A review on the structural dependent optical properties and energy transfer of Mn⁴⁺ and multiple ion-codoped complex oxide phosphors

Abstract

The tetravalent manganese Mn⁴⁺ ions with a 3d³ electron configuration as luminescence centers in solid-state inorganic compounds have been widely investigated because they emit bright light in the red to far-red region when they are excited by light with a wavelength in the UV to blue light region. Herein, we present an overview of the recent developments of Mn⁴⁺ and multiple ion such as Bi³⁺ and rare earth ion Dy³⁺, Nd³⁺, Yb³⁺, Er³⁺, Ho³⁺, and Tm³⁺ codoped complex oxide phosphors. Most of the specified host lattices of these complex oxide phosphors possess multiple metallic cations, which provide possible substitutions with different codopants and form various luminescence centers with diverse spectra. The luminescence of Mn⁴⁺ and multiple ion-codoped materials spans almost the whole visible light to near infrared (NIR) region. The crystal structures of complex oxide phosphors, the spectroscopic properties of Mn⁴⁺, and the energy transfer between Mn⁴⁺ and multiple ions are introduced and summarized in detail with regard to their practical applications. This review provides an insight into the optical properties of Mn⁴⁺ and the energy transfer process in multiple ion-codoped luminescence materials, which will be helpful in the development of novel excellent materials for applications in the lighting industry.

---

1. Introduction

The optical properties based on the structures of host lattices and the energy transfer between Mn⁴⁺ and multiple ion-codoped complex oxide phosphors described in this review make the identified luminescence materials promising for application in solar energy cells, white light-emitting diodes (WLEDs), indoor lighting for plant cultivation, and temperature sensors, as illustrated in Fig. 1.

Fig. 1 Optical properties, energy transfer, and potential application of Mn⁴⁺ and multiple ion-codoped complex oxide phosphors described in this review.

Solar energy cells and WLEDs are considered as alternative approaches to relieve the energy crisis with the increasing global energy consumption. Solar energy cells using crystalline silicon solar cells have occupied majority of the solar cell market owing to their well-developed techniques and low cost; however, their conversion efficiency should be improved further for their wide commercial applications. It is well known that most of the energy of the solar spectrum is concentrated at wavelengths beyond 900 nm including UV-visible (UV-vis) and NIR light, which cannot be absorbed by the current crystalline silicon solar cells with high efficiency.^1–4

Converting the energy of the solar spectrum at wavelengths beyond 900 nm into the range located 900–1100 nm, which matches the maximum spectral response of the absorption of crystalline silicon, is an important alternative approach to improve the energy conversion efficiency of crystalline silicon solar cells. Recently, much attention has been paid to developing Mn⁴⁺-doped phosphors because Mn⁴⁺ usually shows sharp line emissions in the red-infrared (IR) region due to its unique 3d³ electron configurations. It has been observed that Mn⁴⁺ shows red to far-red photoluminescence, which is assigned to the spin-forbidden ²E_g → ⁴A_2g transition under the excitation of UV or blue light owing to its high effective positive charge and the influence of a strong local crystal-field.^5–12 The reversible conversion of UV-vis into NIR, and NIR into visible light with dual-mode luminescence can be realized by codoping multiple ions such as Nd³⁺/Er³⁺/Yb³⁺ into Mn⁴⁺ ion-doped luminescence materials. The red emission of Mn⁴⁺ can be obtained when it is excited by 980 nm due to the energy transfer from Nd³⁺/Er³⁺/Yb³⁺ to Mn⁴⁺ ions.¹³ The NIR photoluminescence maxima at 1064, 1537, and 980 nm originating from Nd³⁺/Er³⁺/Yb³⁺ ions can be sensitized by Mn⁴⁺ with excitation in the UV-vis region (200–500 nm).^13–16 The conversion of UV-vis light into NIR light at about 1064 nm through energy transfer from Mn⁴⁺ to multiple ions is desirable to improve the conversion efficiency of solar cells by coating the phosphor layer on the surface of a crystalline Si layer.

WLEDs have received extensive attention due to their high energy efficiency, long lifetime, and environmental friendliness. The WLEDs fabricated with blue semiconductor GaN chips and yellow phosphor Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce) can produce cold white light because the red component in their spectra is weak. To meet the requirement for indoor illumination, warm white light with a high color rendering index (CRI > 80) and a low correlated color temperature (CCT < 4000 K) is necessary.^17,18 Accordingly, phosphors with strong absorption in the blue light region and intense emission in the red light region should be co-coated on blue semiconductor GaN chips to produce warm white light. Mn⁴⁺ ions located at octahedral crystallographic sites are favorable luminescent centers and promising for blue GaN-excited warm WLED applications because they have narrow-band red emissions, broad-band blue excitations, and no reabsorption in white light, while being free of expensive rare earth metals.^5–12 Thus, much attention has been paid to developing red phosphors to provide alternatives to the commercial nitride phosphors. In particular, the interest in Mn⁴⁺-doped inorganic phosphors has increased because the Mn⁴⁺ luminescence center usually shows sharp line emissions in the red region with high color purity due to the sharp feature of its emission spectrum.^5–12

Recently, indoor plant cultivation has attracted considerable attention because this advanced technology can exclude the unfavorable influence of the climate and natural damage. To meet the requirement in lighting for indoor plant cultivation, blue-violet light in wavelength range of 420–500 nm is indispensable for chlorophyll A and chlorophyll B, and red-far red light in wavelength range of 640–750 nm is indispensable for phytochrome PR and phytochrome PFR.^19–24 The fabrication of red Mn⁴⁺-doped phosphors in blue LED chip results in a superior performance in lighting for indoor plant cultivation due to the blue light from LED chips and red light from Mn⁴⁺-doped luminescence materials excited by blue light. This type of light device is a promising light source for large scale industrial application because of the energy saving and long working time of LEDs, and low cost of Mn⁴⁺-doped luminescence materials. Bi³⁺ and Mn⁴⁺ codoped oxide phosphors, which emit dual blue and red light upon excitation by near UV (NUV) LEDs, are alternative candidates for application in the agricultural industry to improve the efficiency of photosynthesis. To maintain the electroneutrality of the compound, excess metal ion vacancies and O²⁻ ions in the lattices of complex oxides may be formed for charge compensation.^25–30

The upconverted NIR luminescence of Mn⁴⁺ was realized with the aid of the efficient energy transfer of Yb³⁺ → Ln³⁺ → Mn⁴⁺ in the specially prepared Yb³⁺/Ln³⁺/Mn⁴⁺ (Ln = Er, Ho, Tm) codoped YAlO₃ and its energy transfer efficiency was systematically clarified by its steady-state and time-resolved upconverted emission spectra.³¹ The dual emission based on Mn⁴⁺ and multiple ion (such as Yb³⁺, Ln³⁺, and Mn⁴⁺) codoped phosphors is promising for accurate temperature sensors due to the fact that the thermal quenching mechanisms of Mn⁴⁺ and Ln³⁺ are different.³² Fig. 2 presents a summary of the energy transfer between Mn⁴⁺ and multiple ions, the emission wavelengths, and corresponding electronic transitions of both the donor and acceptor. The octahedral environment-coordinated Mn⁴⁺ ions emit red to far-red emissions in the region of 600 to 700 nm. Thus, tunable spectral emissions from the visible to NIR region can be realized by codoping Mn⁴⁺ and multiple ions.

Fig. 2 Summary of the energy transfer between Mn⁴⁺ and rare earth ions.

In all the host lattice of complex oxides, as summarized in Table 1, Mn⁴⁺ ions perfectly substitute the sites in the centers of the octahedral environment coordinated with six oxygen atoms due to their similar radius and valence, such as Ga³⁺, Al³⁺, Ti⁴⁺, Ta⁵⁺, and Mg²⁺–Te⁶⁺ pairs, and Nb⁵⁺ ions. Multiple cation sites in the complex oxide host lattice provide the possibility for codoping Mn⁴⁺ ions and Bi³⁺ or trivalent rare earth ions. The optical characteristics of Mn⁴⁺ and other ions are strongly dependent on the structural symmetry of the host materials. This review aims to comprehensively present the structural-dependent optical properties based on the energy transfer between Mn⁴⁺ and multiple ions in codoped complex oxide phosphors for potential applications in high-efficient solar energy cells, warm WLEDs, indoor plant cultivation, and temperature sensors.

Table 1 Summary of the substituted sites for Mn⁴⁺ and rare earth (RE) ions, and the PLE and PL position of Mn⁴⁺ in various host lattices

	Mn^4+ doping octahedral centers	RE doping	Excitation of Mn^4+ at 200–500 nm (maximum band)	Emission of Mn^4+ at 650–800 nm (maximum peak)	Ref.
Ca14Zn6Ga10O35	Ga^3+	Ca^2+	313 nm	712 nm	13, 46, 51 and 61–63
Ca14Zn6Al10O35	Al^3+	Ca^2+	460 nm	710 nm	14, 15, 54, 65 and 66
Ca3ZnAl4O10	Al^3+	Ca^2+	467 nm	715 nm	18
Gd2ZnTiO6	Ti^4+	Gd^3+	365 nm	704 nm	33, 74 and 75
La2LiTaO6	TaO6	La^3+	495 nm	709 nm	34
NaMgLaTeO6	Mg^2+ and Te^6+	La^3+	365 nm	705 nm	16
La2MgTiO6	Ti^4+	La^3+	355 nm	710 nm	35, 79 and 80
Ba2LaNbO6	Nb^5+	La^3+	352 nm	677 nm	35
CaAl12O19	Al^3+	Ca^2+	400 nm	654 nm	36, 116 and 117
Mg2TiO4	Ti^4+	Mg^2+	475 nm	657 nm	37
La2ZnTiO6	Ti^4+	La^3+	345 nm	710 nm	38
MgAl2Si2O8	Si^4+	Mg^2+	258 nm	710 nm	39 and 145–147
YAlO3	Al^3+	Y^3+	414 nm	714 nm	32

---

Mn⁴⁺ is isoelectronic with Cr³⁺, but the crystal field at the higher charged Mn⁴⁺ ions is stronger than that of Cr³⁺ and the vibronic emission ²E_g → ⁴A_2g of Mn⁴⁺ is more intense than that of Cr³⁺. The Tanabe–Sugano energy diagram presents the energy splitting of the Mn⁴⁺ ion with an octahedral coordination dependent on the crystal field strength (Fig. 3a).^5–12,38,40 The Stokes shift and the features of the photoluminescence emission and excitation (PL and PLE) spectra of Mn⁴⁺ ions are known to be tunable by changing the crystal-field of the host. The spectral position of Mn⁴⁺ ions can be easily tuned over a wide range from 620 nm to 723 nm by modifying the crystal field environment.^8,41 Mn⁴⁺ ions are inclined to form Mn⁴⁺–Mn⁴⁺ pairs due to the O²⁻ impurities in oxides, which significantly influence the excited state dynamics and reduce the luminescence efficiency of Mn⁴⁺.^36,42

Fig. 3 (a) Tanabe–Sugano diagram for Mn⁴⁺ dependent on the crystal field in complex oxides, (b) typical PLE and PL spectra, and (c) Gaussian curves of PLE spectra of Mn⁴⁺ in Ca₁₄Zn₆Ga₁₀O₃₅ phosphors. Reprinted with permission from ref. 39, Copyright 2017, The Royal Society of Chemistry.

