Local symmetry engineering and Judd–Ofelt decoupling in SrLaInO₄:Eu³⁺ for high-purity red emission toward advanced solid-state lighting

Abstract

A highly efficient red phosphor, SrLaInO₄:Eu³⁺ (SLIO:Eu³⁺), was synthesized via a high-temperature solid-solution route for applications in near-ultraviolet (NUV) excited warm white light-emitting diodes (w-LEDs). Rietveld refinement verified that SLIO crystallizes in an orthorhombic Pca2₁ structure, where Eu³⁺ preferentially accommodates the low-symmetry La³⁺ site with a nine-coordinated [EuO₉] environment, effectively promoting the electric-dipole governed ⁵D₀ → ⁷F₂ transition. Upon 395 nm excitation, the optimized composition (x = 0.20, in mol) delivers intense red emission centered at 615 nm, exhibiting a high color purity of 95%. The optimized phosphor had (0.659, 0.340) Commission Internationale de l'Eclairage chromaticity coordinates, approaching the National Television System Committee red standard. Concentration quenching is governed by a dipole–dipole interaction mechanism, with a calculated critical energy transfer distance of 12.89 Å. Judd–Ofelt spectral analysis yields a notably enhanced Ω₂ value, suggesting increased local structural asymmetry and a stronger covalent character around Eu³⁺. The phosphor also featured a moderate luminescence decay time of 752 μs and a reduced optical band gap from 4.21 to 3.74 eV after Eu³⁺ incorporation with 2 mol. A warm w-LED fabricated using an NUV chip combined with SLIO:0.2Eu³⁺ and commercial blue/green phosphors achieved a correlated color temperature below 3100 K, demonstrating its strong promise as a high-performance red component for next-generation solid-state lighting based on phosphor-converted w-LEDs.

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1. Introduction

Recently, owing to their superior performances, including high luminous efficiency, long cycle life, low energy consumption, and high thermal and chemical stabilities, white light-emitting diodes (w-LEDs) have gained extensive interest.^1–4 Currently, commercial w-LEDs commonly contain a blue InGaN chip along with a yellow Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) phosphor.⁵ Unfortunately, this configuration exhibits a poor color rendering index (CRI < 80) and a high correlated color temperature (CCT > 6000 K), resulting from the shortcoming of red-light content.^6,7 To solve these drawbacks, a possible alternative strategy is to develop multi-component red–green–blue (RGB) white-light systems by combining RGB phosphors with ultraviolet (UV) chips.⁸ Compared with conventional YAG:Ce-based w-LEDs, the RGB phosphor approach enables broader and more balanced spectral coverage, providing improved color rendition, tunable CCT, enhanced luminous quality, and superior visual comfort for warm white lighting applications.⁹

Until now, extensive efforts have been taken to develop high-performance red-emitting phosphors based on rare-earth (RE) activators such as Eu²⁺, Sm³⁺, and Eu³⁺.^10–12 Eu²⁺-activated phosphors (e.g., CaAlSiN₃:Eu²⁺, Sr₂Si₅N₈:Eu²⁺, etc.) exhibit broad-band emission released from the 4f⁶5d¹ → 4f⁷ transition and have been widely used in commercial w-LEDs because of their high luminous efficiency.¹³ However, their emission wavelengths are highly sensitive to the host lattice and crystal field strength, and their chemical stability is generally limited, especially in oxide environments. Similarly, Sm³⁺ phosphors can provide orange-red emission but usually suffer from relatively weak emission intensity and thermal quenching due to forbidden-transition features.¹⁴ In contrast, Eu³⁺-activated materials show narrow-band red emission dominated by the featured ⁵D₀ → ⁷F_J (J = 1–4) transitions, particularly the red ⁵D₀ → ⁷F₂ (∼610 nm) electric-dipole transition, which ensures excellent color purity and spectral stability.¹⁵ In addition, Eu³⁺ exhibits excellent matrix compatibility, tunable site accommodation, and reliable thermal and chemical stability, giving it a significant advantage in next-generation warm w-LED applications.¹⁶ To date, numerous Eu³⁺-activated oxides and fluorides have been investigated as red-emitting phosphors.¹⁷ Among these hosts, oxide-based materials have gained particular interest thanks to their excellent chemical stability, reliable thermal reliability, and environmental safety compared to sulfides or nitrides.^18,19 Therefore, Eu³⁺-doped red phosphors in oxide material systems have emerged as promising candidates to overcome the limitations of Eu²⁺ and Sm³⁺ ions and to achieve a high CRI with improved operational reliability.

Layered perovskite oxides with the general formula AA′BO₄ (A = alkaline earth metal; A′ = RE metal; B = trivalent cation, such as In³⁺, Ga³⁺, or Al³⁺) have become ideal matrix materials for RE-activated phosphors in recent years.^20,21 The relatively rigid BO₆ octahedral framework ensures structural stability and mitigates thermal activation of phonon-assisted relaxation paths. Simultaneously, the flexible A/A′-site allows for stable incorporation and tunable local symmetry, thereby achieving controllable charge compensation, optimized energy levels, and electric dipole transitions for enhanced Eu³⁺ emission.²² Due to their wide bandgap, high chemical stability, and favorable photophysical environment, AA′BO₄-type oxide is an ideal platform for designing red phosphors, which can be used in high-power solid-state lighting. AA′BO₄-type materials exhibited excellent performance and interesting phenomena, which have been widely reported. For example, SrLaGaO₄:Dy³⁺/Eu³⁺, BaLaGaO₄:Sm³⁺, CaGdGaO₄:Bi³⁺, SrLaAlO₄:Yb³⁺, Er³⁺, and CaYAlO₄:Dy³⁺ have been developed for use in the fields such as warm w-LEDs, blue light resource, and high sensitivity thermometers.^23–27

Inspired by the aforementioned literature, we decided to introduce a SrLaInO₄ (SLIO) oxide, which has been reported mainly in the contexts of solar cells and electronic/photovoltaic applications.^28,29 In the luminescence area, the SLIO:Bi³⁺ phosphor has been reported as an X-ray-activated UV-A range persistent luminescent material, whereas its potential as an RE-activated phosphor remains largely underexplored.³⁰ Meanwhile, Eu³⁺-activated ALaBO₄-type layered oxides have been explored as narrow-band emitters but most reports focus on phenomenological luminescence optimization and provide a limited quantitative linkage between crystallographic/site symmetry and hypersensitive electric-dipole emission.^31–34 A quantitative linkage between crystallographic site symmetry and Eu³⁺ hypersensitive emission behavior remains limited, as summarized in Table 1.

