distinct identification codes. Upon excitation, the pristine information can be observed as visible emissions while the encryption data associated with the NIR phosphors remain concealed due to the low sensitivity of human eyes to NIR radiation. However, such hidden information can be retrieved utilizing auxiliary equipment such as NIR cameras and night-vision monoculars, thereby enabling authentication of the label.⁷³ Additionally, these NIR phosphors can be engineered to exhibit PersL, ML, and photo-stimulated luminescence, enabling multidimensional encryption.^108,179,351

In a typical example, Liu et al. pioneered a NIR PersL phosphor consisting of Cr³⁺-activated LiGa₅O₈, which, after charging by 254 nm UV light, showed a photo-stimulated PersL signal that remained detectable even after a long period of 1000 h.¹⁷⁹ Accordingly, the printed patterns composed of this phosphor present localized PersL patterns after passing through a photomask, making them suitable for anti-counterfeiting applications. Additionally, multi-mode emissions within visible and NIR spectrum regions have been proposed for advanced anti-counterfeiting applications using ZnAl_1.4Ge_0.3O_3.7:Cr³⁺.³⁵² This phosphor presented bluish-white and NIR PL at 254 nm excitation, switched to only NIR PersL after the excitation ceased, and further showed NIR luminescence under stimulation of a 980 nm laser.³⁵² This dynamically tunable luminescence behavior significantly enhanced security levels of anti-counterfeiting measures, as evidenced in Fig. 20e.

NIR imaging can effectively eliminate the interference from ambient light, which is favorable for high-resolution fingerprint recognition.^353,354 For instance, Feng et al. have demonstrated the efficacy of Ca₃MgSnGe₃O₁₂:Cr³⁺ phosphor in this application.³⁵⁴ Specifically, the phosphor powder was uniformly dispersed onto the fingerprints left on a substrate, after which the excess powder that did not attach was removed. Fig. 20f displays the fingerprint image captured under natural light with a digital camera, where the detailed patterns are indistinguishable, revealing only the general outline. In contrast, the images captured using an NIR camera under blue-LED excitation distinctly reveal the fingerprint features, such as short ridge, delta, lake, bifurcation, whorl, termination, island, and hook (Fig. 20g), along with sweat pores and scars (Fig. 20h). Therefore, these high-resolution fingerprint images provide extensive details for authentication purposes.

5.3. In vivo bioimaging

Bioimaging aims to visualize and elucidate the real-time spatiotemporal dynamics of biological processes. Often, bioimaging is integrated with bioanalytical methodologies to quantify the local disease progression, including tumors and cancer. It can be further combined with therapeutic approaches to achieve image-guided treatment.³⁵⁵ NIR radiation within the biological windows (NIR-I, 750–950 nm; NIR-II, 1000–1800 nm) is characterized by a large tissue penetration depth, which is favorable for bioimaging.^3,5,356,357

NIR PersL imaging is particularly advantageous. After a brief charging period, these materials emit light continuously without the need for real-time excitation. This self-sustained emission effectively eliminates background interference, thereby enhancing the signal-to-noise ratio (SNR). As elaborated in Section 3.4, chromium-activated phosphors exhibit exceptional NIR PersL properties, making them highly competitive candidates for in vivo bioimaging. Unfortunately, most chromium-activated PersL phosphors require UV-light charging. Owing to the limited tissue penetration capability of UV radiation, ex vivo UV excitation is frequently adopted before the materials are injected into biological tissues, posing constraints on long-term imaging experiments. To overcome this issue, Maldiney et al. established an in vivo bioimaging approach using ZnGa₂O₄:Cr³⁺ as optical probes, which could be recharged in situ using red photons, successfully achieving cell tracking and subsequent visualization of particle biodistribution within live animals.³²⁵Fig. 21a schematically presents the experimental setup, in which mice were exposed to irradiation from an orange/red LED source, followed by PersL acquisition using a photo-counting system. The result indicated a major localization of PersL nanoparticles in the liver, with minimal uptake in the spleen (Fig. 21b). Despite the significantly lower absolute intensity and total emitted light compared to carboxyl-QDs, the PersL bioimaging presented markedly superior SNR due to reduced background interferences. Specifically, the SNR value increased from ∼17 for QDs to 186 for ZnGa₂O₄:Cr³⁺ following intramuscular injection, and from 2.5 to 16 following intravenous injection. In addition to ZnGa₂O₄:Cr³⁺, other gallate spinel PersL nanocrystals, such as MgGa₂O₄:Cr³⁺, Zn₃Ga₂GeO₈:Cr³⁺, and Zn₃Ga₂Ge₂O₁₀:Cr³⁺, have also been extensively employed as optical probes for bioimaging applications.³²⁵