2. Luminescent properties of Mn⁴⁺ in complex oxide phosphors

Mn⁴⁺ ions generally occupy octahedral sites coordinated by eight oxygens in complex oxide phosphors, and the PLE and PL spectra of Mn⁴⁺ ions are located in the range of 200–500 nm and 600–700 nm, respectively. As shown in Fig. 3b and c, the excitation bands located at 350 and 440 nm in the PLE spectrum of Mn⁴⁺ in Ca₁₄Zn₆Ga₁₀O₃₅ (CZGO) are assigned to the spin-allowed transitions of Mn⁴⁺. The three Gaussian peaks at 313, 356, and 462 nm are attributed to the ⁴A_2g → ⁴T_1g, ⁴A_2g → ²T_2g, and ⁴A_2g → ⁴T_2g of Mn⁴⁺ transitions, respectively. The broad band at 291 nm in the PLE spectrum of Mn⁴⁺ is ascribed to both the charge transfer transitions of Mn⁴⁺ → O²⁻ and ⁴A_2g → ⁴T_1g transitions of Mn⁴⁺ ions. Under excitation at 310 nm, the intense red emission is composed of some distinguishable sharp R lines and Stokes/anti-Stokes side-peaks located at 676, 684, 695, 704 and 713 nm due to the different vibrational modes for the 3d³ electrons when Mn⁴⁺ is in the [MnO₆]⁸⁻ octahedral complex, which correspond to the vibronic sidebands of the ²E_g → ⁴A_2g transition of the Mn⁴⁺ ions.⁴³

3. NIR emission of Nd³⁺, Yb³⁺, Er³⁺, Ho³⁺, and Tm³⁺ sensitized by Mn⁴⁺

3.1 Ca₁₄Zn₆Ga₁₀O₃₅ as host lattice for Mn⁴⁺ and multiple ion codoping

3.1.1 Formation of tunable color luminescence centers in Ca₁₄Zn₆Ga₁₀O₃₅. Fig. 4a shows that when viewed from the¹⁰⁰ plane, the unit cells for the crystal structure of Ca₁₄Zn₆Ga₁₀O₃₅ (CZGO) possess a cubic structure with the space group F23 (196) and lattice parameters a = 15.0794 Å and V = 3428.88 ų. According to Pauling's rules, one of these empty containers is filled with octahedral (Ga,Zn)O₆⁻, while the others are half occupied by four corner-linked tetrahedral ZnO₄ sharing a common oxygen atom.⁴⁵ All the edges are shared by various Ca polyhedra. Thus, there are three independent Ca²⁺ sites in CZGO, where two of them have an octahedral geometry and the third is in a seven-coordinated polyhedron. Moreover, the effective ionic radii of the six-coordinated Ga³⁺, Zn²⁺, and Ca²⁺ ions are 0.62, 0.74, and 1.00 Å, respectively. The specific crystal structure of CZGO makes doping multiple ions and forming tunable color luminescence centers possible.⁴⁶

Fig. 4 (a) Crystal structure of Ca₁₄Zn₆Ga₁₀O₃₅, (b) schematic diagram of Mn⁴⁺ ions occupying the octahedral lattice sites of Ga³⁺, and (c) Ln³⁺ ions occupying the Ca²⁺ ion sites in the Ca₁₄Zn₆Ga₁₀O₃₅ host. Reprinted with permission from ref. 44, Copyright 2017, The Royal Society of Chemistry.

Based on the effective ionic radii of cations with different coordination numbers (CN),⁴⁷ trivalent rare earth ions are expected to randomly occupy six- and seven-coordinated Ca²⁺ (CN = 6, r = 1.00 Å and CN = 7, r = 1.06 Å) sites, and Mn⁴⁺ (CN = 6, r = 0.53 Å) ions are preferentially accommodated at the Ga³⁺ (CN = 5, r = 0.62 Å) sites with an octahedral coordination in the crystal structure.⁴¹ Electroneutrality in the Mn⁴⁺ and multiple ion-codoped CAZO phosphors can be easily achieved due to some defects such as the formation of Ca²⁺ vacancies and excess O²⁻ ligands for charge compensation.⁴⁸

3.1.2 Dual mode energy transfer between Mn⁴⁺ and Nd³⁺/Er³⁺/Yb³⁺ in CZGO. The energy transfer efficiency depends on the matching of the energy levels between the excitation wavelength of the acceptor and donor emission frequency.^50,51 Fig. 5a–c depict the spectral overlap between the emission spectrum of Mn⁴⁺ and the excitation spectra of Nd³⁺/Er³⁺/Yb³⁺, which demonstrates that the Mn⁴⁺ ion has a strong possibility of being an effective sensitizer for NIR emission of Nd³⁺/Er³⁺/Yb³⁺ through a non-radiative resonant energy transfer process.⁴⁹

Fig. 5 (a–c) Overlap between the PL spectrum of Ca₁₄Zn₆Ga₁₀O₃₅:Mn⁴⁺ and PLE spectra of Ca₁₄Zn₆Ga₁₀O₃₅:Ln³⁺ (Ln = Nd, Er, and Yb), respectively. (d–f) Changes in the PL spectra of Mn⁴⁺ and multiple ion Nd³⁺/Er³⁺/Yb³⁺ codoped Ca₁₄Zn₆Ga₁₀O₃₅ with a change in the concentration of Nd³⁺/Er³⁺/Yb³⁺, respectively. Reprinted with permission from ref. 49, Copyright 2019, Elsevier BV.

Fig. 5e–g show the emission spectra of Mn⁴⁺ and multiple ions Nd³⁺/Er³⁺/Yb³⁺ codoped CZGO with different doping concentrations of Ln³⁺ ions, respectively. Upon excitation at 313 nm, the NIR emissions of Nd³⁺/Er³⁺/Yb³⁺ such as the emission peaks at 900 and 1075 nm are assigned to the ⁴F_3/2 → ⁴I_9/2 and ⁴F_3/2 → ⁴I_11/2 transitions of Nd³⁺, that at 978 and 1537 nm are ascribed to the ⁴I_11/2 → ⁴I_15/2 and ⁴I_13/2 → ⁴I_15/2 transitions of Er³⁺, and that at 980 nm is caused by the ²F_5/2 → ²F_7/2 transition of Yb³⁺. The emission intensity of Mn⁴⁺ monotonously decreases with an increase in the content of Ln³⁺, which indicates energy transfer occurs from Mn⁴⁺ to Nd³⁺/Er³⁺/Yb³⁺.

The energy transfer process from Mn⁴⁺ to multiple ions, Nd³⁺/Er³⁺/Yb³⁺, in CZGO is illustrated in Fig. 6a. Under excitation of NUV to visible light ranging from 250 to 550 nm, the Mn⁴⁺ ions are excited into their charge transfer or excited states of ⁴T_1g and ⁴T_2g, which then rapidly relax to the metastable state of ²E_g of the Mn⁴⁺ ions. The energy transfer occurs via Mn⁴⁺: ²E_g + Nd³⁺: ⁴I_9/2 → Mn⁴⁺: ⁴A_2g + Nd³⁺: ⁴F_9/2, ⁴F_7/2, ⁴S_3/2 or Mn⁴⁺: ²E_g + Yb³⁺: ²F_7/2 → Mn⁴⁺: ⁴A_2g + Yb³⁺: ²F_5/2. The NIR emissions at 896 and 1064, 1540, and 980 nm are generated by the radiative transitions of the Nd³⁺: ⁴F_3/2, Er³⁺: ⁴I_13/2 and Yb³⁺: ²F_5/2 levels, respectively.⁵²

Fig. 6 (a) Excitation/emission and energy transfer mechanism of Mn⁴⁺ to Ln³⁺ in Ca₁₄Zn₆Ga₁₀O₃₅:Mn⁴⁺,Ln³⁺ (Ln = Nd, Er, and Yb) phosphors. (b) Mechanism of the up-conversion of Er³⁺ and energy transfer from Er³⁺ to Mn⁴⁺. (c) Emission spectra of Mn⁴⁺ and Er³⁺ codoped Ca₁₄Zn₆Ga₁₀O₃₅ upon 980 nm excitation and (d) spectral overlap between the emission of Er³⁺ and excitation of Mn⁴⁺. Reprinted with permission from ref. 49, Copyright 2019, Elsevier BV.

Under 980 nm light excitation, green upconverted emission peaks at 551 and 561 nm attributed to the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺ are produced, as shown in Fig. 6b. Therefore, the red emission centered at 712 nm of Mn⁴⁺ and Er³⁺ codoped CZGO phosphor produced by excitation at 980 nm is ascribed to the energy transfer from Er³⁺ to Mn⁴⁺.

The green and red emission centered at 551 (561) and 661 nm can be ascribed to the transitions of ²H_11/2 → ⁴I_15/2 (⁴S_3/2 → ⁴I_15/2) and ⁴F_9/2 → ⁴I_15/2 of Er³⁺, respectively, via the multiple non-radiative multiphonon relaxations from the ⁴F_7/2 to H_11/2, ⁴S_3/2 and ⁴F_9/2 levels.⁵³ The deep red emission ascribed to the transition of ²E_g → ⁴A_2g of Mn⁴⁺ is attributed to the energy transfer from Er³⁺ to Mn⁴⁺, as illustrated in the corresponding mechanism diagram in Fig. 6c. The spectral overlap observed between the emission spectrum of Er³⁺ and the excitation spectrum of Mn⁴⁺ makes the reversal energy transfer from Er³⁺ to Mn⁴⁺ possible, as presented in Fig. 6d.

3.2 Ca₁₄Zn₆Al₁₀O₃₅ as host lattice for Mn⁴⁺ and multiple ion codoping

3.2.1 Formation of tunable color luminescence centers in Ca₁₄Zn₆Al₁₀O₃₅. Fig. 7 shows the unit cell structure and the coordination environment of the cation sites of a typical Ca₁₄Zn₆Al₁₀O₃₅ (CZAO) compound. CZAO has a cubic structure with the space group F23. In the crystal structure of CZAO, Ca²⁺ has three different coordination environments, where two of them are coordinated to six oxygen atoms, forming a distorted octahedron, while the third is in a seven-coordinated polyhedron and the average Ca–O distance is equal to 2.498Å.^54,55 In addition, four of the five independent positions occupied by Zn and Al are in the tetrahedral coordination, with the average Zn–O distance of 1.951 Å and average Al–O distances of 1.719, 1.794 and 1.891 Å, respectively. The positions are in an octahedron coordination, and the one-fifth positions occupied by Al and Zn are octahedral coordinations.^56–59 The Ca²⁺ site is likely to be replaced by a small amount of Nd³⁺/Yb³⁺ ions without significant structural changes due to the similar ion radii between Ca²⁺ and Nd³⁺/Yb³⁺ (Ca²⁺: r = 0.100 nm; Nd³⁺: r = 0.098 nm; and Yb³⁺: r = 0.086 nm).

Fig. 7 Schematic of the crystal structure of Ca₁₄Zn₆Al₁₀O₃₅. Reprinted with permission from ref. 14, Copyright 2016, The Royal Society of Chemistry.

3.2.2 Energy transfer between Mn⁴⁺ and Nd³⁺/Er³⁺/Yb³⁺ in CZAO. Under excitation by UV to visible light from 250 to 550 nm, intense NIR emissions are produced at 900 and 1060 nm originating from the Nd³⁺: ⁴F_3/2/⁴I_9/2 and Nd³⁺: ⁴F_3/2/⁴I_11/2 in Mn⁴⁺ and Nd³⁺-codoped phosphors. The emission at 980 nm in the Mn⁴⁺, Yb³⁺ codoped samples is ascribed to the Yb³⁺: ²F_5/2/²F_7/2 transitions.⁶⁰ The energy transfer based on the strong absorption of Mn⁴⁺ and spin-allowed transitions of Nd³⁺/Yb³⁺ through dipole–dipole interaction is illustrated in Fig. 8. The shapes of the PLE spectra of both the Mn⁴⁺/Nd³⁺ and Mn⁴⁺/Yb³⁺ codoped samples monitored at 1060 nm and 980 nm, respectively, are quite similar to that of the Mn⁴⁺ single-doped sample (Fig. 8a–c). Only weak and discrete PLE peaks in the visible region caused by the f–f transitions of Nd³⁺ appear in the Nd³⁺ single-doped sample and no PLE peak in the visible region is observed in the Yb³⁺ single-doped sample (Fig. 8a and b), respectively. Thus, the characteristics of the above PLE spectra demonstrate that the NIR luminescence of Nd³⁺/Yb³⁺ in Mn⁴⁺ and multiple ion Nd³⁺/Yb³⁺ codoped CZAO is generated by the energy transfer from Mn⁴⁺ to Nd³⁺/Yb³⁺ ions.^61–63

Fig. 8 (Left) PLE spectra and/or (right) PL spectra for (a) Ca_13.75Zn₆Al_9.8O₃₅:Mn_0.2,Nd_0.25 and Ca_13.75Zn₆Al₁₀O₃₅:Nd_0.25, (b) Ca_13.2Zn₆Al_9.8O₃₅:Mn_0.2, Yb_0.8 and Ca_13.2Zn₆Al₁₀O₃₅:Yb_0.8, and (c) Ca₁₄Zn₆Al_9.8O₃₅:Mn_0.2. Spectral overlap between the emission spectrum of Mn⁴⁺ and the excitation spectrum of (d) Nd³⁺ monitored at 1060 nm and (e) Yb³⁺ monitored at 980 nm. Reprinted with permission from ref. 14, Copyright 2016, The Royal Society of Chemistry.

