Structure–luminescence correlations in Eu³⁺-activated Ba₂LaVO₆ and Ba₂GdVO₆ double-perovskite phosphors

Abstract

Eu³⁺-activated double perovskites have attracted increasing attention as red-emitting phosphors owing to their compositional tunability and structurally versatile host lattices. In this work, Ba₂La_1−xVO₆:xEu³⁺ and Ba₂Gd_1−xVO₆:xEu³⁺ (x = 0 and 2.5–30 mol%) phosphors were synthesized via a solid-state reaction route and systematically investigated to elucidate host-dependent luminescence behavior. X-ray diffraction combined with Rietveld refinement confirmed single-phase orthorhombic Pnma symmetry for both systems, with the La-based host exhibiting a more flexible and polarizable lattice, while the Gd-based host forms a more compact and rigid framework. Distinct host-dependent microstructural features are observed by scanning electron microscopy (SEM) for the La-based and Gd-based phosphors. X-ray photoelectron spectroscopy (XPS) was further employed to verify the elemental composition and the trivalent oxidation state of Eu ions in representative high-doping compositions. Photoluminescence studies reveal pronounced differences in emission characteristics: Ba₂LaVO₆:Eu³⁺ stabilizes an abnormal near-UV-excited ⁵D₀ → ⁷F₄-dominated orange-red emission with high dopant tolerance up to 30 mol% Eu³⁺, whereas Ba₂GdVO₆:Eu³⁺ exhibits the conventional ⁵D₀ → ⁷F₂-dominated red emission with earlier concentration quenching beyond 15 mol%. Temperature-dependent photoluminescence measurements further reveal distinct thermal quenching behaviors, reflected in different quenching onsets and half-intensity temperatures (T_0.5), which are closely linked to the lattice rigidity of the two hosts. CIE analysis shows that Ba₂LaVO₆:Eu³⁺ emits orange-red light with moderate color purity due to enhanced ⁵D₀ → ⁷F₄ emission, whereas Ba₂GdVO₆:Eu³⁺ produces deeper red emission dominated by the ⁵D₀ → ⁷F₂ transition. Overall, this comparative study establishes a clear structure–rigidity–luminescence correlation in Eu³⁺-activated Ba₂MVO₆ (M = La, Gd) phosphors, providing fundamental insight into host-controlled emission tuning in double-perovskite systems.

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1. Introduction

Rare-earth-doped phosphor materials have attracted considerable attention owing to their outstanding optical properties, including sharp emission lines, long lifetimes, high color purity, and robust chemical stability. Such features make them suitable for diverse applications in solid-state lighting, displays, lasers, bio-imaging, and security technologies.^1–12 Among various activator ions, Eu³⁺ is especially important due to its strong red emission originating from the ⁵D₀ → ⁷F_J (J = 0–6) transitions, which are crucial for achieving warm-white or deep-red components in white light-emitting diodes (WLEDs).^13–15

In recent years, double perovskite-type oxides with the general formula A₂BB′O₆ have emerged as versatile host lattices for luminescent materials. Their wide band gaps, high structural stability, and compositional flexibility allow fine-tuning of both crystal and optical properties. In particular, substitutions at the A-site (alkaline-earth or rare-earth ions) and B/B′-sites (transition metal or rare-earth ions) provide a powerful means to modulate the local crystal field and energy transfer dynamics. Such structural adaptability renders double perovskites highly promising platforms for photonic applications.^14–28 Within this family, vanadate-based double perovskites have drawn special interest because the VO₆ units can enhance excitation efficiency through charge-transfer transitions, while rare-earth ions at the A-site or B-site further tailor the optical behavior.^25–28 In this context, although Eu³⁺-doped vanadates have been extensively studied,^29–33 investigations on Eu³⁺-doped double perovskites remain scarce, with Ba₂GdVO₆:Eu³⁺ reported only by Meng et al.²⁸ On the other hand, Gd³⁺ is well recognized as an efficient sensitizer, transferring excitation energy to Eu³⁺, whereas La³⁺—with its larger ionic radius and more flexible coordination environment—induces greater lattice distortion and modifies the branching ratios of Eu³⁺ emissions. These contrasting roles highlight the importance of a comparative study between La-based and Gd-based hosts for understanding host–activator interactions. Notably, an unusual dominance of the ⁵D₀ → ⁷F₄ transition has been reported in several La-based hosts; for example, Eu³⁺-doped BaLaGaO₄ and Eu³⁺-doped NaLa₂F₃(CN₂) ₂ exhibit anomalously intense ⁵D₀ → ⁷F₄ emission,^34,35 while (La, Mg)-substituted SrAl₁₂O₁₉ phosphors display pronounced ⁵D₀ → ⁷F₄ photoluminescence under specific La/Mg ratios.³⁶ Such findings suggest unique site-selective excitation mechanisms and emphasize the need for detailed structural–optical correlation studies. Moreover, the influence of host lattice volume—larger for La³⁺ and smaller for Gd³⁺—on concentration quenching, energy transfer, and thermal stability remains insufficiently clarified.