Table 1 Literature comparison of SrLaInO₄-related systems and several Eu³⁺-activated ALaBO₄-type phosphors

System	Application focus	Main emission (nm)	CIE coordinates (x, y)	Symmetry-related analysis	Ref.
SrLaInO4:Fe^3+	Solar cells	Not reported	Not reported	No	28
SrLaInO4:Ti^4+	Ionic/proton conductivity	Not reported	Not reported	No	29
SrLaInO4:Bi^3+	Persistent luminescence	565	Not reported	Qualitative	30
BaLaInO4:Eu^3+	w-LEDs	612	(0.6321, 0.3669)	Qualitative	31
BaLaGaO4:Eu^3+	w-LEDs and Plant growth	700	(0.6479, 0.3506)	J–O analysis (Brief)	32
SrLaGaO4:Eu^3+	w-LEDs	618	(0.647, 0.353)	Qualitative	33
SrLaAlO4:Eu^3+	Red phosphor	625	(0.6582, 0.3266)	Qualitative	34
SrLaInO4:Eu^3+	w-LEDs	615	(0.6801, 0.3197)	J–O analysis (Structure-guided)	This work

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In this regard, SLIO offers an intrinsically low-symmetry A/A′-site framework that can accommodate Eu³⁺ in a nine-coordinated environment, enabling a symmetry-driven enhancement of the ⁵D₀ → ⁷F₂ transition. Furthermore, by combining Rietveld refinement, spectroscopy, and Judd–Ofelt (J–O) analysis, we demonstrate a local-symmetry enhancement vs. lattice rigidity decoupling (enhanced Ω₂ with relatively stable Ω₄), providing a design principle beyond conventional dopant adjustment.³⁵ As an AA′BO₄-type oxide material, it provides a low-symmetry local environment for trivalent dopants (e.g., Eu³⁺), which is conducive to strong electric dipole transitions (⁵D₀ → ⁷F₂). Furthermore, the presence of Sr²⁺ and La³⁺ in the A/A′-site framework enables charge compensation and flexible lattice tuning, which is beneficial for stabilizing Eu³⁺ luminescent centers. In addition, indium (In)-based oxides possess a wide bandgap and good structural rigidity, making SLIO a promising matrix material for blue or near-UV (NUV) excited red phosphors.

In this work, by utilizing the solid-solution reaction method, a series of SLIO:xEu³⁺ phosphors were developed. The effects of Eu³⁺ content on the crystal structure, luminescence properties, decay behavior, and thermal stability were systematically studied. Site accommodation and local symmetry around Eu³⁺ ions were analyzed to elucidate the relationship between structural features and photoluminescence (PL) performance. The results demonstrate that SLIO:Eu³⁺ exhibits strong red emission under UV excitation, excellent color purity, and reliable thermal robustness, highlighting that it can be a promising material for warm w-LEDs and solid-state lighting applications.

2. Experimental section

2.1. Material synthesis

All the phosphor samples in this work were prepared via a conventional solid-state reaction method. An SLIO host and a series of SrLa_1−xEu_xInO₄ (SLIO:xEu³⁺, x = 0.1, 0.2, 0.3, 0.4, and 0.5, in mol) phosphors were designed and synthesized. The nominal compositions were deliberately designed using a La-deficient incorporation strategy, in which the molar amount of La precursor was reduced by x and that of Eu precursor was increased correspondingly to maintain a constant A′-site incorporation. Accordingly, stoichiometric amounts of SrCO₃ (99%), La₂O₃ (99%), In₂O₃ (99%), and Eu₂O₃ (99%) were weighed based on the nominal SrLa_1−xEu_xInO₄ compositions. The precursors were thoroughly mixed and ground in an agate mortar for 30 min to ensure compositional homogeneity, followed by sintering at 1300 °C for 8 h in an alumina crucible under ambient air. After cooling to room temperature, the sintered products were ground into fine powders for subsequent characterization studies.

2.2. w-LED fabrication

w-LED devices were fabricated utilizing polydimethylsiloxane (PDMS) (silica epoxy gel A and B at a ratio of 10 : 1), the optimized SLIO:0.2Eu³⁺ phosphor, commercial BaMgAl₁₀O₁₇:Eu²⁺ blue phosphor (BAM:Eu²⁺) and green BaSc₂Si₃O₁₀:Eu²⁺ phosphor (BSS:Eu²⁺). For the w-LED device, 0.1 g of SLIO:0.2Eu³⁺ phosphor, 0.05 g of BAM:Eu²⁺, and 0.07 g of BSS:Eu²⁺ were mixed with 1 g of PDMS solution. The as-prepared slurries were applied onto the 395 nm emitting chips and dried in an oven at 120 °C for 2 h.