The energy transfer efficiency depends on the spectral matching of the excitation of the acceptor and emission spectra of the donor. As shown in Fig. 8d, good spectral overlap can be observed between the ²E_g emission of Mn⁴⁺ and the ⁴F_9/2, ⁴F_7/2, and ⁴S_3/2 excitations of Nd³⁺. It can be seen from Fig. 8e that although there is a relatively large energy gap between the excited state ²E_g of Mn⁴⁺ and ²F_5/2 of Yb³⁺, an efficient energy transfer from Mn⁴⁺ to Yb³⁺ can still occur in the Mn⁴⁺ and Yb³⁺ codoped samples with strong electron-phonon coupling.⁶⁴ Therefore, the NIR luminescence of Yb³⁺ may be mainly generated by phonon-assisted energy transfer from Mn⁴⁺ to Yb³⁺. The excitation/emission and energy transfer pathways for the Mn⁴⁺ and codoped Nd³⁺/Yb³⁺ ion couples in CZAO are quite similar to that in the host lattice of CZGO.^14,49

NIR emissions from Nd³⁺/Yb³⁺ have been observed in Mn⁴⁺ and Nd³⁺/Yb³⁺ codoped CZAO phosphors. The intensity of the NIR emissions of Nd³⁺/Yb³⁺ increases initially with an increase in the content of rare earth ions Nd³⁺/Yb³⁺, and then decreases gradually as a result of concentration quenching.⁶² The NIR luminescence intensity is enhanced by 338 times at 1060 nm for Ca_13.75Zn₆Al_9.4O₃₅:Mn_0.6,Nd_0.25 and 306 times at 980 nm for Ca_13.2Zn₆Al_9.4O₃₅:Mn_0.6,Yb_0.8, respectively, which is attributed to the efficient energy transfer from Mn⁴⁺ to the Nd³⁺/Yb³⁺ ions, respectively.^65,66 Fig. 9a and b illustrate the excitation spectra of Mn⁴⁺ and emission spectra of Er³⁺ in Mn⁴⁺ and/or Er³⁺ codoped samples with various doping concentrations. The two broad and intense excitation bands (monitored at Mn⁴⁺ 710 nm emission) correspond to the spin-allowed transitions ⁴A_2g → ⁴T_1g and ⁴A_2g → ⁴T_2g of Mn⁴⁺ (Fig. 9a). The weak and discrete excitation peaks (monitored at Er³⁺ 1540 nm emission) are ascribed to the transitions from ⁴I_15/2 to ⁴G_11/2, ⁴F_5/2, ⁴F_7/2, ²H_11/2, and ⁴S_3/2 of Er³⁺. From the excitation spectrum of the Mn⁴⁺ and Er³⁺ codoped sample monitored at the 1540 nm emission of Er³⁺ in Fig. 9b, it can be seen that not only broad and intense excitation bands ascribed to Mn⁴⁺ ions but also the superimposed excitation peaks assigned to the ⁴I_15/2 to ²H_11/2 and ⁴S_3/2 transitions of Er³⁺ appear, indicating the energy transfer from Mn⁴⁺ to Er³⁺.

Fig. 9 Excitation and emission spectra of Ca_14−xZn₆Al_10−yO₃₅:Mn_y,Er_x (y = 0.0, 0.6; x = 0.0, 1.0) in (a) NIR and (b) IR regions. Reprinted with permission from ref. 4, Copyright 2016, The Royal Society of Chemistry.

3.3 Complex hexoxides as host lattices for Mn⁴⁺ and multiple ion codoping

As shown in Fig. 10a,⁶⁵ Gd₂ZnTiO₆ (GZT) crystallizes in a double-perovskite monoclinic structure with the space group P21/n, with the cell parameters of a = 5.3664(9) Å, b = 5.6631(9) Å, c = 7.6847(9) Å and β = 90.294(2)°. In the crystal structure of GZT, the Zn²⁺ and Ti⁴⁺ ion centers are at two slantwise octahedral sites surrounded by six oxygen atoms, and the Gd³⁺ ion occupies the decahedron site coordinated with twelve oxygen atoms. La₂LiTaO₆ is built up of alternating strands of LiO₆ and slightly disordered TaO₆ with La³⁺ located in the cavities of the interconnected network of octahedral sites, as shown in Fig. 10b.^67–70

Fig. 10 Crystal structure of (a) Gd₂ZnTiO₆. Reprinted with permission from ref. 65, Copyright 2014, The Chemical Society of Japan. (b) La₂LiTaO₆, Reprinted with permission from ref. 34, Copyright 2014, Springer Nature. (c) NaMgLaTeO₆ Reprinted with permission from ref. 16, Copyright 2018, The Royal Society of Chemistry.

According to the doping rule that with a similar radius and the same valence of the dopants and host cationic ions, Mn⁴⁺ ions perfectly enter the centers of the octahedral environment coordinated with six oxygen atoms and the trivalent rare earth ions can occupy the Gd³⁺ and/or La³⁺ sites in the host lattices of complex hexoxides, respectively. NaMgLaTeO₆ crystallizes in a monoclinic system with the P12₁/m1(11) space group, as depicted in Fig. 10c.^16,71,72 Both Mg²⁺ and Te⁶⁺ are located at the six-fold sites to form MgO₆ and TeO₆ octahedra with a shared oxygen atom, respectively. Moreover, the La/Gd and Na/K atoms are coordinated with twelve oxygen atoms to form polyhedral La/GdO₁₂ and Na/KO₁₂. These four types of polyhedra connect closely to construct the space framework of this crystal structure.⁷³ The Mg²⁺ and Te⁶⁺ sites at the centers of the octahedra are expected to be substituted by Mn⁴⁺ ions and red luminescence centers of Mn⁴⁺ are formed. In the Mn⁴⁺ and Er³⁺ coped GZT sample, efficient energy transfer from Mn⁴⁺ to Er³⁺ was observed, and the mechanism is quite similar to that in Mn⁴⁺ and Er³⁺ codoped CZAO.¹⁴

It can be seen from Fig. 11a that the emission spectrum of Gd₂ZnTiO₆:yMn⁴⁺,0.02Er³⁺ (y = 0, 0.002) is excited at 335 nm, corresponding to the ⁴A_2g → ⁴T_1g of Mn⁴⁺, and in that of Gd₂ZnTiO₆:0.002Mn⁴⁺,2xEr³⁺ (x = 0, 0.005) are excited at 379 nm, corresponding to ⁴A_2g → ⁴T_1g of Mn⁴⁺ and ⁴I_15/2 → ⁴G_11/2 of Er³⁺.⁷⁵ Only the characteristic emission peaks (²E_g) of Mn⁴⁺ can be observed and no characteristic visible emission peaks (²H_11/2/⁴S_3/2) of Er³⁺ for the GZT:0.002Mn⁴⁺,0.02Er³⁺ sample in the emission excited at 335 nm. Spectral overlap exists between the emission for Er³⁺ (²H_11/2/⁴S_3/2) and the absorption for Mn⁴⁺ (⁴A_2g), which provides a possible energy transfer pathway from Mn⁴⁺ to Er³⁺.⁷⁶ The emission intensity of ²E_g of Mn⁴⁺ upon the codoping of Er³⁺ in GZT is much stronger than that of Mn⁴⁺ single-doped GZT under the common excitation wavelength of 379 nm, which indicates that energy back transfer occurs from Er³⁺ (²H_11/2/⁴S_3/2) to Mn⁴⁺ (⁴A₂) under the common excitation wavelength of 379 nm (see Fig. 11b).

Fig. 11 (a) Emission spectra of Gd₂ZnTiO₆:yMn⁴⁺,0.02Er³⁺ (y = 0, 0.002) excited at 335 nm, and (b) Gd₂ZnTiO₆:0.002Mn⁴⁺,2xEr³⁺ (x = 0, 0.005) excited at 379 nm. Reprinted with permission from ref. 74, Copyright 2017, The Royal Society of Chemistry.

The IR emission at 1529 nm is ascribed to the ⁴F_9/2 (⁴I_9/2) → ⁴I_13/2 transition of Er³⁺ through energy transfer from Mn⁴⁺ in the Mn⁴⁺ and Er³⁺ codoped GZT phosphor and the corresponding mechanism is illustrated in Fig. 12a. The Mn⁴⁺ ions are excited into their excite states under irradiation by short-wavelength light in the region of 250–550 nm, and then the energy transfer of ²E (Mn⁴⁺) → ⁴F_9/2, ⁴I_9/2 (Er³⁺) happens between the Mn⁴⁺ and Er³⁺ ions to populate the ⁴F_9/2 and ⁴I_9/2 levels of Er³⁺ followed by nonradiative relaxation to ⁴I_13/2. Finally, IR emission at 1529 nm is produced by radiative transition from ⁴I_13/2 to ⁴I_15/2 of Er³⁺.

Fig. 12 (a) Electron transitions and mutual sensitized energy transfer scheme between Mn⁴⁺ and Er³⁺ in the Gd₂ZnTiO₆ matrix. Reprinted with permission from ref. 74, Copyright 2017, The Royal Society of Chemistry. (b) PLE, (c) visible, and (d) NIR spectra of Gd₂ZnTiO₆:0.3%Mn⁴⁺,yYb³⁺. Reprinted with permission from ref. 33, Copyright 2018, Elsevier BV.

Far-red (FR) and near-infrared (NIR) double-wavelength emissions have been observed in the Mn⁴⁺ and Yb³⁺ codoped GZT phosphor, which are expected to application in LEDs towards plant cultivation.^77–79 The PLE and PL spectra of the Mn⁴⁺ and Yb³⁺ codoped samples are shown in Fig. 12b–d. The shapes and positions of both PLE spectra (Fig. 12b) monitored at emission 704 nm from the ²E_g → ⁴A_2g transition of Mn⁴⁺ and that at 980 nm from the Yb³⁺ transition ²F_5/2 → ²F_7/2 are similar to that of Mn⁴⁺ singly doped GZT, which indicates that energy transfer between Mn⁴⁺ and Yb³⁺ occurs in the codoping systems. Under the excitation of 365 nm light, both FR emission from Mn⁴⁺ and NIR emission from Yb³⁺ are observed in Fig. 12c and d. The FR emission intensity of Mn⁴⁺ gradually decreases with an increase in the content of Yb³⁺, whereas the NIR emission intensity first increases and then decreases due to the concentration quenching effect, which further prove the occurrence of energy transfer from Mn⁴⁺ to Yb³⁺.

The similar energy transfer from Mn⁴⁺ to Yb³⁺ has been also observed in Mn⁴⁺ and Yb³⁺ codoped La₂MgTiO₆ samples. Broad excitation bands from 250 nm to 550 nm corresponding to the absorptions involving the ⁴A_2g → ⁴T_1g, and ⁴A_2g → ⁴T_2g transitions of Mn⁴⁺ monitored at 710 nm and Yb³⁺ ions monitored at 980 nm were observed in the PLE of La_1.91MgTi_0.998O₆:Mn_0.002,Yb_0.09 sample, as shown in Fig. 13a.^36,80,81 The excitation spectrum monitored at 980 nm of Yb³⁺ emission is similar to that monitored at 710 nm of the Mn⁴⁺ emission in the La_1.91MgTi_0.998O₆:Mn_0.002,Yb_0.09 sample, which clearly proves that energy transfer from Mn⁴⁺ to Yb³⁺ takes place in the Mn⁴⁺ and Yb³⁺ codoped La₂MgTiO₆ samples when the Mn⁴⁺ ions are excited.

Fig. 13 (a) Excitation spectra of Mn⁴⁺ monitored at 710 nm and Yb³⁺ monitored at 980 nm in La_1.91MgTi_0.998O₆:Mn_0.002,Yb_0.09 sample, emission spectra of (b) La_2−xMgTi_1−yO₆:Mn_y,Yb_x, (c) La_2−xMgTi_1−yO₆:Mn_y,Yb_x samples pumped by 460 nm light. Reprinted with permission from ref. 35, Copyright 2018, Elsevier BV.