Fig. 21 NIR bioimaging. (a) Schematic representation of in vivo bioimaging realized by in situ excitation of ZnGa₂O₄:Cr³⁺ PersL nanoparticles. (b) Comparative imaging analysis employing ZnGa₂O₄:Cr³⁺ (left) and carboxyl-QDs (right) as probes. Reproduced with permission.³²⁵ Copyright 2014, Springer Nature. (c) PersL decay curves of Na_0.5Gd_0.5TiO₃:Cr³⁺ and ZnGa₂O₄:Cr³⁺ phosphors under periodical photo-stimulation by a 695 nm laser. The irradiation powers were 2.5, 45, and 2.1 × 10³ µW mm⁻², respectively. (d) Comparative images of ZnGa₂O₄:Cr³⁺, Zn₃Ga₂GeO₈:Cr³⁺, Na_0.5Gd_0.5TiO₃:Cr³⁺, Zn₃Ga₂Ge₂O₁₀:Cr³⁺, and LiGa₅O₈:Cr³⁺ phosphors after being in situ excited by 368, 427, 652, and 737 nm LEDs at 19 µW mm⁻². Reproduced with permission.¹⁸² Copyright 2020, American Chemical Society. (e) UC-PersL decay mappings and intensities of Zn_1.2Ga_1.6Ge_0.2O₄:Cr³⁺ and Zn_1.1Ga_1.8Sn_0.1O₄:Cr³⁺ phosphors. (f) Bioimaging as a function of time after 700 nm LED excitation employing Zn_1.2Ga_1.6Ge_0.2O₄:Cr³⁺ as a probe. Reproduced with permission.³⁵⁸ Copyright 2024, American Chemical Society. (g) Diagrammatic presentation of bioimaging based on ultrasound-induced luminescence and traditional fluorescence imaging. Reproduced with permission.³⁵⁹ Copyright 2024, Springer Nature.

Despite these achievements, the PersL intensity generally undergoes a significant reduction with increasing excitation wavelength. For instance, the PersL excitation intensity of Zn₃Ga₂Ge₂O₁₀:Cr³⁺ reduces to approximately 1% under 600 nm excitation compared to UV excitation. To expand the excitability, Huang et al. recently unveiled a perovskite-structured Na_0.5Gd_0.5TiO₃:Cr³⁺ with two orders of magnitude stronger PersL intensity than ZnGa₂O₄:Cr³⁺ at 652 nm red light excitation via a one-photon charging process.¹⁸²Fig. 21c comparatively displays the NIR PersL decay curves of Na_0.5Gd_0.5TiO₃:Cr³⁺ and ZnGa₂O₄:Cr³⁺ under periodic photo-stimulation of a 695 nm red light. The PersL decay intensities of Na_0.5Gd_0.5TiO₃:Cr³⁺ consistently surpassed those of ZnGa₂O₄:Cr³⁺, even with increasing the charging irradiance up to 2.1 × 10³ µW mm⁻², which was beyond the safety exposure limit of 2 × 10³ µW mm⁻². Similarly, under in situ excitation with a 652 nm LED (19 µW mm⁻²), only Na_0.5Gd_0.5TiO₃:Cr³⁺ could afford detectable PersL signals through a 12-mm thick muscle layer (Fig. 21d). This behavior underscored the excellent performance of Na_0.5Gd_0.5TiO₃:Cr³⁺ for bioimaging.