In this work, Ba₂La_1−xVO₆:xEu³⁺ and Ba₂Gd_1−xVO₆:xEu³⁺ (x = 0–30 mol%) phosphors were synthesized via a conventional solid-state reaction method and systematically investigated to establish a clear host-dependent structure–optical relationship. Their crystal structures were refined using X-ray diffraction (XRD) and Rietveld analysis, while scanning electron microscopy (SEM) was employed to examine the microstructural features. Comprehensive photoluminescence (PL), excitation (PLE), decay dynamics, and Judd–Ofelt analyses were performed to elucidate how the local lattice environments of La³⁺ and Gd³⁺ govern Eu³⁺ emission characteristics. In addition, X-ray photoelectron spectroscopy (XPS) was utilized to verify the elemental composition and oxidation state of Eu³⁺ ions in representative high-doping compositions. Distinct from most previously reported Eu³⁺-activated double perovskites, the Ba₂LaVO₆ host stabilizes an abnormal ⁵D₀ → ⁷F₄-dominated deep-red emission and exhibits an unusually high Eu³⁺ solubility without severe concentration quenching up to 30 mol%, whereas Ba₂GdVO₆ shows conventional ⁵D₀ → ⁷F₂-dominated emission with earlier quenching. Furthermore, temperature-dependent PL measurements were carried out to correlate lattice rigidity with thermal quenching behavior and non-radiative activation energy. By directly comparing La-based and Gd-based hosts within the same vanadate double-perovskite framework, this study provides new insight into excitation–emission mechanism control and dopant-tolerant lattice design, offering practical guidelines for the development of high-performance red-emitting phosphors for photonic applications.

2. Experimental

Ba₂La_1−xVO₆:xEu³⁺ and Ba₂Gd_1−xVO₆:xEu³⁺ (x = 0, 2.5, 5, 10, 15, 20, and 30 mol%) phosphor series were synthesized via a conventional solid-state reaction method. High-purity starting materials, including BaCO₃ (99%), La₂O₃ (99.99%), Gd₂O₃ (99.99%), V₂O₅ (99.9%) and Eu₂O₃ (99.9%) were used without further purification. Stoichiometric amounts corresponding to Ba₂M_1−xVO₆:xEu³⁺ (M = La, Gd) compositions were accurately weighed and thoroughly mixed in an agate mortar to achieve compositional homogeneity. The mixed powders were pressed into pellets and placed in alumina crucibles for sintering. The Eu³⁺-doped Ba₂LaVO₆ and Ba₂GdVO₆ samples were calcined at 1350 °C and 1325 °C for 3 h, respectively, followed by natural cooling to room temperature.

Phase identification was performed using powder X-ray diffraction (XRD; D2 PHASER, Bruker Corp., Germany) with Cu Kα radiation (λ = 1.5406 Å) over the 2θ range of 20–80° at a scanning rate of 2° min⁻¹. Rietveld refinement was applied to extract lattice parameters and confirm phase purity. Microstructural features and grain morphology were examined using field-emission scanning electron microscopy (FE-SEM; XL 30S, Philips Corp., Netherlands). The elemental composition and chemical states of the samples were defined using X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Inc. U.S.) Al-Kα (1486.6 eV). Photoluminescence (PL), photoluminescence excitation (PLE), and decay measurements were carried out using a fluorescence spectrometer (FS5, Edinburgh Instruments, UK) equipped with a 450 W ozone-free xenon lamp. Emission and excitation slit widths were kept constant during all measurements to ensure data consistency. For lifetime analysis, short microsecond excitation pulses were generated using pulsed xenon flash lamps (µ_F1 and µ_F2, 5 W and 60 W), and decay signals were recorded using a time-correlated single-photon counting (TCSPC) module. Temperature-dependent PL analysis was conducted in the range of 300–550 K using the FS5 spectrometer equipped with a controlled heating stage. Thermal activation energies (E_a) were extracted from Arrhenius fitting of the normalized emission intensity data. All photoluminescence measurements were repeated at least three times under identical experimental conditions to ensure reproducibility, and the reported trends represent averaged results. All optical characterizations were performed at room temperature.