Fig. 13b and c exhibit the emission spectra of the La_2−xMgTi_1−yO₆:Mn_y,Yb_x and La_2−xMgTi_1−yO₆:Mn_y,Yb_x samples pumped by 460 nm light. The NIR emission band with the highest peak at 990 nm is from the ²F_5/2 → ²F_7/2 transition of Yb³⁺ ions and its emission is strongly dependent the concentrations of Yb³⁺ ions.^35,82,83 The integrated intensity of the NIR emission band centered at 990 nm increases initially with an increase in the concentration of Yb³⁺ ions.

Energy transfer from Mn⁴⁺ to Yb³⁺ occurs in the Mn⁴⁺ and Yb³⁺ codoped Ba₂LaNbO₆ (BLNO) samples, as illustrated in Fig. 14.³⁵ The spectral shapes and positions of the excitation spectra monitored at 677 nm (Mn⁴⁺ emission) and 998 nm (Yb³⁺ emission) remain the same, but their intensities are different, which indicates that energy transfer from Mn⁴⁺ to Yb³⁺ occurs in the Mn⁴⁺ and Yb³⁺ codoped BLNO, as show in Fig. 14a and b. The emission centered at 998 nm is consistent with the infrared light needed for bacterial chlorophyll.^22,84 The intensity of the Mn⁴⁺ emission at 677 nm decreases, while that of the Yb³⁺ emission at 998 nm increases due to the transfer of energy from Mn⁴⁺ to Yb³⁺.

Fig. 14 (a) PLE and (b) PL spectra of Ba₂LaNbO₆:Mn⁴⁺,yYb³⁺ phosphors. (c) Decay curves of Ba₂LaNbO₆:Mn⁴⁺,yYb³⁺. (d) Energy transfer schematic diagram of Mn⁴⁺ and Yb³⁺ codoped system. Reprinted with permission from ref. 35, Copyright 2019, Elsevier BV.

Fig. 14c shows the decay lifetimes of BLNO:0.003Mn⁴⁺,yYb³⁺, which decrease with an increase in the Yb³⁺ concentration, thus proving the occurrence of energy transfer from Mn⁴⁺ to Yb³⁺ in the phosphor. According to the mechanism of energy transfer of Mn⁴⁺ and Yb³⁺ based on Fig. 14d,^35,85,86 the Mn⁴⁺ ions are excited from the ground state (⁴A_2g) to excited states (⁴T_1g, 2T_2g, and ⁴T_2g) under UV light excitation, and then relax to the ²E_g state. The energy can be transferred from the ²E_g state of Mn⁴⁺ to the ²F_5/2 level of Yb³⁺ through nonradiative transition, thereby producing the NIR emission observed at 998 nm.

The energy transfer from Mn⁴⁺ to Nd³⁺ occurs in the Mn⁴⁺ and Nd³⁺ codoped (Na,K)Mg(La,Gd)TeO₆ samples, as illustrated in Fig. 15.¹⁶ Upon excitation at 365 nm UV, both emissions from Mn⁴⁺ and Nd³⁺ are observed, and the Mn⁴⁺ emission intensity and the corresponding decay time of Mn⁴⁺ at 705 nm decrease monotonously with an increase in Nd³⁺ concentration, which strongly confirms the efficient energy transfer from the Mn⁴⁺ to Nd³⁺ ions in these samples.^87,88

Fig. 15 (a) PL emission spectra (λ_ex = 365 nm) of NaMgLaTeO₆:0.02Mn⁴⁺,xNd³⁺ and (b) corresponding decay curves for NaMgLaTeO₆:0.02Mn⁴⁺,xNd³⁺ (λ_ex = 365 nm, λ_em = 705 nm). Reprinted with permission from ref. 16, Copyright 2018, The Royal Society of Chemistry.

The energy transfer processes of Mn⁴⁺ → Nd³⁺ → Yb³⁺ occurring in the Mn⁴⁺, Nd³⁺ and Yb³⁺ codoped NaMgLaTeO₆ (NMLTO) samples are illustrated in Fig. 16.¹⁶ The emission spectra of NML:0.02Mn⁴⁺,0.30Yb³⁺ excited at 365 nm contains both the Mn⁴⁺ emission band at around 705 nm due to the Mn^{4+ 2}E_g → ⁴A_2g transition, and the Yb³⁺ emission band with a maximum at around 1003 nm attributed to the Yb^{3+ 2}F_5/2 → ²F_7/2 transition. The excitation spectrum (200–900 nm) monitored at 1003 nm clearly contains the Mn⁴⁺ absorption band, suggesting energy transfer from Mn⁴⁺ to Yb³⁺ ions.^89–91 In the Mn⁴⁺, Nd³⁺, and Yb³⁺ codoped NMLTO sample, the emission spectra of the obviously present bands from all three ions Mn⁴⁺, Nd³⁺, and Yb³⁺ in the range of 600–1300 nm upon 365 nm UV excitation.^92,93 The emission intensity of Nd³⁺ decreases monotonously with an increase in Yb³⁺ concentration, which illustrates the possibility of energy transfer from the Nd³⁺ to Yb³⁺ ions as shown in Fig. 16a–c.

Fig. 16 PL excitation and emission spectra of (a) NaMgLaTeO₆:0.02Mn⁴⁺,0.30Yb³⁺, (b) NaMgLaTeO₆:0.02Mn⁴⁺,0.01Nd³⁺,aYb³⁺, and (c) decay curves of NaMgLaTeO₆:0.02Mn⁴⁺,0.01Nd³⁺,aYb³⁺ (λ_ex = 580 nm, λ_em = 911 nm). (d) Partial coordination environment in the NaMgLaTeO₆ structure and schematic energy-level diagram illustrating the possible energy transfer processes in the NaMgLaTeO₆:Mn⁴⁺,Nd³⁺,Yb³⁺ materials. Reprinted with permission from ref. 16, Copyright 2018, The Royal Society of Chemistry.

Fig. 16d shows an overview of the partial electronic energy level diagram of Mn⁴⁺, Nd³⁺, and Yb³⁺ in NMLTO and a schematic diagram illustrating the possible energy transfer processes occurring in Mn⁴⁺, Nd³⁺, and Yb³⁺ codoped NMLTO.¹⁶ The energy at the Mn⁴⁺ excited state ²E_g can be transferred to the Nd³⁺ levels ⁴F_7/2 and ⁴S_3/2 via the Forster resonant energy transfer process to produce the emissions at 910 and 1072 nm.^90,94

The NIR emissions of Nd³⁺ at 910 and 1072 nm from the ⁴F_7/2 and ⁴S_3/2 levels, respectively, increases and the red emission of Mn⁴⁺ at 705 nm from the ²E_g excited state decreases with an increase in the concentration of Mn⁴⁺, which indicates the energy transfer from Mn⁴⁺ to Nd³⁺.^68,95 Then the excited ⁴F_7/2 and ⁴S_3/2 energy levels of Nd³⁺ can relax nonradiatively to the ⁴F_5/2 and ²H_9/2 Nd³⁺ energy levels, and transfer the energy to the ²F_5/2 Yb³⁺ excited state and enhance the Yb³⁺ emission.

As can be seen in Fig. 17, the excitation spectra of the Mn⁴⁺, Nd³⁺ and Yb³⁺ codoped NMLTO samples match well with the solar spectrum in the UV and visible regions, and the emission bands are located at the ideal 930–1100 nm region for excellent response for crystal silicon solar energy cells.^68,96 Thus, the Mn⁴⁺, Nd³⁺ and Yb³⁺ codoped NMLTO sample has potential for the effective broadband spectral conversion of UV/visible light to the NIR band utilizing the energy transfer processes of Mn⁴⁺ → Nd³⁺ → Yb³⁺.

Fig. 17 Solar spectrum and PL excitation and emission spectra of NaMgLaTeO₆:0.02Mn⁴⁺,0.01Nd³⁺,aYb³⁺, NaMgGdTeO₆:0.01Mn⁴⁺,0.02Nd³⁺,bYb³⁺ and KMgLaTeO₆:0.006Mn⁴⁺,0.03Nd³⁺,cYb³⁺. Reprinted with permission from ref. 16, Copyright 2018, The Royal Society of Chemistry.

4. Tunable multiple emissions via energy transfer in a single host lattice

4.1 Energy transfer between Dy³⁺ and Mn⁴⁺

As displayed in Fig. 18a, the PLE spectrum of the Ca_13.88Al₁₀Zn₆O₃₅:0.12Dy³⁺ phosphor monitored at 576 nm consists of a series of sharp peaks with the strongest absorption at 351 nm due to the ⁶H_15/2 → ⁶P_7/2 transition of Dy³⁺. Under excitation at 351 nm, the PL spectrum consists of two dominant peaks at around 482 nm (blue) and 576 nm (yellow), corresponding to the ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions of Dy³⁺, respectively.^97–99 As shown in Fig. 18b, significant spectral overlap was observed between the PLE of Mn⁴⁺ and PL of Dy³⁺, indicating that effective energy transfer from Dy³⁺ to Mn⁴⁺ is expected.

Fig. 18 (a) PLE and PL spectra of Ca_13.88Al₁₀Zn₆O₃₅:0.12Dy³⁺, (b) spectral overlap between the PLE of Mn⁴⁺ and the PL of Dy³⁺, (c) schematic level diagram for the cross-relaxation process and energy transfer process from Dy³⁺ to Mn⁴⁺. Reprinted with permission from ref. 97, Copyright 2016, Kluwer Academic Publishers.

The energy transfer process from Dy³⁺ to Mn⁴⁺ is elucidated according to the schematic energy level diagram in Fig. Fig. 18c. In the cross-relaxation processes, the Dy³⁺ ions at the ⁴F_9/2 level can be de-excited to the ⁶F_9/2/⁶H_7/2, ⁶H_9/2/⁶F_11/2, or ⁶F_1/2 level, while the ions at the ⁶H_15/2 ground state will accept the energies excited simultaneously to the ⁶F_3/2, ⁶F_5/2, and ⁶H_9/2/⁶F_11/2 levels. Although the energy level ⁴F_9/2 of Dy³⁺ (20 747 cm⁻¹) is higher than the ²E_g energy level of Mn⁴⁺ (14 025 cm⁻¹), the energy transfer from the ⁴F_9/2 level of Dy³⁺ to the ²E level of Mn⁴⁺ may be realized via the assistance of phonons.^97,100–104

The PL spectra of Ca_13.88Al_10−yZn₆O₃₅:0.12Dy³⁺,yMn⁴⁺ (y = 0, 0.01, 0.05, 0.10, 0.15, 0.20, and 0.25) upon excitation at 351 nm and the change in the emission intensities of Dy³⁺ and Mn⁴⁺ with the concentration of Mn⁴⁺ are presented in Fig. 19.⁹⁷ The emissions at 482 and 576 nm are due to the ⁴F_9/2 → ⁶H_J/2 (J = 15, 13) transitions of Dy³⁺, and the red emission with a multi-peak structure in the wavelength range of 650 to 750 nm corresponds to the vibronic emission ²E_g → ⁴A_2g of Mn⁴⁺. The emission intensity of Mn⁴⁺ increases, whereas that of Dy³⁺ is simultaneously found to decrease monotonically with an increase in concentration of Mn⁴⁺, indicating that the energy transfer from Dy³⁺ to Mn⁴⁺ is efficient.^105–108

Fig. 19 (a) PL spectra of Ca_13.88Al_10-yZn₆O₃₅:0.12Dy³⁺,yMn⁴⁺ (y = 0, 0.01, 0.05, 0.10, 0.15, 0.20, and 0.25) under the excitation at 351 nm and (b) emission intensities of Dy³⁺ and Mn⁴⁺ as a function of the concentration of Mn⁴⁺. Reprinted with permission from ref. 97, Copyright 2016, Kluwer Academic Publishers.