2.3. Characterization studies

XRD analysis was performed on a Bruker D8 Advance diffractometer equipped with Cu Kα radiation (λ = 0.154056 nm) to determine the phase composition and crystal structure of the synthesized samples. The surface morphology and microstructural characteristics were analyzed using a high-resolution field-emission scanning electron microscope (HR-FESEM, Carl Zeiss), coupled with energy-dispersive X-ray spectroscopy (EDS) for elemental mapping and compositional investigation. The valence states and chemical environments of elements were characterized by X-ray photoelectron spectroscopy (XPS) on a Thermo Fisher Scientific K-Alpha spectrometer. PL excitation (PLE) and PL emission spectra were recorded using a Scinco FluoroMate FS-2 fluorescence spectrophotometer. The thermal durability of the as-obtained materials was evaluated by collecting temperature-dependent PL emission spectra from 303 to 483 K, controlled using a NOVA ST540 temperature-regulated stage.

2.4. Rietveld refinement details

Rietveld refinements of the XRD patterns were performed using the GSAS-II software.³⁶ The background was fitted using a polynomial function with 12 adjustable coefficients, and the peak profile was modeled using a pseudo-Voigt function, which is a linear combination of Gaussian and Lorentzian functions. During the refinement of structural information, fractional atomic coordinates (e.g., 0, 1/8, 1/4, 1/3, 1/2, and 1) were fixed to their initial values, and only non-fractional atomic coordinates were refined.

3. Results and discussion

3.1. Structure and morphology

Fig. S1 of supporting information (SI) shows the powder XRD patterns of the SLIO host and all the SLIO:xEu³⁺ samples with different doping concentrations. When the dopant content x is lower than 0.3, all the diffraction peaks can be indexed to the standard SLIO phase (ICSD 169079), indicating single-phase formation in the low- and mid-doping range.³⁷ Considering the comparable valence states and ionic radius, Eu³⁺ ions (CN = 9, 1.12 Å) show a preference for the La³⁺ sites (CN = 9, 1.21 Å) rather than Sr²⁺ (CN = 9, 1.31 Å) or In³⁺ (CN = 6, 0.80 Å) sites.^12,23,38 This site preference can be further rationalized by calculating the ionic radius mismatch percentage according to the following formula:¹⁰

Here, R_s and R_d are the ionic radii of the acceptor and dopant ions, respectively. The as-obtained ionic radius mismatch (D_r) between Eu³⁺ and La³⁺ ions is 7.4%, which is far below the commonly accepted threshold of 30%, indicating that Eu³⁺ ions can be stably incorporated into the La³⁺ sites. As highlighted by the enlarged views near the (113) reflection, the diffraction peak shifts gradually toward higher 2θ angles with increasing Eu³⁺ content up to x = 0.3, consistent with lattice contraction induced by Eu³⁺ incorporation. When x is beyond 0.3, additional reflections attributable to secondary orthorhombic EuInO₃ with a Pbnm space group become visible.³⁹ Meanwhile, the (113) peak of the SLIO main phase shows a slight shift back toward lower angles, suggesting that the high Eu³⁺ ion concentration leads to phase separation, which reduces the effective Eu³⁺ incorporation in the remaining SLIO lattice. The Rietveld refinements of the XRD patterns for the as-obtained SLIO host and the optimal SLIO:0.2Eu³⁺ phosphor are illustrated in Fig. 1(a) and (b). All the diffraction peaks for both samples can be well matched with the standard pattern, confirming the successful synthesis process of the target phase and the effective integration of dopant Eu³⁺ ions into the SLIO lattice. To further analyze the crystal structure, Rietveld refinement was conducted utilizing GSAS-II, and the corresponding reliability factors are provided in Table S1 in the SI. The refinement parameters, including R_wp (6.85% and 7.26%), R_p (4.90% and 5.21%), and χ² (0.978 and 0.938), fall within acceptable ranges, indicating reliable refinement quality and high phase purity of the samples.⁵ Fig. 1(c) shows the crystal structure of the SLIO:0.2Eu³⁺ phosphor and the corresponding polyhedral units. The SLIO host belongs to orthorhombic crystal system with a Pca2₁ space group. Upon incorporation of La³⁺ by slightly smaller Eu³⁺ ions, a subtle contraction of the crystal lattice occurs owing to the reduced ionic radius.⁴⁰ This observation is consistent with the refined cell volumes listed in Table S1 in the SI, where the lattice volume decreases from 433.625 ų for the pristine SLIO host to 432.233 ų at 0.2 mol Eu³⁺ doping.

Fig. 1 Rietveld refinement of the XRD patterns for (a) the SLIO host and (b) the SLIO:0.2Eu³⁺ phosphor. (c) Schematic crystal structure of the SLIO and coordination environments of constituent cations.

To analyze the elemental composition and chemical states in SLIO:Eu³⁺ phosphors, XPS measurements were conducted over the binding energy range of 0–1350 eV, as shown in Fig. 2(a). Signals corresponding to Sr, La, In, Eu, and O were clearly detected, along with a minor C 1s peak at 284.8 eV, which was used for charge calibration.⁴¹ The high-resolution spectra of individual elements are presented in Fig. 2(b–f). As depicted in Fig. 2(b), the Sr 3d spectrum exhibits two peaks at 134.75 and 132.94 eV, assigned to Sr 3d_3/2 and Sr 3d_5/2, respectively.⁴² The La 3d spectrum (Fig. 2(c)) showed characteristic doublet features corresponding to La 3d_3/2 (850.92 and 854.48 eV) and La 3d_5/2 (834.17 and 837.72 eV).²⁴ The satellite peaks originate from the final-state charge-transfer process between La 3d core holes and La 4f/O 2p orbitals, giving rise to unscreened (3d⁹4f⁰) and screened (3d⁹4f¹L) states (L denotes a ligand hole), consistent with La³⁺ in oxide environments.⁴³ As shown in Fig. 2(d), the In 3d spectrum exhibited two peaks at 451.20 and 443.65 eV, attributing to In 3d_3/2 and In 3d_5/2.⁴⁴ The Eu 3d core-level spectrum (Fig. 2(e)) showed two dominant peaks at 1162.90 and 1134.17 eV, attributed to Eu³⁺ 3d_3/2 and 3d_5/2, associated with the 3d⁹4f⁶ configuration.⁴² The energy separation between the two peaks (28.73 eV) is consistent with literature values (∼30 eV). Additional weak features at 1154.24 and 1125.19 eV are earmarked to Eu²⁺ 3d_3/2 and 3d_5/2, respectively.¹¹ Although no reducing environment was employed during synthesis, trace Eu²⁺ states can still appear, in agreement with previous reports on Eu-doped oxide material systems.⁴⁵ The O 1s spectrum (Fig. 2(f)) can be deconvoluted into three components peaked at 529.49, 531.21, and 533.39 eV, corresponding to lattice oxygen, oxygen vacancies, and surface-adsorbed oxygen species, respectively.⁴⁶