3. Results and discussions

3.1 XRD, SEM and XPS results

The crystal structure and phase purity of the synthesized Ba₂La_1−xVO₆:xEu³⁺ and Ba₂Gd_1−xVO₆:xEu³⁺ (x = 0, 2.5, 5, 10, 15, 20, and 30 mol%) samples were analyzed by powder X-ray diffraction (XRD) in the 2θ range of 10°–80°, as shown in Fig. 1 and 2, respectively. The diffraction peaks of both series can be well indexed to the orthorhombic double-perovskite structure with space group Pnma, in good agreement with the standard JCPDS card No. 48-1036 reported for Ba₂GdVO₆,²⁸ confirming that the target phase was successfully formed without detectable secondary phases within the sensitivity of the instrument. Importantly, no significant peak shifts or additional impurity reflections were observed up to 30 mol% Eu³⁺ substitution in the Gd-based series, consistent with the very small ionic radius mismatch between Eu³⁺ (r = 0.947 Å, CN = 6) and Gd³⁺ (r = 0.938 Å, CN = 6). However, a slight shift of the diffraction peaks toward lower 2θ angles can be discerned for the La-based samples, which can be attributed to the larger ionic radius of La³⁺ (r = 1.032 Å, CN = 6) compared to Gd³⁺, resulting in a modest lattice expansion. Considering the identical trivalent charge state, Eu³⁺ ions are assumed to substitute La³⁺ or Gd³⁺ sites in the Ba₂LaVO₆ and Ba₂GdVO₆ lattices, respectively. This isovalent substitution is further supported by the absence of secondary phases and the small magnitude of the observed peak shifts, indicating a smooth incorporation of Eu³⁺ into the rare-earth sublattice rather than the V⁵⁺ site.

Fig. 1 X-ray diffraction (XRD) patterns of Ba₂La_1−xVO₆:xEu³⁺ phosphors with x = 0, 2.5, 5, 10, 15, 20, and 30 mol%, confirming single-phase orthorhombic perovskite formation.

Fig. 2 X-ray diffraction (XRD) patterns of Ba₂Gd_1−xVO₆:xEu³⁺ phosphors with x = 0, 2.5, 5, 10, 15, 20, and 30 mol%, indexed to the orthorhombic Pnma structure.

To further validate the structural assignment, Rietveld refinements were carried out for representative compositions of Ba₂LaVO₆ and Ba₂GdVO₆ (Fig. 3a and b). The calculated profiles reproduce the experimental patterns with low residuals, confirming the suitability of the structural models and reaffirming the orthorhombic Pnma symmetry for both hosts. As summarized in Table 1, Ba₂LaVO₆ refined to lattice parameters a = 9.91 Å, b = 7.89 Å, c = 7.38 Å, V = 577.04 ų with residual factors R_p = 6.14%, R_wp = 8.13%, χ² ≈ 2.21, whereas Ba₂GdVO₆ refined to a = 9.83 Å, b = 7.86 Å, c = 7.29 Å, V = 563.25 ų with R_p = 4.33%, R_wp = 5.56%, χ² ≈ 1.66. These values highlight the reliability of the refinements, as both R_p and R_wp remain below 10% with acceptable χ² values close to 2. The comparison clearly shows that the La-based host exhibits a larger unit-cell volume (∼2.45% higher than Ba₂GdVO₆), which can be ascribed to the larger ionic radius of La³⁺ relative to Gd³⁺. Moreover, the deviation among the a, b, and c parameters is slightly more pronounced in Ba₂LaVO₆, reflecting a more anisotropic and distorted framework. In contrast, Ba₂GdVO₆ crystallizes in a more compact and rigid lattice, consistent with its smaller volume and more symmetric configuration. In summary, the combined XRD and Rietveld refinement analyses confirm that both Ba₂LaVO₆:Eu³⁺ and Ba₂GdVO₆:Eu³⁺ phosphors crystallize in a single phase orthorhombic Pnma double perovskite structure, with successful incorporation of Eu³⁺ into the host lattices. The distinct differences in unit-cell volume, degree of orthorhombic distortion, and structural rigidity between the two hosts provide a robust structural basis for explaining their divergent photoluminescence behavior, particularly the contrasting dominance of the ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₄ transitions.

Fig. 3 Rietveld refinement profiles of the undoped double-perovskite hosts refined in the orthorhombic Pnma space group: (a) Ba₂LaVO₆ and (b) Ba₂GdVO₆. Observed, calculated, and difference patterns are shown.

Table 1 Rietveld refinement parameters of Ba₂LaVO₆ and Ba₂GdVO₆

Sample type	Rietveld refinement parameters
Ba2LaVO6	Symmetry	Orthorhombic
	Space group	Pnma
	a (Å)	9.91
	b (Å)	7.89
	c (Å)	7.38
	V (Å^3)	577.04
	Rp	6.14%
	Rwp	8.13%
	χ^2	2.21%
Ba2GdVO6	Symmetry	Orthorhombic
	Space group	Pnma
	a (Å)	9.83
	b (Å)	7.86
	c (Å)	7.29
	V (Å^3)	563.25
	Rp	4.32%
	Rwp	5.56%
	χ^2	1.66%