4.2 Tunable dual emissions for Bi³⁺ and Mn⁴⁺ codoped phosphors

It was found that both the blue light from Bi³⁺ and red light from Mn⁴⁺ are produced in all the Bi³⁺ and Mn⁴⁺ codoped CZAO samples, as illustrated in Fig. 20a. The emission band from 400 nm to 550 nm with a maximum at 410 nm is ascribed to the ³P₁ → ¹S₀ transition of the Bi³⁺ ions, while that from 650 nm to 750 nm is ascribed to the ²E_g → ⁴A_2g emission of the Mn⁴⁺ ions.^25,109 The intensity of the blue emission decreases and that of the red emission increases with an increase in the Mn⁴⁺ concentration, as shown in Fig. 20b, which indicates the occurrence of energy transfer from Bi³⁺ to Mn⁴⁺. The dual-emission color can be tuned by changing the Bi³⁺/Mn⁴⁺ ratio.

Fig. 20 (a) Emission spectra (λ_ex = 351 nm) of samples of Ca₁₄Zn₆Al₁₀O₃₅:0.5% Bi³⁺,x% Mn⁴⁺ (x = 0.05, 0.1, 0.2, 0.3, 0.35, 0.4 or 0.5) and (b) dependence of the luminescence intensities of the red emission from Mn⁴⁺ and blue emission from Bi³⁺ on the Mn⁴⁺ doping concentrations. Reprinted with permission from ref. 25, Copyright 2017, The Royal Society of Chemistry. (c) PLE spectrum of Ca₃ZnAl₄O₁₀:0.008Mn⁴⁺ and PL spectrum of the Ca₃ZnAl₄O₁₀:0.008Bi³⁺ phosphor. (d) Absorption spectra of chlorophyll A, chlorophyll B, and phytochromes PR and PPR, and (e) PL spectra of Bi³⁺ and Mn⁴⁺ in Ca₃ZnAl₄O₁₀. Reprinted with permission from ref. 26, Copyright 2018, The Royal Society of Chemistry.

A similar energy transfer from Bi³⁺ to Mn⁴⁺ was also observed in the Bi³⁺ and Mn⁴⁺ codoped CZAO phosphor due to the spectral overlap in the PLE of Mn⁴⁺ and PL of CZAO:0.008Bi³⁺, as shown in Fig. 20c. Under the same excitation source, Bi³⁺ and Mn⁴⁺ codoped CZAO phosphors show dual emissions, where the blue-violet emission is mainly from the ³P₁ → ¹S₀ transition of Bi³⁺ and the far red emission is attributed to the ²E_g → ⁴A_2g transition of Mn⁴⁺.^26,110,111 As presented in Fig. 20d and e, the blue emission of Bi³⁺ matches the absorption spectra of chlorophyll A and chlorophyll B, while the red emission from Mn⁴⁺ matches the absorption spectra of phytochrome PR and phytochrome PFR, which indicate that the phosphor has potential for application in plant growth LED lighting.

The energy transfer process from Bi³⁺ to Mn⁴⁺ realized in Bi³⁺ and Mn⁴⁺ codoped La₂MgTiO₆ (LMTO) phosphors is illustrated in Fig. 21. The absorption bands from 275 to 375 nm in the PLE spectra for LMT:0.005Bi³⁺ in Fig. 21a are ascribed to the ¹S₀ → ¹P₁ and ¹S₀ → ³P₁ transitions of Bi³⁺. A blue emission (375–500 nm) with a maximum at 417 nm of Bi³⁺ is detected, which is due to the ³P₁ → ¹S₀ transitions. The strong red emission band from 650 to 750 nm with an emission peak at 710 nm is observed owing to the ²E_g → ⁴A_2g transition of Mn⁴⁺. The spectral overlap between the emission spectrum of Bi³⁺ and the excitation spectra of Mn⁴⁺ provides strong evidence for the energy transfer between Bi³⁺ and Mn⁴⁺. The emission intensity of Bi³⁺ gradually decreases and that of Mn⁴⁺ presents a monotonous increase with an increase in the Mn⁴⁺ doping concentration, which indicates that energy transfer occurs in the Bi³⁺ and Mn⁴⁺ codoped LMTO phosphors, as shown in Fig. 21b.

Fig. 21 (a) PLE and PL spectra of La₂MgTiO₆:0.005Bi³⁺ (blue) and La₂MgTiO₆:0.002Mn⁴⁺ (cyan). (b) PL spectra of La₂MgTi_(1-z)O₆:0.005Bi³⁺,zMn⁴⁺ (0 ≤ z ≤ 0.01). The inset shows the integrated intensity of the Bi³⁺ and Mn⁴⁺ emission as a function of the concentration of Mn⁴⁺. Reprinted with permission from ref. 112, Copyright 2018, The Royal Society of Chemistry. (c) Schematic illustration of the electronic transitions and energy transfer process in Ca₃ZnAl₄O₁₀:Bi³⁺,Mn⁴⁺. Reprinted with permission from ref. 26, Copyright 2018, The Royal Society of Chemistry.

The electronic transitions and the energy transfer process in the Bi³⁺ and Mn⁴⁺ codoped phosphors are illustrated the schematic energy level diagram shown in Fig. 21c.²⁶ The Bi³⁺ ions are initially excited from the ground state ¹S₀ to the excited state ³P₁, ³P₂, and ¹P₁ or even the conduction bands under the irradiation of UV light. Then, the Bi³⁺ ions relax to the lowest excited state of ³P₁ and return to the ¹S₀ ground state through radiative transition and yield blue emission. Simultaneously, the Bi³⁺ ions in the ³P₁ state can also transfer their energy to the adjacent Mn⁴⁺ ions and promote the Mn⁴⁺ ions from the ⁴A_2g ground state to the ⁴T_2g, ²T_2g, and ⁴T_1g energy levels and relax to the ²E_g level through a nonradiative transition and then produce red emission when they return to the ⁴A_2g ground state.^113–115 The energy transfer occurring between Bi³⁺ and Mn⁴⁺ eventually lead to an enhancement in the far-red emission of Mn⁴⁺.

5. Red emitting phosphors for plant growth LED lights

5.1 Enhanced red emission of Mn⁴⁺ by codoping rare earth ions

The Dy³⁺ and Mn⁴⁺ codoped Ca₁₄Ga_10−mAl_mZn₆O₃₅ (CGAZO:Dy³⁺,Mn⁴⁺) phosphor can exhibit strong far-red emission, which has potential application for plant growth LED lighting.⁴⁴ As shown in Fig. 22a, the three absorption bands A (200–290 nm), B (290–420 nm), and C (420–550 nm) of the phosphors in the UV-vis absorption spectra can be attributed to the host lattice absorption, charge transfer transition of Mn⁴⁺-O^2-, and spin-allowed transitions ⁴A_2g → ⁴T_1g and ⁴A_2g → ⁴T_2g of the Mn⁴⁺ ions, respectively.¹¹⁶ The absorption intensity of bands A and C decrease, but that of band B is enhanced with elevated Al³⁺ concentrations, which indicates that the absorption intensity of the phosphor powder is enhanced in the ultraviolet light range, but reduced slightly in the blue light range.

Fig. 22 UV-vis absorption spectra of (a) Ca₁₄Ga_10−mAl_mZn₆O₃₅ with different dopants. (b) PL spectra of Ca₁₄Ga_10−mAl_mZn₆O₃₅:0.12Dy³⁺,xMn⁴⁺ phosphor. (c) Energy level, electron transitions and energy transfer schematic diagram of Dy³⁺, Mn⁴⁺ in Ca₁₄Ga_10−mAl_mZn₆O₃₅ matrix. Reprinted with permission from ref. 44, Copyright 2017, The Royal Society of Chemistry.

As shown in Fig. 22b, the PL intensity of the Mn⁴⁺ activator increased, whereas that of the Dy³⁺ sensitizer simultaneously decreased monotonically with an increase in the concentration of Mn⁴⁺ ions, which demonstrates that energy transfer from Dy³⁺ to Mn⁴⁺ occurred in the Dy³⁺ and Mn⁴⁺-co-activated CGAZO, as described using Fig. 22c. The Dy³⁺ ions are excited to their ⁶P_7/2 or ⁶P_5/2 or ⁴I_13/2 excited states or conduction band under irradiation of near UV light and nonradiatively relax to their ⁴F_9/2 state. The energy transfer process between the Dy³⁺ and Mn⁴⁺ ions occurs via ⁴F_9/2 (Dy³⁺) → ²E_g (Mn⁴⁺) and the Mn⁴⁺ ions return from the lowest excited level ²E_g (Mn⁴⁺) to the ⁴A_2g ground state (Mn⁴⁺) through a radiative transition, which produces the far-red light emission at 715 nm.

5.2 Enhanced red emission of Mn⁴⁺ by codoping Bi³⁺

The red emission of the Mn⁴⁺ ions in the phosphors based on the CaAl₁₂O₁₉,^117,118 Mg₂TiO₄,^37,119–121 and La₂ATiO₆ (ref. 112) (A = Mg, Zn) host lattices can be dramatically enhanced by the incorporation of Bi³⁺ codopant. The spectral profiles of the excitation and emission spectra of Mn⁴⁺ with or without codoping Bi³⁺ ions in these Mn⁴⁺ doped phosphors are quite similar. Therefore, it can be speculated that the synergetic effect of codoping Bi³⁺ plays a key role in the modification of the crystal structure and the luminescence efficiency of Mn⁴⁺. Thus, the strategy for enhancing the luminescence performance of Mn⁴⁺ plays a pivotal role in the development of highly efficient red-emitting phosphors.^122–125

6. Luminescent thermometers based on Mn⁴⁺ and multiple ion-doped materials

By employing the highly temperature-sensitive Mn⁴⁺ luminescence as the temperature detecting signal, while the temperature-insensitive rare earth ion (Eu³⁺, Tb³⁺ or Dy³⁺) emission was used as a reference signal, Mn⁴⁺ and multiple rare earth ion codoped phosphors exhibited an excellent temperature sensing performance with absolute and relative sensitivities as high as 0.114–0.441 K⁻¹ and 2.32–4.81% K⁻¹, respectively, which indicate their potential application in luminescent thermometers.^126–128

In Fig. 23a and b, the bright red luminescence of the Eu³⁺ and Mn⁴⁺ codoped YAG samples originated from both the transitions ⁵D₀ → ⁷F_J of Eu³⁺ and ²E_g → ⁴A_2g of Mn⁴⁺. With an increase in temperature, the luminescence of Mn⁴⁺ weakens quickly, whereas that of Eu³⁺ exhibits a slight decrease. As shown in Fig. 23c, the remarkable change in I_Mn/I_Eu with a variation in temperature measured on the cycling process of heating-cooling can almost be restored to the original states after the heating-cooling cycle.^130–134 As confirmed in Fig. 23d, this temperature-dependent I_Mn/I_Eu is repeatable and reversible after several cycling experiments. Therefore, a highly sensitive temperature determination can be expected if the Mn⁴⁺ emission is employed as the detection signal of temperature, while the Eu³⁺ emission is used as the reference signal.

Fig. 23 Temperature-dependent (a) PL spectra of a Mn⁴⁺/Eu³⁺:YAG sample recorded from 303 K to 393 K. (b) PL intensities for Mn⁴⁺ and Eu³⁺. (c) Emission mapping upon the cycling process of heating and cooling. (d) Temperature-induced switching of FIR between Mn⁴⁺ and Eu³⁺ (alternating between 293 K and 393 K). Reprinted with permission from ref. 122, Copyright 2016, The Royal Society of Chemistry. (e) Configurational coordinate diagrams of the Mn⁴⁺/Eu³⁺/Tb³⁺/Dy³⁺ emitting centers in the Y₃Al₅O₁₂ host, showing the energy-level crossing relaxation (ELCR) quenching mechanism for the Mn⁴⁺ activator and the multi-phonon de-excitation (MPD) quenching mechanism for the Eu³⁺/Tb³⁺/Dy³⁺ centers. Reprinted with permission from ref. 129, Copyright 2016, The Royal Society of Chemistry.