Recently, high-energy X-rays have been explored as an excitation source to improve the bioimaging efficiency. For example, Xue et al. showcased a PersL duration of 6 h under minimal X-ray radiation power (45 kVp, 0.5 mA) in ZnGa₂O₄:Cr³⁺ nanocrystals, which could be efficiently reactivated in situ by highly penetrative X-rays for bioimaging.³⁶⁰ In parallel efforts, the concept of upconversion persistent luminescence (UC-PersL) has been advanced for effective bioimaging. Pan et al. pioneered the UC-PersL system by integrating an up-converting ion pair of Yb³⁺ and Er³⁺ ions into the Zn₃Ga₂GeO₈:Cr³⁺ matrix, achieving a NIR PersL duration of over 24 hours.^185–187 Recently, by incorporating Ge⁴⁺ and Sn⁴⁺ into isomorphic ZnGa₂O₄:Cr³⁺, for instance, Yang et al. enhanced UC-PersL intensity due to augmentation of lattice defects (Fig. 21e).³⁵⁸ Specifically, the PersL intensities of Zn_1.2Ga_1.6Ge_0.2O₄:Cr³⁺ and Zn_1.1Ga_1.8Sn_0.1O₄:Cr³⁺ phosphors were approximately 3.97 × 10⁸ p s⁻¹ Sr⁻¹ cm⁻² and 4.01 × 10⁸ p s⁻¹ Sr⁻¹ cm⁻², representing enhancements by 1.92 and 1.94 folds compared to ZnGa₂O₄:Cr³⁺, respectively. Consequently, the bioimaging manifested an SNR of 230 utilizing Zn_1.2Ga_1.6Ge_0.2O₄:Cr³⁺ as an optical probe (Fig. 21f).

In addition to photo-stimulated PersL, recent studies have explored NIR emission triggered by mechanical stimulation, e.g., ultrasonic waves, for in vivo bioimaging, as schematically shown in Fig. 21g. In 2013, Terasaki et al. pioneered the applications of ultrasonic waves as non-invasive stimulation for biological imaging and succeeded in the detection of the light emission concomitant with ultrasonic wave irradiation utilizing ML phosphors.^361,362 Similarly, Wu et al. realized bioimaging and optogenetics by employing ML nanocrystals as light-emitting sources within the brain under stimulation of ex vivo focused ultrasonic waves.³⁶³ These studies indicated that NIR ML nanoparticles hold significant promise for advancing in vivo bioimaging under mechanical stimulation.

Apart from light (photon) emission, the energy absorbed by the host could be alternatively released as heat (phonons). These non-radiative processes have inspired a burgeoning interest in photothermal therapy, i.e., using heat generated by light to destroy targeted cells.^364–366 Specifically, the intra-configuration transition of Cr³⁺ ions presents a relatively high absorption cross-section (10⁻¹⁹–10⁻²⁰ cm²), which is favorable for high-efficiency light-to-heat conversion.^367–369 For example, GdAl₃(BO₃)₄:Cr³⁺ phosphor, characterized by an ultrahigh phonon energy of 1350 cm⁻¹, showcases a temperature increase of 210 °C under 600 mW cm⁻² blue laser irradiation by approximately 50 s.³⁷⁰ NaGdF₄:Cr³⁺ also presented a maximal external light-to-heat conversion efficiency of 26 × 10⁻³ L g⁻¹ cm⁻¹ at a Cr³⁺ doping concentration of 15%, ascribed to its high absorption coefficient. However, the practical applications of the chromium-activated matrices in photothermal therapy in vivo are pending further advancement.

5.4. Optical sensing

Temperature is a crucial thermodynamic variable that influences both biological processes and industrial operations, making accurate temperature measurement a long-standing research focus. The ratiometric luminescent thermometers, particularly luminescence intensity ratio (LIR) for optical sensing, have recently gained prominence as an effective non-invasive technique for temperature measurement. This technique offers high spatial resolution and exceptional detection sensitivity with rapid thermal response.^371–375

Chromium-activated matrices are particularly promising for this application because the ²E and ⁴T₂ excited states of Cr³⁺ are thermally coupled, especially in intermediate crystal fields (see Sections 2.2 and 3.1).^376–378 Governed by the Boltzmann law,⁴⁴ the LIR of the ⁴T₂ → ⁴A₂ and ²E → ⁴A₂ transitions can provide absolute temperature measurements across an extensive range. Back et al. pioneered to summarize the relative sensitivity value (S_r, % K⁻¹) as a function of energy gap (ΔE) between ⁴T₂ and ²E excited states for Cr³⁺-activated Boltzmann thermometers, as depicted in eqn (11).³⁷⁹