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Fig. 4(a–d) and 5(a–d) present SEM micrographs of Ba₂La_1−xVO₆:xEu³⁺ and Ba₂Gd_1−xVO₆:xEu³⁺ ceramics (x = 0, 5, 15, 30 mol%), respectively. In Fig. 4(a–d), the La-based series exhibits an aggregated and porous structure composed of irregular morphologies arising from much finer subunits, with indistinct grain boundaries. In the examined Eu³⁺ doping range (0–30 mol%), La-based samples retain their clustered morphology, while no change in the uniform grain size trend is observed. In Fig. 5(a–d), the Gd-based series exhibits a markedly different microstructure, characterized by well-defined, faceted polyhedral grains with clear grain boundaries and a comparatively dense packing. The microstructure remains faceted with increasing Eu³⁺ content, and no significant change in grain size is observed, similar to the La-based structure. In summary, the SEM results indicate that the Gd-based host forms a more consolidated, grain-resolved ceramic microstructure, whereas the La-based host shows a more agglomerated and porous texture under the present synthesis conditions. These microstructural differences may contribute to variations in light scattering and emission extraction, while the dominant differences in emission pathway and concentration tolerance are primarily governed by the host-dependent local lattice environment discussed in the structural and optical sections.

Fig. 4 SEM micrographs of La-based ceramic samples: (a) undoped Ba₂LaVO₆, (b) Ba₂La_0.95VO₆:0.05Eu³⁺, (c) Ba₂La_0.85VO₆:0.15Eu³⁺, and (d) Ba₂La_0.70VO₆:0.30Eu³⁺.

Fig. 5 SEM micrographs of Gd-based ceramic samples: (a) undoped Ba₂GdVO₆, (b) Ba₂Gd_0.95VO₆:0.05Eu³⁺, (c) Ba₂Gd_0.85VO₆:0.15Eu³⁺, and (d) Ba₂Gd_0.70VO₆:0.30Eu³⁺.

X-ray photoelectron spectroscopy (XPS) was employed to verify the elemental composition and oxidation state of europium ions in the representative high-doping concentrations. Fig. 6a and c present the wide-scan (survey) XPS spectra for Ba₂La_1−xVO₆:0.3Eu³⁺ and Ba₂Gd_1−xVO₆:0.15Eu³⁺ samples, respectively. The survey spectra clearly confirm the presence of all constituent elements (Ba, La/Gd, V, O, and Eu) without detectable impurity-related signals, indicating successful incorporation of Eu into both host lattices. The high-resolution Eu 3d core-level spectra of Ba₂La_1−xVO₆:0.3Eu³⁺ and Ba₂Gd_1−xVO₆:0.15Eu³⁺ are shown in Fig. 6b and d, respectively. In both cases, the Eu 3d spectra exhibit characteristic doublet features corresponding to the Eu³⁺ 3d_5/2 and Eu³⁺ 3d_3/2 components, which are consistent with the trivalent Eu³⁺ oxidation state reported in the literature.^11,12 Importantly, no additional components or phase features associated with Eu²⁺ ions are observed, confirming that Eu is stabilized exclusively in the trivalent state in both La-based and Gd-based hosts. The similarity of the Eu 3d binding energies in 30 mol% Eu³⁺ (Ba₂LaVO₆) and 15 mol% Eu³⁺ (Ba₂GdVO₆) further indicates that the local chemical environment of Eu³⁺ is comparable in both lattices, despite their different structural rigidity. Combined with the survey spectra, these results support the substitution of Eu³⁺ ions at the rare-earth (La³⁺/Gd³⁺) sites rather than at the Ba²⁺ or V⁵⁺ positions, in agreement with the isovalent charge state and compatible ionic radii. In addition, the quantitative elemental compositions derived from the XPS survey spectra are summarized in the corresponding tables for the La-based (Fig. 6a) and Gd-based (Fig. 6c) samples. The measured atomic percentages are in good agreement with the nominal stoichiometry within the experimental uncertainty of XPS analysis, supporting the reliable incorporation of Eu³⁺ into the host lattices. Overall, the XPS analysis provides direct evidence for the successful incorporation of Eu³⁺ into the double-perovskite framework without the formation of secondary Eu-containing phases, corroborating the structural and optical results discussed above.

Fig. 6 X-ray photoelectron spectroscopy (XPS) analysis of representative high-doping samples: (a) survey spectrum and elemental composition of Ba₂Gd_0.70VO₆:0.30Eu³⁺, (b) high-resolution Eu 3d spectrum of Ba₂Gd_0.70VO₆:0.30Eu³⁺, (c) survey spectrum and elemental composition of Ba₂Gd_0.85VO₆:0.15Eu³⁺, and (d) high-resolution Eu 3d spectrum of Ba₂Gd_0.85VO₆:0.15Eu³⁺.