The photon generation and energy transfer between Mn⁴⁺ and rare earth ions in Mn³⁺, Mn⁴⁺, and Nd³⁺ codoped YAG nanocrystals can be illustrated by an energy level diagram, as presented in Fig. 23e. The Mn⁴⁺ ions are excited from the ⁴A_2g ground state to the ⁴T₂ excited state, followed by nonradiative multiphonon relaxation, leading to population of the ²E_g state, and then emit red emission at 670 nm, which is ascribed to the radiative electronic ²E_g → ⁴A_2g transition of Mn⁴⁺. The appearance of an intersection point between the ⁴T₂ parabola and the ⁴A_2g parabola at ΔE (activation energy, in this case ΔE₁ = 2506 cm⁻¹) is due to the strong electron-phonon coupling. The value of ΔE₁ is associated with the distortion of the Mn⁴⁺ energy states, which is strongly dependent on the crystal field. With an increase in the temperature, the population of higher vibrational states gradually increases up to the moment when the provided thermal energy is sufficiently high to overcome the intersection point (ΔE₁), above which electrons from the ²E_g level are transferred through ⁴T_2g to the ⁴A_2g ground state via nonradiative multiphonon relaxation. In contrast, rare earth ions are expected to be less affected by luminescence temperature quenching because their energy diagram usually consist of numerous f energy states due to low electron-phonon coupling. Therefore, Mn⁴⁺ and rare earth ions codoped in a single host lattice can be applied in a luminescent thermometer.^135–138

The structural coordinate diagram in Fig. 23e proposes a possible mechanism for elucidating the high temperature sensitivity of Mn⁴⁺ and rare earth ion (such as Eu³⁺/Tb³⁺ and Dy³⁺) codoped samples. The Mn⁴⁺ luminescence is easily thermally quenched through an energy-level crossing relaxation (ELCR) between the ⁴T_2g excited state and the ⁴A_2g ground state due to the role of strong electron–phonon coupling. The thermal quenching of rare earth ions is completely different to that of Mn⁴⁺ since there is no crossing point between the excited states and the ground state of rare earth ions because their 4f orbitals are shielded from the surroundings by the filled ⁵S₂ and ⁵P₆ orbitals,^139,140 and consequently the multi-phonon deexcitation (MPD) mode is the dominant mechanism responsible for the thermal-quenching of rare earth ions. The thermal-quenching probability of Eu³⁺, Tb³⁺, and Dy³⁺ luminescence is quite low because the required phonon numbers to bridge the energy gaps of Eu³⁺, Tb³⁺ and Dy³⁺ are 16, 21 and 10, respectively.

The representative thermal evolution of the emission spectra of Y₃Al₅O₁₂:Mn³⁺, Mn⁴⁺, Nd³⁺ nanocrystals presented in Fig. 24a indicates that the emission intensity of both the ²E_g → ⁴A_2g emission band of Mn⁴⁺ and the ⁴F_3/2 → ⁴I_9/2 band of Nd³⁺ decreases with an increase in temperature. In contrast, the ⁵T₂ → ⁵E′′ emission of Mn³⁺ exhibits different behavior. The upper lying ⁵T₂ state of Mn³⁺ can be populated via phonon-assisted energy transfer with the phonon absorption. The probability of this process increases with temperature according to the Miyakawa–Dexter theory.^31,141,142 On the other hand, Kuck et al. explained the increase in the Mn³⁺ emission at elevated temperatures in terms of the thermal population from the ³T₁ state.¹⁴³

Fig. 24 (a) Thermal evolution of 20 nm Y₃Al₅O₁₂:0.1% Mn,1% Nd³⁺ nanocrystal emission spectra. (b) Impact of temperature on LIR for different Mn concentrations of Y₃Al₅O₁₂:Mn³⁺, Mn⁴⁺, Nd³⁺ nanocrystals. (c) Thermal evolution of S1 for thermometers with different Mn concentrations. (d) Dependence of sensitivity on manganese concentration at T = 273 K. (e) UTR of thermometers with different Mn concentrations. Reprinted with permission from ref. 31, Copyright 2018, Pergamon Press Ltd.

The thermal evolution of LIR₁ for the series of 20 nm nanocrystals with different manganese concentrations is presented in Fig. 24b. In the low temperature range, LIR₁ increases with temperature, reaching the maximum at T = 400 K. A further increase in temperature causes a reduction in the value of LIR₁. In the low temperature range (below 350 K), the LIR₂ value is thermally independent, which is related to the high thermal stability of the Mn⁴⁺ luminescence at low temperatures. This shows that the emission intensity of the ⁵T₂ state of Mn³⁺ increases at low temperatures, while that of the ²E_g state of Mn⁴⁺ becomes stable.

The thermoluminescence glow curves of the all the Mn⁴⁺ and rare earth ion (La³⁺, Gd³⁺, Dy³⁺, and Ho³⁺) codoped MgAl₂Si₂O₈ host phosphors recorded after β- and α-irradiation are shown in Fig. 25.³⁹ All the phosphors exhibit one main peak at about 261 ± 3 °C for β-irradiation and many satellite peaks in the low temperature range up to 200 °C. Furthermore, the α-irradiated phosphors had one main peak at about 245–252 °C and the same satellite peaks. The addition of La³⁺, Gd³⁺, Dy³⁺, and Ho³⁺ dopants in the MgAl₂Si₂O₈:Mn⁴⁺ phosphor did not cause any new TL peaks, but the peak intensities changed. In addition, the Dy³⁺ and Gd³⁺ co-doped phosphors had relatively high peak intensities compared with the other phosphors. The main peaks were shifted towards the lower temperature region when the phosphors were exposed to α-irradiation.^144–147 The TL curves of the β- and α-irradiated phosphors exhibited substantial changes, which can be associated with the type of radiation. Therefore, the TL peak positions of MgAl₂Si₂O₈:Mn⁴⁺ with codoping La³⁺, Gd³⁺, Dy³⁺, and Ho³⁺ activators did not change for α- and β-irradiation.

Fig. 25 TL glow curves of all the phosphors recorded after 1 h of (a) α-irradiation and (b) 37.5 Gy β-irradiation. Reprinted with permission from ref. 39, Copyright 2018, John Wiley & Sons Inc.

The upconversion (UC) luminescence of Mn⁴⁺ can be realized by energy transfer from Yb³⁺ to Er³⁺: ²H_11/2/⁴S_3/2, ⁴F_9/2, Ho³⁺: ⁵S₂/⁵F₄, ⁵F₅ and Tm³⁺: ¹G₄, and further to Mn⁴⁺: ²T_2g and ²E_g exited states in Mn⁴⁺, Yb³⁺, and Er³⁺/Ho³⁺/Tm³⁺ codoped YAlO₃.³² The different influence of temperature on the emission spectra and decay behaviors of Mn⁴⁺ and rare earth ions exhibits their possible application in optical thermometry. Fig. 26a shows down the converted PL and PLE spectra for Mn⁴⁺ single doped and Yb³⁺/Ln³⁺ (Ln = Er, Ho, Tm) codoped YAlO₃. The PL spectrum of Mn⁴⁺ exhibits two emission bands centered at 694 and 714 nm, which are assigned to the spin-forbidden transition ²E_g → ⁴A_2g of Mn⁴⁺. The PLE spectrum monitored at 714 nm consists of two strong excitation peaks centered at 340 and 484 nm, which are attributed to the spin-allowed ⁴A_2g → ⁴T_1g and ⁴A_2g → ⁴T_2g transitions of Mn⁴⁺, respectively.^148–152

Fig. 26 (a) PL (λ_ex = 484 nm) and PLE (λ_em = 714 nm) spectra of single Mn⁴⁺-doped YAlO₃ and (b) UC emission spectra of Yb³⁺/Ln³⁺/Mn⁴⁺ codoped YAlO₃ (Ln = Er, Ho, Tm) under 980 nm laser excitation. Reprinted with permission from ref. 32 and ¹⁴⁸, Copyright 2017, Elsevier BV.

The obvious spectral overlap between the Ln³⁺ emission band and Mn⁴⁺ excitation band indicates the possible resonant energy transfer from Ln³⁺ to Mn⁴⁺. As evidenced in Fig. 26b, an extra NIR emission band at around 714 nm assigned to the ²E_g → ⁴A_2g transition of Mn⁴⁺ is observed for all these Yb³⁺/Ln³⁺/Mn⁴⁺ tri-doped samples, confirming the existence of Ln³⁺ → Mn⁴⁺ (Ln = Er, Ho, Tm) energy transfer.

Similar behavior was observed in the Mn⁴⁺ and Tb³⁺ codoped Sr₄Al₁₄O₂₅ nanocrystalline phosphor. The intense red emission associated with the ²E → ⁴A₂ electronic transition of Mn⁴⁺ ions was drastically quenched, while the ⁵D₄ → ⁷F₅ emission of Tb³⁺ remained almost thermally independent above 100 °C. The combination of the thermally quenched luminescence from the Mn⁴⁺ ions to the almost temperature-independent emission from Tb³⁺ provided a sensitive luminescent thermometer (SR = 2.8%/°C at 150 °C) with strong emission color variability. Thus, the developed thermochromic luminescent nanomaterials based on codoped Mn⁴⁺ and Tb³⁺ possess the high application potential for thermal sensing and mapping.¹⁵³

7. Challenges and perspectives

Mn⁴⁺ and multiple ion-codoped complex oxide phosphors have high stability, abundant starting materials, simple synthetic technology (solid state sintering), and tunable luminescence spectra covering the full visible light region from blue to red, and extending to the NIR region. The challenges and perspectives of the future work focusing on Mn⁴⁺ and multiple ion-codoped materials are proposed as follows:

(1) Applying the developed Mn⁴⁺ and multiple ion-codoped phosphors for the fabrication of WLED devices, solar energy cells, etc.

(2) Enhancement of the luminescence efficiency of Mn⁴⁺ by optimizing the synthetic parameters including codoping some content of multiple ions.

(3) Discovery of novel host lattice materials with multiple crystals sites to accommodate various dopants and luminescence centers in a single host lattice.

(4) Improvement of the efficiency of energy transfer between Mn⁴⁺ and multiple ion-codoped phosphors to obtain tunable luminescence spectra.

8. Conclusions

This review summarized the recent research progress of Mn⁴⁺ and multiple ion such as Bi³⁺ and rare earth ions Dy³⁺/Nd³⁺/Yb³⁺/Er³⁺/Ho³⁺/Tm³⁺ codoped phosphors in the complex oxide host lattice, including their structural-dependent optical properties, energy transfer mechanism, and potential optical applications. Thus, these Mn⁴⁺-and multiple ion-codoped phosphors are potential candidates for application in the fields of solar energy cells, WLEDs, indoor plant cultivation, and temperature sensors. This review provides extensive insight for developing novel Mn⁴⁺-doped phosphors with desirable functional properties from an application point of view and helps to reveal the underlying energy transfer mechanism between Mn⁴⁺ and multiple ions.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We are grateful to the financial support from National Natural Science Foundation (NSF) of China (51572200) and NSF of Key Projects of NSF of Zhejiang Province (LZ20E020003), and Wenzhou Major Scientific and Technological Innovation Project (ZG2020025).