Fig. 2 (a) XPS survey scan spectrum and high-resolution core-level spectra (b–f) of Sr 3d, La 3d, In 3d, Eu 3d, and O 1s for the SLIO:0.2Eu³⁺ phosphor.

The morphology of all the SLIO:xEu³⁺ phosphors was measured by HR-FESEM analysis at 10 k×, and the corresponding images are presented in Fig. 3(a). All the samples exhibit similar morphological characteristics, consisting of irregularly shaped particles of sizes in the micrometer range and a certain degree of particle agglomeration, which is typical for phosphors synthesized via the high-temperature solid-state reaction method.⁴⁷ Notably, no obvious changes in particle morphology or grain size are observed with increasing Eu³⁺ ion concentration, indicating that Eu³⁺ doping has a negligible effect on the crystal growth behavior of the SLIO host. Although morphology can influence luminescence properties in some phosphor systems, the HR-FESEM results suggest that all the SLIO:xEu³⁺ samples possess very similar particle morphology and micro-scale grain sizes.^48,49 Therefore, morphology is unlikely to be the primary factor affecting the luminescence behavior in this work. The EDS spectrum shown in Fig. 3(b) confirmed the presence of Sr, La, In, Eu, and O elements in the sample. The homogeneous distribution of these elements, including Eu³⁺ dopant ions, is further verified by the elemental mapping images (inset of Fig. 3(b)), indicating that Eu³⁺ ions are successfully introduced into the SLIO host lattice without noticeable agglomeration or phase separation.

Fig. 3 (a) HR-FESEM micrographs of SLIO:xEu³⁺ phosphors at 10 k×. (b) EDS spectrum with elemental mapping images of Sr, La, In, Eu, and O (inset).

3.2. UV-Vis and band gap studies

The diffuse reflectance spectroscopy (DRS) spectra of the SLIO host and SLIO:0.2Eu³⁺ phosphor were recorded to explain the influence of Eu³⁺ incorporation on the optical absorption performance of the SLIO matrix, as presented in Fig. 4(a). Moreover, the optical band gap energy (E_g) was derived from the DRS data using the following equations:⁵⁰

Fig. 4 (a) DRS spectra of the SLIO host and SLIO:0.2Eu³⁺ phosphor. (b) Tauc plots of the SLIO and SLIO:0.2Eu³⁺ phosphor.

Herein, R_∞ and F(R_∞) represent the reflectance coefficient and the Kubelka–Munk function, respectively, while h, v, E_g, and A correspond to the Planck constant, photon frequency, band gap energy, and a proportion constant. The parameter n depends on the nature of the electronic transition: for direct allowed and direct forbidden transitions, n = 1/2 and 3/2, respectively, while for indirect allowed and indirect forbidden transitions, n = 2 and 3.⁵¹ Since SLIO is a direct band gap material, n was taken as 1/2 in this work. As compared in Fig. 4(b), the estimated band energies of the un-doped SLIO host and the SLIO:0.2Eu³⁺ sample are approximately 4.21 and 3.74 eV, respectively. The results indicate that even a low concentration of Eu³⁺ doping leads to a narrowing of the band gap in SLIO, which is helpful to enhanced luminescence performance in the SLIO:Eu³⁺ phosphors.

3.3. PL properties

Fig. 5(a) shows the PL emission and PLE spectra of the optimal SLIO:0.2Eu³⁺ phosphor. PLE spectra, studied with the emission wavelength at 615 nm, released a broad band centered at 306 nm, corresponding to the O²⁻ → Eu³⁺ charge-transfer band (CTB).¹⁶ In addition, several acute f–f transition peaks locating from 350 to 550 nm were observed at 364, 383, 395, 403, 415, 466, and 535 nm, attributing to the energy transitions from the Eu³⁺ ground state ⁷F₀ or ⁷F₁ to the excited states ⁵G_2,3, ⁵L₆, ⁵D₄, ⁵D₃, ⁵D₂, and ⁵D₁, respectively.¹⁰ The emission spectrum exhibited characteristic Eu³⁺ emissions at 582, 595, 615, 657, and 707 nm, originating from the ⁵D₀ → ⁷F_J (J = 0–4) transitions.³² As shown in Fig. 5(b), the phosphor showed strong red emission under excitations at 306, 395, and 466 nm, with 395 nm, located in the NUV range, indicating that the BLIO:Eu³⁺ phosphor can be excited by the commercial NUV chip. Fig. 5(c) shows the Eu³⁺ ion concentration-dependent PL emission spectra of SLIO:xEu³⁺. All the PL excitation and emission spectra were recorded under a fixed excitation wavelength of 395 nm. The emission spectra collected under this condition were used for subsequent J–O analysis. The emission intensity initially increased and reached a maximum at x = 0.2, then gradually decreased with further Eu³⁺ addition. This trend is clearly demonstrated in Fig. 5(d), suggesting a concentration-dependent luminescence behavior. The decline beyond 0.2 mol Eu³⁺ is the result of concentration quenching. This means that exceeding dopant ion precipitate enables non-radiative energy transfer pathways, thereby reducing emission intensity.²²

Fig. 5 (a) Excitation and emission spectra of the SLIO:0.2Eu³⁺ phosphor. (b) Emission spectra of the SLIO:0.2Eu³⁺ phosphor at different excitation wavelengths. (c) Eu³⁺ ion concentration-dependent PL emission spectra. (d) Normalized emission intensity as a function of dopant content.