3.2 Spectral properties and Judd–Ofelt parameters

Fig. 7a and c shows the excitation spectra of Ba₂LaVO₆:Eu³⁺ (monitored at 696 nm, ⁵D₀ → ⁷F₄) and Ba₂GdVO₆:Eu³⁺ (monitored at 611 nm, ⁵D₀ → ⁷F₂), respectively. The dominant excitation band for Ba₂LaVO₆:Eu³⁺ occurs at 395 nm, attributed to the Eu^{3+ 7}F₀ → ⁵L₆ transition, while weaker shoulders are observed in the 360–385 nm region, and an extremely weak O²⁻ → Eu³⁺ charge transfer band (CTB) between 250–320 nm. This pattern indicates that the population of the ⁵D₀ state proceeds mainly through direct 4f–4f excitation and selectively populates Eu³⁺ sites with a high branching into ⁷F₄, consistent with the abnormal-emission behavior observed in the PL spectra. In contrast, Ba₂GdVO₆:Eu³⁺ displays its strongest excitation at 466 nm (⁷F₀ → ⁵D₂), together with a well-resolved CTB in the UV range. The relative suppression of the 395 nm line and the presence of a strong CTB suggest more efficient host to Eu³⁺ energy transfer and a different local crystal field around Eu³⁺ in the Gd-based lattice compared with the La-based one. Consequently, these results indicate site-selective and host-dependent excitation proceses: monitoring ⁷F₄ (696 nm) highlights near-ultraviolet ⁷F₀ → ⁵L₆ excitation in Ba₂LaVO₆:Eu³⁺, while monitoring ⁷F₂ (611 nm) highlights blue ⁷F₀ → ⁵D₂ excitation in Ba₂GdVO₆:Eu³⁺ with the aid of a stronger charge transfer band (CTB). Practically, Ba₂LaVO₆:Eu³⁺ is most efficiently excited by near-UV light (∼395 nm) and yields deep-red emission with a reinforced ⁵D₀ → ⁷F₄ component, while Ba₂GdVO₆:Eu³⁺ is efficiently excited by blue light (∼465–470 nm) and exhibits the conventional ⁵D₀ → ⁷F₂-dominated orange-red emission.

Fig. 7 PLE spectra of (a) Ba₂LaVO₆:Eu³⁺ (monitored at 696 nm) and (c) Ba₂GdVO₆:Eu³⁺ (monitored at 611 nm), and the corresponding PL emission spectra of (b) Ba₂LaVO₆:Eu³⁺ under 395 nm excitation and (d) Ba₂GdVO₆:Eu³⁺ under 466 nm excitation. Insets show the integrated emission intensity versus Eu³⁺ concentration; (b) 30 mol% for the La-based host and (d) concentration quenching beyond 15 mol% for the Gd-based host.

Fig. 7b and d represent the photoluminescence (PL) spectra of Ba₂LaVO₆:Eu³⁺ and Ba₂GdVO₆:Eu³⁺, recorded under their most efficient excitation wavelengths at 395 nm and 466 nm, respectively. In both hosts, the characteristic Eu³⁺ emission transitions from the ⁵D₀ state are resolved at 591–594 nm (⁵D₀ → ⁷F₁, magnetic-dipole), 615–611 nm (⁵D₀ → ⁷F₂, electric-dipole), 651–654 nm (⁵D₀ → ⁷F₃), and 696–707 nm (⁵D₀ → ⁷F₄). The inset plots in Fig. 7b and d show that the integrated emission intensity of the La-based host increases monotonically with Eu³⁺ concentration up to 30 mol%, whereas the Gd-based host exhibits concentration quenching beyond 15 mol% Eu³⁺, respectively. For Ba₂LaVO₆:Eu³⁺, the ⁵D₀ → ⁷F₄ transition (696 nm) dominates the spectrum (Fig. 7b), exceeding the hypersensitive ⁵D₀ → ⁷F₂ transition—an unusual trend that corresponds to abnormal emission and agrees with the excitation-dependent intensity ratios where I_(⁷F4)/I_(⁷F2) > 1 under 395 nm excitation. In contrast, Ba₂GdVO₆:Eu³⁺ excited at 466 nm (Fig. 7d) exhibits the conventional Eu³⁺ pattern with a dominant ⁵D₀ → ⁷F₂ emission and a weaker ⁵D₀ → ⁷F₄ transition. In this context, consistent with the PLE analysis, the CTB contribution is pronounced in Ba₂GdVO₆:Eu³⁺ but strongly suppressed in Ba₂LaVO₆:Eu³⁺, favoring direct f–f excitation in the latter. This behavior indicates that the La-based lattice can accommodate high Eu³⁺ contents, likely due to its greater structural flexibility and reduced probability of non-radiative cross-relaxation. In contrast, Ba₂GdVO₆:Eu³⁺ exhibits a clear onset of concentration quenching beyond 15 mol%, reflecting enhanced non-radiative energy transfer between closely spaced Eu³⁺ ions in its more rigid and compact lattice framework. This interpretation is further supported by the systematic decrease in the asymmetry ratio R_F2/F1 = I_{(⁵D0 → ⁷F2)}/I_{(⁵D0 → ⁷F1)} with increasing Eu³⁺ concentration, which declines from 7.44 (2.5 mol%) to 2.74 (30 mol%). The reduction in R_F2/F1 suggests a progressive decrease in local asymmetry around Eu³⁺ ions and a relative weakening of the hypersensitive ⁵D₀ → ⁷F₂ transition, consistent with increased ion–ion interactions and earlier concentration quenching. In summary, Ba₂LaVO₆:Eu³⁺ exhibits deeper-red emission with a reinforced ⁵D₀ → ⁷F₄ component and superior dopant tolerance within the investigated concentration range, whereas Ba₂GdVO₆:Eu³⁺ displays conventional orange-red emission dominated by the ⁵D₀ → ⁷F₂ transition but undergoes an earlier onset of concentration quenching. These findings highlight the strong structure–optical interplay in Eu³⁺-activated double perovskites, where lattice flexibility and local symmetry govern both emission characteristics and dopant tolerance.