References

1. T. Trupke, M. A. Green and P. Würfel, J. Appl. Phys., 2002, 92, 1668–1674.
2. M. A. Green, Third generation photovoltaics: advanced solar energy conversion, Berlin, Springer, 2003.
3. X. Y. Huang, S. Y. Han, W. Huang and X. G. Liu, Chem. Soc. Rev., 2013, 42, 173–201.
4. J. Chen, J. Liu, H. Yin, S. Jiang, H. Yao and X. Yu, J. Am. Ceram. Soc., 2016, 99, 141–145.
5. Q. Zhou, L. Dolgov, A. M. Srivastava, L. Zhou, Z. L. Wang, J. X. Shi, M. D. Dramićanin, M. G. Brik and M. M. Wu, J. Mater. Chem. C, 2018, 6, 2652–2671.
6. Q. Wang, Z. Y. Yang, H. Y. Wang, Z. K. Chen, H. L. Yang, J. Yang and Z. L. Wang, Opt. Mater., 2018, 85, 96–99.
7. H. Ming, S. F. Liu, L. L. Liu, J. Q. Peng, J. X. Fu, F. Du and X. Y. Ye, ACS Appl. Mater. Interfaces, 2018, 10, 19783–19795.
8. D. Q. Chen, Y. Zhou and J. S. Zhong, RSC Adv., 2016, 6, 86285–86296.
9. S. Adachi, J. Lumin., 2018, 202, 263–281.
10. H. Sijbom, R. Verstraete, J. Joos, J. Poelman and D. Philippe, Opt. Mater., 2017, 7, 3332–3362.
11. Z. G. Xia and Q. L. Liu, Mater. Sci., 2016, 84, 59–117.
12. P. P. Dang, D. J. Liu, G. G. Li, A. A. Al Kheraif and J. Lin, Adv. Opt. Mater., 2020, 8, 1901993.
13. Y. Lv, Y. H. Jin, T. Sun, J. L. Su, C. L. Wang, G. F. Ju, L. Chen and Y. H. Hu, Dyes Pigm., 2019, 161, 137–146.
14. X. J. Gao, W. B. Xia, T. J. Chen, X. L. Yang, X. L. Jin and S. G. Xiao, RSC Adv., 2016, 6, 7544–7552.
15. X. J. Gao, W. Li, X. L. Yang, X. L. Jin and S. G. Xiao, J. Phys. Chem. C, 2015, 119, 28090–28098.
16. K. Li and R. V. Deun, J. Mater. Chem. C, 2018, 6, 7302–7310.
17. W. J. Chung and Y. H. Nam, ECS J. Solid State Sci. Technol., 2020, 9, 016010.
18. X. L. Shi, Y. L. Huang, Y. Jie, L. Shi, X. B. Qiao and H. J. Seo, J. Rare Earths, 2010, 28, 693–696.
19. J. X. Hu, T. H. Huang, Y. P. Zhang, B. Lu, H. Q. Ye, B. J. Chen, H. P. Xia and C. Y. Ji, Dalton Trans., 2019, 48, 2455–2466.
20. Q. Sun, S. Y. Wang, B. Devakumar, L. L. Sun, J. Liang, X. Y. Huang and Y. C. Wu, J. Alloys Compd., 2019, 785, 1198–1205.
21. L. Jia, B. Devakumar, L. L. Sun, Q. Sun, S. Y. Wang, B. Li, D. Q. Chen and X. Y. Huang, Ceram. Int., 2019, 45, 4564–4569.
22. Z. Z. Lu, Y. B. Meng, L. L. Wen, M. X. Huang, L. Y. Zhou, L. J. Liao and D. T. He, Dyes Pigm., 2019, 160, 395–402.
23. Z. W. Zhou, J. M. Zheng, R. Shi, N. M. Zhang, J. Y. Chen, R. Y. Zhang, H. Suo, E. M. Goldys and C. F. Guo, ACS Appl. Mater. Interfaces, 2017, 9, 6177–6185.
24. X. Chen, M. Z. Chi, L. L. Xing, X. Xie, S. M. Liu, Y. R. Liang, M. T. Zheng, H. Hu, H. W. Dong, Y. L. Liu, S. P. Jiang and Y. Xiao, ACS Sustainable Chem. Eng., 2019, 7, 5845–5855.
25. L. Li, Y. X. Pan, Z. Chen, S. M. Huang and M. M. Wu, RSC Adv., 2017, 7, 14868–14875.
26. Z. Zhou, Y. Zhong, M. Xia, N. Zhou, B. Lei, J. Wang and F. Wu, J. Mater. Chem. C, 2018, 6, 8914–8922.
27. Y. Ding, N. Guo, X. Lv, H. T. Zhou, L. Wang, R. Z. Ouyang, Y. Q. Miao and B. Q. Shao, J. Am. Ceram. Soc., 2019, 102, 7436–7447.
28. D. Huang, P. Dang, Y. Wei, B. Bai, H. Lian, Q. Zeng and J. Lin, Mater. Res. Bull., 2020, 124, 110743.
29. D. Huang, P. Dang, H. Lian, Q. Zeng and J. Lin, Inorg. Chem., 2019, 58, 15507–15519.
30. Y. C. Chen, Q. Wang, Z. F. Mu, J. Q. Feng, D. Y. Zhu and F. G. Wu, Optik, 2019, 179, 1035–1041.
31. K. Trejgis and L. Marciniak, Chem. Phys. Lett., 2018, 20, 9574–9581.
32. D. Q. Chen, W. Xu, S. Yuan, X. Y. Li and J. S. Zhong, J. Mater. Chem. C, 2017, 5, 9619–9628.
33. J. M. Xiang, J. Y. Chen, N. M. Zhang, H. B. Yao and C. F. Guo, Dyes Pigm., 2018, 154, 257–262.
34. L. Wang, L. Yuan, Y. D. Xu, R. L. Zhou, B. Y. Qu, N. Ding, M. Shi, B. Zhang, Y. Q. Chen, Y. Jiang, D. Wang and J. Y. Shi, Appl. Phys. A, 2014, 117, 1777–1783.
35. W. Li, T. J. Chen, W. B. Xia, X. L. Yang and S. G. Xiao, J. Lumin., 2018, 194, 547–550.
36. J. Lu, Y. X. Pan, J. G. Wang, X. A. Chen, S. M. Huang and G. K. Liu, RSC Adv., 2013, 14, 4510–4513.
37. Z. X. Qiu, T. T. Luo, J. L. Zhang, W. L. Zhou, L. P. Yu and S. X. Lian, J. Lumin., 2015, 158, 130–135.
38. J. H. Ou, X. L. Yang and S. G. Xiao, Mater. Res. Bull., 2020, 124, 110764.
39. E. Uzun, E. Öztürk and N. K. Ozpozan, Luminescence, 2018, 33, 1346–1357.
40. S. Adachiz, ECS J. Solid State Sci. Technol., 2020, 9, 026003.
41. M. G. Brik, S. J. Camardello and A. M. Srivastava, ECS J. Solid State Sci. Technol., 2015, 4, R39–R43.
42. Y. X. Pan and G. K. Liu, Enhancement of phosphor efficiency via composition modification, Opt. Lett., 2008, 33, 1–3.
43. A. M. Srivastava and W. W. Beers, J. Electrochem. Soc., 1996, 143, L203–L205.
44. Z. Zhou, M. Xia, Y. Zhong, S. J Gai, S. X. Huang, Y. Tian, X. Y. Lu and N. Zhou, J. Mater. Chem. C, 2017, 5, 8201–8210.
45. I. D. Brown, The Chemical Bond in Inorganic Chemistry: The Bond Valence Model, IUCr Monographs on Crystallography, Oxford University Press, 2002.
46. B. Jiang, F. F. Chi, L. Zhao, X. T. Wei, Y. H. Chen and M. Yin, J. Lumin., 2019, 206, 234–239.
47. R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr., 1976, 32, 751–767.
48. X. D. Zhou, E. L. Wang, X. D. Lao, Y. M. Wang and X. K. Sun, Opt. Mater., 2020, 107, 110044.
49. Y. B. Wu, Y. Lv, K. B. Ruan and Z. Xie, Dalton Trans., 2018, 47, 15574–15582.
50. S. J. Qiu, H. W. Wei, X. M. Wang, S. Zhang, M. M. Wang, Y. Y. Wang, L. Xu and H. Jiao, J. Lumin., 2020, 226, 117426.
51. C. Yang, Z. F. Zhang, G. C. Hu, R. Cao, X. J. Liang and W. D. Xiang, J. Alloys Compd., 2017, 694, 1201–1208.
52. F. Vetrone, J. C. Boyer, J. A. Capobianco, A. Speghini and M. Bettinelli, Appl. Phys. Lett., 2002, 80, 1752–1754.
53. M. R. N. Soares, T. Holz, F. Oliveira, F. M. Costaa and T. Monteiro, RSC Adv., 2015, 5, 20138–20147.
54. T. Hasegawa, S. W. Kim, T. Abe, S. Kumagai, R. Yamanashi, K. Seki, K. Uematsu, K. Toda and M. Sato, Chem. Lett., 2016, 45, 1096–1098.
55. L. Li, Y. X. Pan, Y. Huang, S. M. Huang and M. M. Wu, J. Alloys Compd., 2017, 724, 735–743.
56. R. D. Shannon, Acta Crystallogr., Sect. A: Found. Adv., 1976, 32, 751–767.
57. J. J. Zhou, Y. C. Teng, X. F. Liu, S. Ye, X. Q. Xu, Z. J. Ma and J. R. Qiu, Opt. Express, 2010, 18, 21663–21668.
58. V. Singh, R. P. S. Chakradhar, I. Ledoux-Rak, L. Badie, F. Pelle and S. Ivanova, J. Lumin., 2009, 129, 1375–1380.
59. J. Lv, Y. Huang, Y. Tao and H. J. Seo, J. Alloys Compd., 2010, 500, 134–137.
60. K. Li and R. V. Deun, Dyes Pigm., 2019, 162, 990–997.
61. Z. F. Liao, H. F. Xu, W. R. Zhao, H. X. Yang, J. Y. Zhong, H. Zhang, Z. G. Nie and Z. K. Zhou, Chem. Eng. J., 2020, 395, 125060.
62. H. H. Liu, L. Yuan, X. F. Wu, X. Y. Hou, M. Tang, C. M. Hou, H. W. Chen and S. H. Feng, J. Mater. Chem. C, 2020, 8, 9615–9624.
63. F. Q. Sun, R. R. Xie, L. Guan and C. Y. Zhang, J. Lumin., 2016, 180, 251–257.
64. T. Miyakawa and D. L. Dexter, Phys. Rev. B: Solid State, 1970, 1, 2961–2969.
65. K. Seki, K. Uematsu, K. Toda and M. Sato, Chem. Lett., 2014, 43, 1213–1215.
66. W. Lv, W. Z. Lv, Q. Zhao, M. M. Jiao, B. Q. Shao and H. P. You, Inorg. Chem., 2014, 53, 11985–11990.
67. Q. Sun, S. Y. Wang, B. Devakumar, L. L. Sun, J. Liang and X. Y. Huang, ACS Omega, 2019, 8, 13474–13480.
68. K. Hayashi, H. Noguchi and M. Ishii, Mater. Res. Bull., 1986, 21, 401–406.
69. K. Hayashi, H. Noguchi and S. Fujiwara, Mater. Res. Bull., 1986, 21, 289–293.
70. G. Blasse and B. C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994.
71. L. I. Kazakova, A. B. Dubovsky, G. V. Semenkovich and O. A. Ivanova, Radiat. Meas., 1995, 24, 359–360.
72. M. Nyman, M. A. Rodriguez, L. E. Shea-Rohwer, J. E. Martin and P. P. Provencio, J. Am. Chem. Soc., 2009, 131, 11652–11653.
73. K. Li, H. Z. Lian, R. V. Deun and M. G. Brik, Dyes Pigm., 2019, 162, 214–221.
74. J. S. Liao, Q. Wang, H. R. Wen, H. L. Yuan, S. J. Liu, J. X. Fu and B. Qiu, J. Mater. Chem. C, 2017, 5, 9098–9105.
75. H. Chen, H. Lin, Q. M. Huang, F. Huang, J. Xu, B. Wang, Z. B. Lin, J. C. Zhou and Y. S. Wang, J. Mater. Chem. C, 2016, 4, 2374–2381.
76. R. P. Cao, X. O. Yang, Y. M. Jiao, X. T. Wang, Q. L. Hu, T. Chen, C. X. Liao and Y. J. Li, J. Lumin., 2019, 209, 1–7.
77. S. W. Le, J. M. Seo, M. K. Lee, J. H. Chun, P. Antonisamy, M. V. Arasu, T. Suzuki, N. A. Al-Dhabi and S. J. Kim, Ind. Crops Prod., 2014, 54, 320–326.
78. G. G. Li, D. L. Geng, M. M. Shang, C. Peng, Z. Y. Cheng and J. Lin, J. Mater. Chem., 2011, 21, 13334–13344.