Three primary interaction mechanisms, including exchange interaction, electrostatic multipolar interaction, and radiative reabsorption, may contribute to concentration quenching in RE-activated phosphors. Radiative reabsorption typically occurs when there is a significant overlap between the PL emission and PLE spectra.⁵² As no obvious overlap was observed for the SLIO:Eu³⁺ phosphors, the concentration quenching behavior is mainly governed by exchange or electrostatic multipolar interactions, rather than radiative reabsorption. To further understand the quenching mechanism, the critical distance (R_c) between Eu³⁺ activators was estimated based on the Blasse theory using the following equation:⁴⁶

Here, V (unit-cell volume), x_c (critical dopant concentration), and Z (number of cation sites per unit cell) are parameters for determining the critical distance. For the SLIO:Eu³⁺ phosphors, V = 449.35 ų, x_c = 0.2, and Z = 2. Based on these parameters, the critical distance (R_c) between Eu³⁺ ions was obtained to be 12.89 Å. Since exchange interaction typically dominates when R_c < 5 Å, the significantly larger R_c value obtained in this work implies that the concentration-quenching process is mainly governed by electrostatic multipolar interactions rather than exchange interactions.¹⁵ For electrostatic multipolar interactions, three types of interactions may occur (dipole–dipole (d–d), dipole–quadrupole (d–q), and quadrupole–quadrupole (q–q)). The dominant mechanism is determined by analyzing the interaction parameter Q according to the following relation:¹⁸

In this expression, x and I mean the dopant concentration and the corresponding PL emission intensity, respectively. According to the theoretical model, the dominant interaction mechanism is associated with the value of the parameter Q: d–d, d–q, and q–q interactions correspond to Q values of 6, 8, and 10, respectively. When x exceeds the critical concentration (x_c), the above relationship can be linearized and rewritten as eqn (3):¹⁷

Based on this equation, the linear fitting of the log(I/x) versus log(x) plot is shown in Fig. 6(a). The slope is determined to be −1.81, corresponding to a calculated Q value of approximately 5.43, close to 6, showing that the concentration-quenching behavior of SLIO:Eu³⁺ phosphors is predominantly governed by d–d interactions. In addition to multipolar interactions, particle morphology can also influence luminescence behavior by affecting light scattering and surface-related non-radiative processes.^53,54 However, since all the samples exhibit very similar particle morphology and comparable grain sizes, the influence of morphology on the concentration-dependent PL properties is expected to be minimal in the present system.

Fig. 6 (a) Log(I/x) vs. log(x) dependence with linear fitting. (b) Variation of orange/red emission components and O/R ratio with Eu³⁺ content. (c) CIE chromaticity coordinates of the SLIO:0.2Eu³⁺ phosphor with NTSC standard red and commercial Y₂O₂S:Eu³⁺ phosphors. (d) Proposed energy-level scheme of Eu³⁺ in the SLIO host.

The evolution of the magnetic-dipole (MD, ⁵D₀ → ⁷F₁) and electric-dipole (ED, ⁵D₀ → ⁷F₁) transition percentages plotted against Eu³⁺ ion concentration is presented in Fig. 6(b).⁵⁵ With increasing Eu³⁺ content, the orange-to-red emission ratio (O/R) gradually increased, indicating a modification of the local crystal field environment surrounding Eu³⁺ ions. Typically, the ⁵D₀ → ⁷F₁ transition is a parity-allowed MD transition, and its intensity is nearly insensitive to the site symmetry. In contrast, the ⁵D₀ → ⁷F₂ transition is a hypersensitive ED transition, strongly dependent on the asymmetry of the local coordination environment. As more Eu³⁺ ions preferentially accommodate La³⁺ lattice sites, the inversion symmetry around the Eu³⁺ centers becomes increasingly disrupted, leading to the enhancement of the ED transition. Consequently, the emission shifts toward the dominant ⁵D₀ → ⁷F₂ red-emission band, suggesting a gradual color evolution toward red with increasing Eu³⁺ ion concentration.

Fig. 6(c) shows the Commission Internationale de l'Eclairage (CIE) chromaticity coordinates of the Eu³⁺-activated SLIO phosphor at the optimal dopant content. Based on the PL spectra, the chromaticity point of the SLIO:0.2Eu³⁺ sample is calculated to be (0.659, 0.340), which is very close to the National Television System Committee (NTSC) standard for red emission (0.670, 0.330).⁵⁶ This value is also better than the widely used commercial Y₂O₂S:Eu³⁺ phosphors (0.622, 0.351), highlighting the excellent CRI capability of the present material. To further assess the color performance, the color purity was calculated, which can be defined as follows:²⁰