To quantitatively assess the anomalous emission behavior in Ba₂LaVO₆:Eu³⁺, the PL spectra obtained under different excitation wavelengths are displayed in Fig. 8a, whereas Fig. 8b depicts the emission intensity ratio R_F4/F2 = I_{(⁵D0 → ⁷F4)}/I_{(⁵D0 → ⁷F2)} as a function of excitation wavelength. This ratio provides an effective indicator of excitation-dependent site selectivity related to the local coordination symmetry around Eu³⁺ ions; while the hypersensitive ⁵D₀ → ⁷F₂ transition is strongly enhanced in distorted and asymmetric environments, the ⁵D₀ → ⁷F₄ transition is generally less sensitive to local asymmetry and becomes relatively more pronounced when Eu³⁺ ions occupy more symmetric or rigid lattice sites. Upon excitation in the UV-near-UV region (263–395 nm), R_F4/F2 increases monotonically from 0.80 (263 nm) to 1.55 (395 nm), corresponding to an overall ∼1.9-fold enhancement. This trend indicates that near-UV excitation—particularly the ⁷F₀ → ⁵L₆ transition at 395 nm—preferentially addresses Eu³⁺ sites that are inferred to possess higher local symmetry, thereby reinforcing the branching probability toward the ⁷F₄ manifold. This excitation-dependent behavior is more clearly visualized in Fig. 8b, where the R_F4/F2 ratio increases continuously from 263 to 395 nm and then decreases under blue excitation. As a result, Ba₂LaVO₆:Eu³⁺ exhibits an abnormal emission pattern under 395 nm excitation, where the ⁵D₀ → ⁷F₄ emission (∼700 nm) dominates over the hypersensitive ⁵D₀ → ⁷F₂ transition (∼611 nm), yielding R_F4/F2 > 1. Such dominance of the ⁷F₄ transition is uncommon for Eu³⁺-activated phosphors and suggests that a significant fraction of Eu³⁺ ions reside in relatively symmetric and rigid coordination environments within the La-based lattice, which favor radiative decay into the ⁷F₄ level. In contrast, under blue-region excitation the R_F4/F2 ratio decreases to 1.42 at 416 nm (≈⁷F₀ → ⁵D₃) and further to 1.21 at 466 nm (≈⁷F₀ → ⁵D₂). These excitation pathways populate a broader distribution of Eu³⁺ sites, including more distorted and lower-symmetry environments, thereby enhancing the electric-dipole-allowed ⁵D₀ → ⁷F₂ transition relative to ⁵D₀ → ⁷F₄. Consequently, the emission spectrum gradually reverts toward the conventional Eu³⁺ luminescence pattern dominated by the ⁷F₂ transition. The excitation-dependent evolution of the R_F4/F2 ratio clearly demonstrates that the anomalous ⁵D₀ → ⁷F₄ dominance observed in Ba₂LaVO₆:Eu³⁺ is not an intrinsic characteristic of Eu³⁺ emission but rather a site-selective phenomenon governed by excitation wavelength and the symmetry and rigidity of the La-based host lattice.

Fig. 8 (a) PL emission spectra of Ba₂LaVO₆:Eu³⁺ phosphors recorded under different excitation wavelengths, (b) integrated emission intensity of the ⁵D₀ → ⁷F₄ transition as a function of excitation wavelength.