Considering an average error of 100 cm⁻¹ for the Dq value estimation, S_r could be considered reliable within ±0.1% K⁻¹. In line with this relationship, they found that α-Ga₂O₃:Cr³⁺ phosphor with a ΔE of 657 cm⁻¹ exhibited an S_r of 1.05% K⁻¹ at 300 K, whereas β-Ga₂O₃:Cr³⁺ phosphor with a reduced ΔE of 399 cm⁻¹ manifested an S_r of 0.64% K⁻¹ at 300 K.³⁷⁹ Besides, they further developed Bi₂Ga₄O₉:Cr³⁺ phosphor for optical thermometer application, which displayed a ΔE value of 307 cm⁻¹. Their research evidenced its applicability in the LIR technique over 77–450 K, achieving a relative sensitivity of 0.7% K⁻¹ within the physiological temperature range.^76,380 Similarly, Li et al. also demonstrated an LIR thermometry application employing Li₂ZnGe₃O₈:Cr³⁺ phosphor as an optical probe, achieving a high S_r of 13.09% K⁻¹ at cryogenic temperature.³⁷⁷

In addition to intensity variation, the luminescence lifetime of Cr³⁺ ions in an intermediate crystal field also rapidly declines with temperature (usually in a pseudo-exponential mode, SI). Chen et al. investigated the temperature-dependent luminescence lifetimes, substantiating their potential for thermometric applications.³⁸¹

Beyond temperature sensing, chromium-activated phosphors are also highly effective for luminescence manometry. The pressure sensitivity is typically characterized by the absolute (S_p,a) and relative sensitivity (S_p,r) parameters, defined as^382,383

in which E(R) represents radiative energy and p denotes the applied pressure. For Cr³⁺ ions in a strong crystal field, the energy of emission through the ²E → ⁴A₂ transition can be described by⁴⁴

Under high pressure, the shortening of cation–ligand bonds enhances the crystal field strength Dq, while the increased orbital overlap reduces the Racah parameters B and C. These competing effects make it difficult to deconvolute the individual contributions of each parameter under external pressure. Moreover, manometry based on the ²E → ⁴A₂ emission exhibits a small spectral shift, resulting in low pressure sensitivity. For example, ruby (α-Al₂O₃:Cr³⁺), which has served as a standard reference for probing pressure in diamond anvil cell-based experiments worldwide since 1972, suffers from a relatively low S_p,a of −0.76 cm⁻¹ kbar⁻¹.³⁸⁴ Recent attempts by Back et al. only obtained a minor improvement S_p,a to −1.06 cm⁻¹ kbar⁻¹ using Bi₂Al₄O₉:Cr³⁺ phosphors.³⁸³

For Cr³⁺ ions in a weak crystal field, the emission is dominated by the ⁴T₂ → ⁴A₂ transition, whose energy is predominantly governed by Dq value and scales inversely with bond length, as described in eqn (5). Consequently, pressure-induced bond compression generally results in a pronounced spectral shift towards shorter wavelengths. Szymczak et al. discovered that Cr³⁺ ions in a weaker crystal field environment render higher sensitivity in the ratiometric pressure sensing.^385,386 Accordingly, they developed a LiScGeO₄:Cr³⁺ manometer at a significantly low Dq/B value of 1.44, where the ⁴T₂ → ⁴A₂ emission shifts linearly under pressure, yielding a high S_p,a of 23.63 nm GPa⁻¹ in the 0–7 GPa pressure range. These observations demonstrate the considerable potential of Cr³⁺-activated NIR phosphors in high-pressure sensing.

6. Conclusions and outlook

In this review, we deliver a comprehensive overview of the recent advancements in chromium-activated phosphors, covering topics from crystal field theory to structural engineering and technological applications. Importantly, deeper insights into the luminescence mechanisms, encompassing the Tanabe–Sugano diagram of Cr³⁺/Cr⁴⁺ in distorted polyhedra, the luminescence behavior of tetrahedrally coordinated Cr³⁺ ions, interaction coupling of the Cr³⁺–Cr³⁺ pair, and locating the ground state within the bandgap of hosts, have revealed how these factors collectively govern key optical properties, including spectral profiles, luminescence efficiency, and luminescence thermal stability. These insights challenge the conventional stereotype that luminescence properties are solely dictated by the local crystal field strength and instead highlight the interplay of multiple structural and electronic parameters. Based on these principles, the emission behaviors of chromium-activated NIR phosphors can be precisely tailored through multiple strategies like crystal field modulation, multiple sites occupation, octahedral distortion, heavy doping, and defect engineering. Looking forward, the burgeoning interest and extensive study in chromium-activated NIR phosphors have been fueled by the new opportunities for multidisciplinary applications, from non-destructive analysis, plant growth lighting, and night vision to bioimaging, information encryption, and sensing. Continued efforts in crystal design and mechanistic understanding will be pivotal in unlocking the full potential of chromium-based NIR phosphors for next-generation photonic technologies.