In this equation, (x, y), (x_i, y_i), and (x_d, y_d) represent the chromaticity coordinates of the phosphor, the standard white light source, and the dominant wavelength point, respectively. The coordinates of the white point and the dominant wavelength (615 nm) are taken as (x_i, y_i) = (0.310, 0.316) and (x_d, y_d) = (0.6801, 0.3197), respectively.⁶ The CIE chromaticity coordinates of SLIO:xEu³⁺ under 306 and 395 nm excitations are listed in Tables S2 and S3 in the SI, respectively. As the Eu³⁺ ion concentration increased, the calculated color purity showed a consistent enhancement under both excitation wavelengths, which is in agreement with the previous O/R ratio analysis, confirming the progressive strengthening of the ⁵D₀ → ⁷F₂ transition with increasing Eu³⁺ content. In addition to chromaticity and color purity, CCT is another important parameter used to evaluate the warmth or coolness of emitted light. Generally, CCT values below 3000 K correspond to warm white light, whereas the values above 4000 K indicate cold white emission. CCT can be obtained by the following equation:²³

CCT = −437n³ + 3601n² − 6861n + 5514.31 (8)

Here, n is calculated based on the inverse slope and the chromaticity epicenter using McCamy's empirical formula:

where x_e and y_e correspond to the chromaticity coordinates of the reference black-body radiation point in the CIE diagram, which are 0.3320 and 0.1858, respectively. As summarized in Tables S2 and S3 in the SI, the CCT values of SLIO:xEu³⁺ phosphors under both 306 nm and 395 nm excitations fall within the range of 2800–3100 K. This indicates that the phosphors exhibit warm light characteristics, which is advantageous compared to conventional high-CCT cold light sources that often cause visual discomfort and reduce lighting quality. The combined results, which include favorable chromaticity coordinates, high color purity, and desirable warm light CCT, demonstrate that Eu³⁺-activated SLIO phosphors possess excellent luminescence properties. Therefore, these materials hold strong potential as promising red-emitting candidates for warm w-LEDs applications.

Fig. 6(d) presents the schematic energy-level diagram of Eu³⁺ ions in the SLIO host, illustrating the excitation and emission processes. Upon excitation with high-energy photons, electrons in Eu³⁺ are promoted from the ground state ⁷F₀ to higher lying 4f⁶5d or ⁵L and ⁵D manifold states. These excited electrons subsequently release non-radiatively to the lowest excited state ⁵D₀, followed by radiative transitions to the ⁷F_J (J = 0–4) multiple levels, giving rise to the characteristic Eu³⁺ emission lines. The excitation spectrum confirms that the phosphor can be effectively excited by NUV commercial LED chips, highlighting its potential for solid-state lighting applications. An equation is used to fit the decay curve:¹⁰

where I and I₀ are the luminescence intensities at the times of t = 0 and t, respectively, A is a constant, and τ is the lifetime. Generally, if the decay time τ can be single-exponential, indicating that there is only one radiative pathway governing this system, and the luminescent centers are in the same coordinate environment.⁵⁷ The contribution of non-radiative channels is constant and avoids significant energy transfer, cross-relaxation, or defect quenching competition. Otherwise, there are at least two different attenuation paths or luminescence center environments. As shown in Fig. 7(a), as the Eu³⁺ doping concentration continuously increases, their decay time has a trend of decline. This phenomenon can be attributed to the reduced interionic distance between Eu³⁺ ions with increasing doping concentration, which promotes more frequent energy transfer among dopant ions and thus enhances the probability of non-radiative transitions.⁵⁸ Moreover, all the decay curves were well-fitted with a single-exponential express, implying that Eu³⁺ ions take the same local lattice environment in the SLIO lattice.

Fig. 7 (a) Luminescence decay time of SLIO:xEu³⁺ phosphors. (b) Temperature-dependent PL emission spectra. (c) Normalized integrated emission intensity at different temperatures. (d) ln(I₀/I − 1) vs. 1/K_BT plot and linear fitting.

3.4. J–O analysis

The J–O analysis was performed using emission spectra measured under identical excitation conditions (λ_ex = 395 nm). The J–O theory gives an effective method for evaluating the local symmetry environment and radiative characteristics of RE ions in solid hosts using a JOES software.³⁵ The three intensity parameters (Ω₂, Ω₄, and Ω₆) reflect the structural and bonding characteristics of the host lattice.⁵⁹ Among them, Ω₂ is highly sensitive to the site symmetry and covalency; lower Ω₂ values typically correspond to more centrosymmetric environments, while an increase in Ω₂ indicates reduced local symmetry and enhanced covalent interaction between Eu³⁺ ions and surrounding O²⁻ ligands. In contrast, Ω₄ and Ω₆ are not directly governed by symmetry but are more strongly influenced by the rigidity, viscosity, and packing density of the host framework, as well as vibronic coupling between Eu³⁺ and its coordinating ligands. Although their physical interpretation is less explicit than that of Ω₂, these two parameters provide valuable insight into the bulk structural properties and lattice dynamics of the matrix. The J–O intensity parameters are commonly derived from transition intensity analysis in PL spectra. In this experiment, they are evaluated based on the intensity ratios between the ED transitions ⁵D₀ → ⁷F_J (J = 2, 4, and 6) and the MD reference transition ⁵D₀ → ⁷F₁, as expressed in eqn (11).⁶⁰ These spectral transitions form the foundation for collecting the J–O parameters and further evaluating the radiative behavior of Eu³⁺ ions in the host lattice:

In this equation, v₁ corresponds to the wavenumber of the MD transition ⁵D₀ → ⁷F₁, while v_J represents the wavenumbers of the ED transitions ⁵D₀ → ⁷F_J (J = 2, 4, and 6). The term S_MD is the MD transition line strength of Eu³⁺ ions, with a constant value of 9.6 × 10⁻⁴² esu² cm², which is considered independent of the host lattice. I₁ and I_J denote the integrated emission intensities of the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F_J transitions, respectively. The squares of the reduced matrix elements of the unit tensor operator terms are introduced from the table values reported by Carnall et al.⁶¹ In addition, the index of refraction n of the host matrix is estimated using the Lorentz–Lorenz relation, which provides an effective approach for related optical polarization with the local dielectric environment of the material:⁶²