79. X. Yin, J. Y. Yao, Y. M. Wang, C. C. Zhao and F. Q. Huang, J. Lumin., 2012, 132, 1701–1704.
80. M. L. Hu, C. X. Liao, L. B. Xia, W. X. You and Z. F. Li, J. Lumin., 2019, 211, 114–120.
81. L. Qin, D. L. Wei, S. L. Bi, C. L. Chen, J. Wang, H. H. Yin and H. J. Seo, Opt. Mater., 2019, 98, 109496–109503.
82. R. Watanabe, Y. Iso and T. Isobe, Ceram. Int., 2019, 2, 4009–4017.
83. U. Rambabu and S. D. Han, Ceram. Int., 2013, 39, 1603–1612.
84. X. Y. Huang and H. Guo, Dyes Pigm., 2018, 152, 36–42.
85. L. Shi, J. X. Li, Y. J. Han, W. L. Li and Z. W. Zhang, J. Lumin., 2019, 208, 201–207.
86. A. J. Fu, A. X. Guan, D. Y. Yu, S. Y. Xia, F. F. Gao, X. S. Zhang, L. Y. Zhou, Y. H. Li and R. G. Li, Mater. Res. Bull., 2017, 88, 258–265.
87. J. Q. Hu, E. H. Song, S. Ye, B. Zhou and Q. Y. Zhang, J. Mater. Chem. C, 2017, 5, 3343–3351.
88. A. B. Hassen, F. I. H. Rhouma, M. Daoudi, J. Dhahri, M. Zaidi and N. Abdelmoula, RSC Adv., 2019, 9, 19285–19296.
89. H. H. Lin, T. Yu, G. X. Bai, M. K. Tsang, Q. Y. Zhang and J. H. Hao, J. Mater. Chem. C, 2016, 4, 3396–3402.
90. M. Rathaiah, P. Haritha, A. D. Lozano-Gorrin, P. Babu, C. K. Jayasankar, U. R. Rodriguez-Mendoza, V. Lavin and V. Venkatramu, Chem. Phys., 2016, 18, 14720–14729.
91. W. Lv, M. M. Jiao, B. Q. Shao, L. F. Zhao, Y. Feng and H. P. You, Dalton Trans., 2016, 45, 466–468.
92. S. Ye, J. J. Zhou, S. T. Wang, R. X. Hu, D. P. Wang and J. R. Qiu, Opt. Express, 2013, 21, 4167–4173.
93. Y. J. Han, S. Wang, H. Liu, L. Shi, J. Y. Zhang, Z. N. Zhang, Z. Y. Mao, D. J. Wang, Z. F. Mu, Z. W. Zhang and Y. Zhao, J. Lumin., 2020, 220, 116968.
94. A. S. Laia, D. A. Hora, M. V. dos S. Rezende, Y. T. Xing, J. J. Rodrigues Jr, G. S. Maciel and M. A. R. C. Alencar, Chem. Eng. J., 2020, 399, 125742.
95. H. Nurhafizah and M. S. Rohani, Solid State Phenom., 2019, 290, 16–21.
96. C. Cao, M. Xue, X. J. Zhu, P. Y. Yang, W. Feng and F. Y. Li, ACS Appl. Mater. Interfaces, 2017, 9, 18540–18548.
97. J. H. Chen, W. R. Zhao, N. H. Wang, Y. J. Meng, S. P. Yi, J. He and X. Zhang, J. Mater. Sci., 2016, 51, 4201–4212.
98. P. You, G. Yin, X. Chen, B. Yue, Z. Huang, X. Liao and Y. Yao, Opt. Mater., 2011, 33, 1808–1812.
99. Q. Liu, Y. Liu, Z. Yang, Y. Han, X. Li and G. Fu, J. Alloys Compd., 2012, 515, 16–19.
100. D. L. Dexter, J. Chem. Phys., 1953, 21, 836–850.
101. G. Q. Zhang, M. S. Molokeev, Q. C. Ma, X. N. Yang, S. Q. Han, Q. Chen, B. N. Zhong and B. Ma, CrystEngComm, 2020, 22, 5809–5817.
102. Y. R. Shi, Y. H. Wang and Z. G. Yang, J. Alloys Compd., 2011, 509, 3128–3131.
103. H. Zhong, X. Li, R. Shen, J. Zhang, J. Sun, H. Zhong, L. Cheng, Y. Tian and B. Chen, J. Alloys Compd., 2012, 517, 170–175.
104. Q. Du, G. Zhou, J. Zhou, X. Jia and H. Zhou, J. Alloys Compd., 2013, 552, 152–156.
105. Z. Yang, C. Hou, G. Duan, F. Yang, P. Liu, C. Wang, L. Liu and G. Dong, J. Alloys Compd., 2014, 604, 346–351.
106. D. L. Monika, H. Nagabhushana, R. H. Krishna, B. M. Nagabhushana, S. C. Sharma and T. J. Thomas, RSC Adv., 2014, 4, 38655–38662.
107. X. Liu, W. Xiang, F. Chen, Z. Hu and W. Zhang, Mater. Res. Bull., 2013, 48, 281–285.
108. G. Q. Wang, X. H. Gong, Y. J. Chen, J. H. Huang, Y. F. Lin, Z. D. Luo and Y. D. Huang, Opt. Mater., 2014, 36, 1255–1259.
109. P. C. Ma, Y. H. Song, B. Yuan, Y. Sheng, C. Y. Xu, H. F. Zou and K. Y. Zheng, Ceram. Int., 2017, 43, 60–70.
110. P. C. Chen, F. W. Mo, A. X. Guan, R. F. Wang, G. F. Wang, S. Y. Xia and L. Y. Zhou, Appl. Radiat. Isot., 2016, 108, 148–153.
111. R. P. Cao, H. D. Xu, W. J. Luo, Z. Y. Luo, S. L. Guo, F. Xia and H. Ao, Mater. Res. Bull., 2016, 81, 27–32.
112. G. C. Xing, Y. X. Feng, M. Pan, Y. Wei, G. G. Li, P. P. Dang, S. S. Liang, M. S. Molokeev, Z. Y. Cheng and J. Lin, J. Mater. Chem. C, 2018, 6, 13136–13147.
113. T. Orihashi, T. Nakamura and S. Adach, RSC Adv., 2016, 6, 66130–66139.
114. Z. B. Xia, Q. Xue, K. F. Zhang, H. P. Zhang and T. He, J. Mater. Sci.: Mater. Electron., 2015, 26, 8078–8082.
115. R. F. Wang, J. Liu and Z. Zhang, J. Alloys Compd., 2016, 688, 332–336.
116. L. Kong, Y. Y. Liu, L. P. Dong, L. Zhang, L. Qiao, W. S. Wang and H. P. You, Dalton Trans., 2020, 49, 1947–1954.
117. Y. J. Zhu, Z. X. Qiu, B. Y. Ai, Y. T. Lin, W. L. Zhou, J. L. Zhang, L. P. Yu, Q. H. Mi and S. X. Lian, J. Lumin., 2018, 201, 314–320.
118. B. Wang, H. Lin, J. Xu, H. Chen and Y. S. Wang, ACS Appl. Mater. Interfaces, 2014, 6, 22905–22913.
119. R. Dittmann, D. Hahn and J. Stade, Luminescence of Crystals, Molecules, and Solutions, 1973, 518–523.
120. Z. G. Xia, R. S. Liu, K. W. Huang and V. Drozd, J. Mater. Chem., 2012, 22, 15183–15189.
121. J. Ueda, K. Kuroishi and S. Tanabe, Appl. Phys. Express, 2014, 7, 062201–062203.
122. M. Zhao, Z. Xia, M. S. Molokeev, L. Ning and Q. Liu, Chem. Mater., 2017, 29, 6552–6559.
123. S. Z. Liao, X. Y. Ji, Y. F. Liu and J. L. Zhang, ACS Appl. Mater. Interfaces, 2018, 10, 39064–39073.
124. W. L. Wu, M. H. Fang, W. Zhou, T. Lesniewski, S. Mahlik, M. Grinberg, M. G. Brik, H. S. Sheu, B. M. Cheng, J. Wang and R. S. Liu, Chem. Mater., 2017, 29, 935–939.
125. S. Adachi, J. Lumin., 2018, 202, 263–281.
126. X. T. Rao, T. Song, J. K. Gao, Y. J. Cui, Y. Yang, C. D. Wu, B. L. Chen and G. D. Qian, J. Am. Chem. Soc., 2013, 135, 15559–15564.
127. D. Wawrzynczyk, A. Bednarkiewicz, M. Nyk, W. Strek and M. Samoc, Nanoscale, 2012, 4, 6959–6961.
128. M. D. Dramićanin, Z. Antić, S. Ćulubrk, S. P. Ahrenkiel and J. M. Nedeljković, Nanotechnology, 2014, 25, 485501–485506.
129. D. Q. Chen, S. Liu, Y. Zhou, Z. Y. Wan, P. Huang and Z. G. Ji, J. Mater. Chem. C, 2016, 4, 9044–9051.
130. D. Q. Chen, W. Xu, Y. Zhou and Y. Chen, J. Alloys Compd., 2016, 676, 215–223.
131. A. F. Pereira, K. U. Kumar, W. F. Silva, W. Q. Santos, D. Jaque and C. Jacinto, Sens. Actuators, B, 2015, 213, 65–71.
132. S. Cúlubrk, V. Lojpur, S. P. Ahrenkiel, J. M. Nedeljkovic and M. D. Dramićanin, J. Lumin., 2016, 170, 395–400.
133. E. C. Ximendes, U. Rocha, K. U. Kumar, C. Jacinto and D. Jaque, Appl. Phys. Lett., 2016, 108, 253103–253107.
134. E. C. Ximendes, W. Q. Santos, U. Rocha, U. K. Kagola, F. SanzRodríguez, N. Fernández, A. daSilva Gouveia-Neto, D. Bravo, A. M. Domingo, B. del Rosal, G. D. S. Brites, L. D. Carlos, D. Jaque and C. Jacinto, Nano Lett., 2016, 16, 1695–1703.
135. D. Q. Chen, Y. Zhou, W. Xu, J. S. Zhong, Z. G. Ji and W. D. Xiang, J. Mater. Chem. C, 2016, 4, 1704–1712.
136. K. S. Kumar, C. G. Lou, A. G. Manohari, H. H. Cao and D. Pribat, RSC Adv., 2017, 7, 24674–24678.
137. Pushpendra, R. K. Kunchala, R. Kalia and B. S. Naidu, Ceram. Interfaces, 2020, 46, 18614–18622.
138. Y. P. Tai, X. Z. Li, X. G. Du, B. L. Pan and G. H. Yuan, RSC Adv., 2018, 8, 23268–23273.
139. M. G. Nikolić, Z. Antić, S. Cúlubrk, J. M. Nedeljkovic and M. D. Dramićanin, Sens. Actuators, B, 2014, 201, 46–50.
140. H. Nakamura, K. Shinozaki, T. Okumura, K. Nomura and T. Akai, RSC Adv., 2020, 10, 12535–12546.
141. Pushpendra, R. K. Kunchala, R. Kalia and B. S. Naidu, RSC Adv., 2020, 10, 14525–14530.
142. T. Miyakawa and D. L. Dexter, Phys. Rev. B: Solid State, 1970, 1, 2961–2969.
143. S. Kück, S. Hartung, S. Hurling, K. Petermann and G. Huber, Spectrochim. Acta, Part A, 1998, 54, 1741–1749.
144. E. Öztürk and N. O. Kalaycioglu, Solid State Phenom., 2015, 230, 217–220.
145. N. O. Kalaycioglu and E. Irir, J. Alloys Compd., 2011, 510, 6–10.
146. E. Uzun, E. Öztürk, N. K. Ozpozan and E. Karacaoglu, J. Lumin., 2016, 173, 73–81.
147. E. Öztürk and N. O. Kalaycioglu, J. Therm. Anal. Calorim., 2014, 115, 573–577.
148. M. Buryi, V. Laguta, M. Nikl, V. Gorbenko, T. Zorenko and Y. Zorenko, CrystEngComm, 2019, 21, 3313–3321.
149. Pushpendra, R. K. Kunchala, S. N. Achary, A. K. Tyagi and B. S. Naidu, Cryst. Growth Des., 2019, 19, 3379–3388.
150. Y. Zhang, W. T. Gong, J. J. Yu, Z. Y. Cheng and G. L. Ning, RSC Adv., 2016, 6, 30886–30894.
151. Pushpendra, R. K. Kunchala, S. N. Achary and B. S. Naidu, ACS Appl. Nano Mater., 2019, 2, 5527–5537.
152. W. Li, X. J. Gao, X. L. Yang, X. L. Jin and S. G. Xiao, J. Alloys Compd., 2016, 664, 181–187.
153. W. Piotrowski, K. Trejgis, K. Maciejewska, K. Ledwa, B. Fond and L. Marciniak, ACS Appl. Mater. Interfaces, 2020, 12, 44039–44048.

---