Judd–Ofelt theory provides a quantitative framework for analyzing the intensity of electron transitions within the 4f orbitals of rare-earth ions. It defines three key parameters, Ω_J (J = 2, 4, 6), to characterize their spectral behavior.^37,38 For Eu³⁺, these parameters are extracted from the emission spectrum via eqn (1):^39–46

where I₁ and I_J are the integrated intensities of the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F_J transitions respectively, V₁ and V_J their transition frequencies, S_MD = 9.6 × 10⁻⁴² esu² cm² the magnetic dipole line strength, e = 4.803 × 10⁻¹⁰ esu the elementary charge, n the refractive index, |〈J‖U^J‖J′〉|² the double reduced matrix elements for unit tensor operators, J and J′ are the total angular momentum of the initial and final states, respectively. Only the electric dipole (ED) transitions ⁵D₀ → ⁷F₂ (U² = 0.0032) and ⁵D₀ → ⁷F₄ (U⁴ = 0.0023), contribute nonzero matrix elements; the ⁵D₀ → ⁷F₆ (U⁶ = 0.0002) band is typically too weak in the PL spectrum and may be omitted with negligible impact on the overall Ω_J values.^44–46 Finally, the spontaneous transition probability (A) is proportional to the corresponding dipole strength and can be expressed in terms of the Judd–Ofelt parameters, as given by eqn (2):^39–46where h and n represent Planck's constant and the refractive index, respectively. The local field corrections χ_ED and χ_MD for the ED and MD transitions are n(n² + 2)²/9 and n³, respectively. S_ED and S_MD are electric dipole and magnetic dipole line strengths (esu² cm²). The electric dipole line strength (S_ED) related to JO parameters can be determined via eqn (3):

The refractive index (n) values can be estimated from the Lorenz–Lorentz formula eqn (4):^44–46

where l_i refers to the atomic number of each element present in the compound's chemical formula, M stands for the molar mass, r_i denotes the specific refraction of the elements, and ρ presents the density of the compound, calculated using the formula ρ = 1.661kM/Σl_ir_i, k is the cell packing coefficient. The refractive indexes for Ba₂LaVO₆ and Ba₂GdVO₆ were determined 2.306 and 2.277, respectively. The Judd–Ofelt intensity parameters (Ω₂ and Ω₄) for Ba₂LaVO₆:Eu³⁺ and Ba₂GdVO₆:Eu³⁺, derived from the emission spectra, are summarized in Table 2. The Ω₂ parameter reflects the local asymmetry and covalency around Eu³⁺ ions, whereas Ω₄ is associated with the polarizability of the ligand field and the rigidity or flexibility of the host lattice.^39–46 In the Ba₂LaVO₆:Eu³⁺ series, Ω₂ shows a slight decrease from 1.824 to 1.651 × 10⁻²⁰ cm² with increasing Eu³⁺ concentration, while Ω₄ decreases from 7.130 to 6.131 × 10⁻²⁰ cm². The relatively low Ω₂ values indicate a comparatively symmetric and weakly covalent Eu³⁺ environment, whereas the persistently higher Ω₄ values signify a highly polarizable and dynamically flexible ligand field. This unusual Ω₄ > Ω₂ relationship directly correlates with the abnormal enhancement of the ⁵D₀ → ⁷F₄ transition, accompanied by suppression of the hypersensitive ⁵D₀ → ⁷F₂ channel. In contrast, Ba₂GdVO₆:Eu³⁺ phosphor series exhibit significantly higher Ω₂ values at low Eu³⁺ concentrations (up to 10.080 × 10⁻²⁰ cm²), followed by a pronounced decrease to 3.602 × 10⁻²⁰ cm² as the dopant content increases. This evolution indicates a progressive transition from a highly distorted local environment toward higher apparent centrosymmetry, consistent with the observed reduction of the asymmetry ratio I_{(⁵D0 → ⁷F2)}/I_{(⁵D0 → ⁷F1)} from 7.44 to 2.74. Meanwhile, the Ω₄ values (4.786–5.086 × 10⁻²⁰ cm²) remain lower than those of the La-based host, reflecting a more rigid and less polarizable lattice framework. As a result, the conventional ⁵D₀ → ⁷F₂ electric-dipole transition dominates in Ba₂GdVO₆:Eu³⁺. The pronounced decrease of Ω₂ with increasing Eu³⁺ concentration in the Gd-based host may be attributed to a combined effect of statistical site occupation and enhanced Eu–Eu interactions in the compact lattice, leading to effective crystal-field averaging. Under such conditions, Ω₂ should be regarded as an effective phenomenological parameter rather than a strictly site-specific descriptor, in agreement with the concurrent reductions in asymmetry ratio, decay lifetime, and quantum efficiency. Beyond this static interpretation, the results point to an excitation-selective and site-dependent emission mechanism. In Ba₂LaVO₆:Eu³⁺, near-UV excitation (⁷F₀ → ⁵L₆) efficiently populates the ⁵D₀ level within a polarizable lattice, selectively stabilizing radiative decay toward the ⁷F₄ manifold and giving rise to the observed abnormal emission. In contrast, the increasingly rigid and centrosymmetric environment in Ba₂GdVO₆:Eu³⁺ energetically favors the electric-dipole-allowed ⁵D₀ → ⁷F₂ transition while simultaneously enhancing non-radiative Eu–Eu energy transfer at higher dopant levels.