In the Lorentz–Lorenz equation, l_i is the number of electrons of the composition element in the material chemical formula, M is the molar mass of the compound, r_i is the molar refraction, and ρ is the theoretical density. Based on this relationship, n of the host lattice was estimated to be 2.002. Owing to the characteristic 4f–4f energy level configuration of Eu³⁺ ions, the J–O intensity parameters Ω₂ and Ω₄ can be reliably derived from the emission spectra. However, the ⁵D₀ → ⁷F₆ transition was not monitored in the PL spectra in this work, preventing the determination of Ω₆. Herein, the Ω₂ and Ω₄ parameters for Eu³⁺-activated SLIO phosphors were calculated using the JOES software package, with the refractive index (n = 2.002) obtained from the Lorentz–Lorenz equation. The practical Eu³⁺ molar concentrations incorporated into the host lattice are summarized in Table 2. Among the J–O parameters, Ω₂ is strongly correlated with the hypersensitive ED transition ⁵D₀ → ⁷F₂ and provides key information on the covalency of the Eu–O bond as well as the asymmetry of the local crystal field around Eu³⁺ ions. In contrast, Ω₄ is more sensitive to the rigidity and electron density distribution of the surrounding ligand framework, where a higher Ω₄ generally indicates reduced ligand electron density. The larger Ω₂ values compared to Ω₄ in SLIO imply that Eu³⁺ ion priority accommodates low-symmetry and highly covalent coordination environments, which agrees with the dominant hypersensitive ⁵D₀ → ⁷F₂ emission observed in PL spectra. This conclusion is in good agreement with the nephelauxetic effect and current reports, and larger Ω₂ values indicate increased nephelauxetic effect (reduced Racah B), confirming stronger Eu–O covalency. In this study, Ω₂ ranges from 8.817 to 9.517 × 10⁻²⁰ cm², while Ω₄ varies from 2.276 to 2.329 × 10⁻²⁰ cm². The relatively close magnitude and small variation of these parameters imply that neither the local coordination asymmetry around Eu³⁺ ions nor the ligand electron density undergoes significant alteration with increasing dopant concentration.

Table 2 J–O intensity parameters, asymmetry ratio, and branching ratio of SLIO:xEu³⁺ phosphors

x (mol)	Ω2 (×10^−20 cm^2)	Ω4 (×10^−20 cm^2)	Asymmetric ratio (R)	βcalc (%) for ^5D0 → ^7F2 transition
0.1	9.493	2.329	6.242	78.4
0.2	9.517	2.294	6.249	78.6
0.3	9.388	2.323	6.176	78.3
0.4	9.077	2.302	5.975	77.8
0.5	8.817	2.276	5.804	77.4

---

The radiative transition probability in J–O analysis is evaluated based on the ⁵D₀ → ⁷F_J (J = 1, 2, 4, and 6) transitions, while the ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₃, and ⁵D₀ → ⁷F₅ transitions are parity-forbidden and thus excluded from the analysis. As mentioned above, the ⁵D₀ → ⁷F₆ emission was not observed in the PL spectra, making the intensity parameter Ω₆ neglected in SLIO:xEu³⁺ phosphors. Consequently, only the ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂, and ⁵D₀ → ⁷F₄ transitions are considered for calculating the radiative transition probabilities. For Eu³⁺ ions, ED transitions originating from the ⁵D₀ level exhibit zero reduced matrix elements except for a few hypersensitive transitions, namely ⁵D₀ → ⁷F₂ (U² = 0.0032), ⁵D₀ → ⁷F₄ (U⁴ = 0.0023), and ⁵D₀ → ⁷F₆ (U⁶ = 0.0002).⁶³ Among these, the first two transitions are utilized. The spontaneous radiative transition probability (A) correlated with the ED component can be expressed as formulated as follows:⁶⁰

where h is Planck's constant, S_MD and S_ED are the MD and ED transition strengths, while χ_MD and χ_ED denote the local field correction factors corresponding to MD and ED transitions. Eqn. (14) and (15) are employed to get the correction factors using the refractive index (n):⁶⁴

χ_MD = n³ (14)

The J–O intensity parameters were employed to calculate the S_ED using eqn (16), while the S_MD remained a constant value of 9.6 × 10⁻⁴² esu² cm², independent of the host matrix:

The calculated radiative transition probability A(J, J′), along with the total radiative transition probability or A_r derived from the J–O intensity parameters (Ω₂ and Ω₄), is summarized in Table 2. Based on these radiative transition probabilities, the branching ratio β_calc for each transition can be determined using the following equation:⁵⁸

As summarized in Table 2, across all the Eu³⁺ ion concentrations investigated (0.1–0.5 mol), the branching ratio of the most dominant transition ⁵D₀ → ⁷F₂ falls within the range of 77.4%–78.6%.

According to previous reports, a branching ratio exceeding 50% suggests that the corresponding transition has the potential to support laser emission. In addition, the quantum efficiency, a crucial parameter for evaluating phosphor performance, can be determined using the following equation:⁶⁵

where τ is the experimental lifetime and τ_r is the calculated radiative lifetime. As shown in Table S4 in the SI, the η_QE values of SLIO:xEu³⁺ phosphors are high, indicating that radiative transition is the predominant pathway while non-radiative losses remain limited. What should be emphasized is that some of the η_QE values exceed 100%, which is physically unrealistic and does not imply photon yields greater than unity.⁵⁴ Such results originate from the inherent assumptions of J–O analysis. In particular, these deviations may arise from the neglect or underestimation of weak non-radiative relaxation pathways, the potential overestimation of radiative transition probabilities associated with large Ω₂ parameters in highly distorted coordination environments, and decay time measurements that may be affected by energy migration or the site-selective emission process.⁵⁸ In this framework, a η_QE over 100% only suggests that the radiative decay dominates and that non-radiative contributions are negligible within the limitations of the J–O model. In summary, the η_QE values derived from J–O analysis remain consistent with the structural and optical characteristics discussed in this work.