Despite these encouraging achievements, challenges and opportunities remain in the development of high-performance chromium-activated phosphors and their associated devices, including but not limited to:

(1) Full-spectrum tunability covering NIR-I and NIR-II. The overtones of chemical functional groups manifest continuous absorption characteristics in both the NIR-I and NIR-II regions. Accordingly, it is essential to develop a continuous full-spectrum for non-destructive analysis applications. Strategies such as octahedral distortion and heavy doping can improve the luminescence efficiency of Cr³⁺. Besides, energy transfer from concentrated Cr³⁺ ions is favorable for the sensitization of Ni²⁺ and Ln³⁺ luminescence, such as LiGa₅O₈:Cr³⁺/Ni²⁺,¹²⁷ Ca₃Al₂Ge₃O₁₂:Cr³⁺/Ni²⁺,¹⁴² and Gd₃Sc_1.5Al_0.5Ga₃O₁₂:Cr³⁺/Yb³⁺,¹²⁵ thereby facilitating efficient NIR-II luminescence. Nevertheless, an NIR spectral gap persists in the range of 900–1000 and 1300–1600 nm. Despite the potential of Cr⁴⁺ ions to bridge this NIR-II spectral gap, comprehensive studies on their luminescence properties (including PL, PersL, and ML) are scant. Additionally, the reported luminescence efficiency of Cr⁴⁺ ions is significantly low, with an EQE below 20%. It is therefore critical to continue the exploration of novel synthesis routes and host materials. Due to the preferential occupation of Cr⁴⁺ ions at tetrahedral sites, the potential crystal structures deserving attention include pyroxene, garnet, and olivine, and their derivatives, which are characterized by their [M–O₄] tetrahedral sites.

(2) Surface passivation of nanoscale phosphors for the promoted performance. Nanoscale phosphors have great potential for applications in biomedical imaging, diagnosis, and micro-LEDs. Yet, their luminescence properties decline significantly as particle size decreases due to their large surface areas, which are typically recognized as the main quenching sites for excitation energy. Therefore, surface passivation methodologies, such as surface coating (e.g., silane coupling agents,³²⁵ silica,³⁸⁷ and polymers^281,388,389), organic solvent-mediated soft chemical synthesis,^390,391 and epitaxial shell growth,^392–394 are desired for mitigating non-radiative processes induced by surface defects.

(3) Mechanism revelation for Cr³⁺-concentrated systems and Cr³⁺-based ML phenomenon. Cr³⁺ ions typically demonstrate significant luminescence concentration quenching at high doping concentrations. Interestingly, recent studies have reported atypical luminescence behaviors in concentrated Cr³⁺ systems, such as altered emission peaks, quantum efficiencies, and lifetimes. However, the underlying luminescence mechanisms remain inadequately understood. Accordingly, comprehensive structural and spectroscopic investigations are imperative to elucidate the structure–property relationships for further advancing the luminescence performance. In addition, as an emerging avenue for mechano-to-light energy conversion, ML materials require further developments, particularly in the exploration of novel high-performance hosts. An in-depth analysis of frictional behavior remains imperative to fully elucidate the ML mechanisms of chromium-activated phosphors.

(4) Materials development using computational tools. The advancement of chromium-activated phosphors has traditionally been constrained by labor-intensive trial-and-error experimentation. Emerging computational technologies, such as machine learning and artificial intelligence, provide a promising data-driven paradigm for materials design. By integrating large-scale datasets with advanced spectral theories, these approaches are expected to rationally accelerate the discovery of new phosphors with tailored luminescence properties.

Author contributions

Shengqiang Liu data curation, visualization, writing – original draft, writing – review & editing; Leipeng Li writing – review & editing; Quanlin Liu conceptualization, supervision; Bing Chen and Feng Wang conceptualization, supervision, writing – review & editing.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

No primary research results, software or code have been included, and no new data were generated or analysed as part of this review.

The calculations of the crystal field strength for Cr³⁺ and Cr⁴⁺ ions, estimation of trap depths (Fig. S1), and luminescence decay behaviors of Cr³⁺ ions (Fig. S2) are included in the supplementary information (SI). See DOI: https://doi.org/10.1039/d5cs00957j.

Acknowledgements

This work was supported by the Research Grants Council of Hong Kong (Project No. CityU RFS2021-1S03) and the National Natural Science Foundation of China (12474401, 52572152 and 62205155).

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