Overall, the J–O analysis confirms that Eu³⁺ ions accommodate non-centrosymmetric and moderately covalent environments in the SLIO lattice, with stable local coordination unaffected by dopant concentration, which is consistent with the high red emission efficiency and strong ⁵D₀ → ⁷F₂ transition majority. When compared with representative reports summarized in Table 1, the present SLIO:Eu³⁺ material system distinguishes itself by quantitatively linking local symmetry to luminescence behavior through a structure-guided J–O analysis.

3.5. Thermal stability

To assess the thermal stability of SLIO:Eu³⁺ phosphors, temperature-dependent PL emission spectra of the optimally doped sample were monitored in the range of 303–483 K, as shown in Fig. 7(b). With increasing temperature, the PL intensity gradually decreased due to thermal quenching, while the peak positions remained unchanged, suggesting that the emission centers are stable under thermal excitation. The reduction in emission intensity at elevated temperature is mainly attributed to enhanced non-radiative relaxation processes that compete with radiative transitions. The normalized integrated emission intensities as a function of temperature are plotted in Fig. 7(c). Based on the temperature-dependent luminescence behavior, the activation energy (ΔE) for thermal quenching was evaluated using the Arrhenius equation:⁴¹

In this equation, c is a constant, I₀ denotes the emission intensity at room temperature, and I is the luminescence intensity at temperature T. The Boltzmann constant is represented by K_B (8.63 × 10⁻⁵ eV K⁻¹). As illustrated in the figure, the Arrhenius expression can be rearranged into a linear form, yielding a plot of ln(I₀/I − 1) versus 1/K_BT, which enables the determination of the activation energy for thermal quenching:

The dependence between ln(I₀/I − 1) and 1/K_BT is shown in Fig. 7(d). It can be seen that the ΔE value was obtained to be 0.186 eV for the SLIO:0.2Eu³⁺ phosphor as the slope is −0.186.

3.6. w-LED application

To evaluate the practical applicability of the synthesized SLIO:Eu³⁺ phosphors in solid-state lighting, a warm w-LED prototype was fabricated by combining SLIO:Eu³⁺ with commercial BAM:Eu²⁺ and BSS:Eu²⁺ phosphors, coating the phosphor blend onto a 395 nm NUV LED chip. The electroluminescence (EL) spectrum of the packaged device is presented in Fig. 8(a). The as-assembled w-LED had a low CCT (3518 K) and high CRI (84.7), which indicates that it has a warm white light and meets the requirements of lighting and display applications. As shown in Fig. 8(b), the resulting w-LED exhibited the CIE chromaticity coordinates of (0.4009, 0.3833), verifying that the red emission from SLIO:Eu³⁺ is well-suited for constructing warm white lighting. The photograph of the operating device (inset of Fig. 8(b)) further confirmed bright and visually uniform light output. The radar chart revealed that the CRI factors were much higher than commercial w-LEDs, especially filling the drawbacks in the red range (R9 rose from 30 to 67), enhancing color fidelity compared to conventional commercial w-LEDs.⁶⁶ Such high CRI values reflect the device's strong capability to reproduce natural colors with high accuracy under illumination. To examine the electrical reliability of the phosphor under practical driving conditions, EL spectra were recorded over a current range of 50–300 mA (Fig. 8(d)). With increasing drive current, the emission intensity increased proportionally, while the spectral shape and peak positions remained nearly unchanged, showing no discernible spectral distortion or wavelength shift. This strongly demonstrates the outstanding electrical and spectral stability of SLIO:Eu³⁺, even under high current injection. Notably, these results indicate that SLIO:Eu³⁺ phosphors possess excellent luminescence performance, superior color quality, and robust operational stability, making them a promising candidate for NUV-excited w-LED applications. Its ability to maintain color fidelity and emission intensity under elevated electrical load further highlights its great potential for next-generation solid-state lighting, particularly in high-power, high-CRI, and warm white illumination systems.

Fig. 8 (a) EL spectrum of the fabricated w-LEDs. (b) CIE chromaticity coordinates (inset: device photograph). (c) Color-rendering comparison with a commercial w-LED. (d) Emission spectra under different driving currents.

4. Conclusion

In summary, a new red-emitting phosphor SrLaInO₄:Eu³⁺ (SLIO:Eu³⁺) was developed and evaluated for NUV-driven warm w-LED applications. Structural characterization confirmed that the Eu³⁺ ions were preferentially accommodated into the non-centrosymmetric La³⁺ sites, effectively activating the hypersensitive ⁵D₀ → ⁷F₂ transition and producing spectrally narrow red emission. In addition, the band gap narrowing induced by Eu³⁺ incorporation contributes to enhanced absorption in the NUV region. The optimal concentration of Eu³⁺ ions in the SLIO host was determined as x = 0.2 mol, and outstanding luminescence performance was observed, including a high color purity of 95%. Concentration quenching was analyzed to originate from electric dipole–dipole interactions, while J–O analysis revealed a large Ω₂/Ω₄ ratio, further confirming the highly asymmetric local environment surrounding Eu³⁺. A warm w-LED device integrating SLIO:Eu³⁺ demonstrated stable warm white light output with a high CRI and low CCT, highlighting its potential as a high-purity red component. These results suggest SrLaInO₄:Eu³⁺ as a potential phosphor for the design of next-generation red-emitting materials in advanced solid-state lighting and display systems.

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.

Data availability

The data are available from the corresponding author on reasonable request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5qi02595h.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (RS-2025-23963824).

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