Table 2 Judd–Ofelt parameters (Ω₂, Ω₄) and branching ratios (β) for Ba₂LaVO₆:Eu³⁺ and Ba₂GdVO₆:Eu³⁺ phosphor series

Eu^3+ conc. (x mol%)	Ba2La1−xVO6:xEu^3+				Ba2Gd1−xVO6:xEu^3+
	Ω2 (10^−20 cm^2)	Ω4 (10^−20 cm^2)	Eu^3+ transitions	β (%)	Ω2 (10^−20 cm^2)	Ω4 (10^−20 cm^2)	Eu^3+ transitions	β (%)
2.5	1.824	7.130	^5D0 → ^7F1	19.64	10.080	4.786	^5D0 → ^7F1	9.42
			^5D0 → ^7F2	27.35			^5D0 → ^7F2	74.23
			^5D0 → ^7F4	53.01			^5D0 → ^7F4	16.35
5	1.777	6.447	^5D0 → ^7F1	20.84	7.467	5.288	^5D0 → ^7F1	11.42
			^5D0 → ^7F2	28.29			^5D0 → ^7F2	66.67
			^5D0 → ^7F4	50.87			^5D0 → ^7F4	21.91
10	1.666	6.730	^5D0 → ^7F1	20.75	6.207	6.240	^5D0 → ^7F1	12.32
			^5D0 → ^7F2	26.39			^5D0 → ^7F2	59.79
			^5D0 → ^7F4	52.86			^5D0 → ^7F4	27.89
15	1.699	6.405	^5D0 → ^7F1	21.17	5.739	6.208	^5D0 → ^7F1	12.92
			^5D0 → ^7F2	27.47			^5D0 → ^7F2	57.98
			^5D0 → ^7F4	51.36			^5D0 → ^7F4	29.10
20	1.738	6.315	^5D0 → ^7F1	21.20	5.385	5.242	^5D0 → ^7F1	14.07
			^5D0 → ^7F2	28.12			^5D0 → ^7F2	59.20
			^5D0 → ^7F4	50.68			^5D0 → ^7F4	26.73
30	1.651	6.131	^5D0 → ^7F1	21.83	3.602	5.086	^5D0 → ^7F1	17.67
			^5D0 → ^7F2	27.50			^5D0 → ^7F2	49.74
			^5D0 → ^7F4	50.67			^5D0 → ^7F4	32.59

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To contextualize these findings, Table 3 compares the Ω₂ and Ω₄ parameters obtained in this study with representative Eu³⁺-doped hosts reported in the literature.^47–60 Notably, Ba₂La_0.70VO₆:0.30Eu³⁺ (Ω₂ = 1.651, Ω₄ = 6.131 × 10⁻²⁰ cm²) falls into the class of systems where Ω₄ > Ω₂. Similar behavior has been documented for other compounds such as CoNb₂O₆:Eu³⁺, Li₂Zr(PO₄)₂:Eu³⁺, LiZnPO₄:Eu³⁺, and LiCdPO₄:Eu³⁺.^47–54 The occurrence of Ω₄ > Ω₂ is relatively uncommon for Eu³⁺ systems, and its manifestation in Ba₂LaVO₆:Eu³⁺ is particularly relevant because it directly accounts for the abnormal dominance of the ⁵D₀ → ⁷F₄ transition. In contrast, Ba₂GdVO₆:Eu³⁺ (15 mol%) follows the more conventional Ω₂ > Ω₄ regime (Ω₂ = 5.739, Ω₄ = 6.208 × 10⁻²⁰ cm²), consistent with strong local asymmetry and the dominance of the hypersensitive ⁵D₀ → ⁷F₂ transition. This trend aligns well with numerous reported Eu³⁺-doped systems such as Ca₂GdVO₆:Eu³⁺, Sr₂GdVO₆:Eu³⁺, Sr₂SiO₄:Eu³⁺, Sr₂MgSi₂O₇:Eu³⁺, and BaWO₄:Eu³⁺,^55–60 where high Ω₂ values correlate with conventional red emission behavior. Thus, the Ω₄-dominated La-based host favors anomalous ⁵D₀ → ⁷F₄ emission via lattice polarizability, while the Ω₂-dominated Gd-based host exhibits conventional crystal-field-controlled emission.

Table 3 Comparison of Judd–Ofelt intensity parameters (Ω₂, Ω₄) for Eu³⁺-doped phosphors, highlighting Ω₄-dominated and Ω₂-dominated emission regimes

Phosphor	Eu^3+ concentration (%)	Ω2 (10^−20 cm^2)	Ω4 (10^−20 cm^2)	Ref.
Ba2LaVO6:Eu^3+	30	1.651	6.131	This study
Ba2GdVO6:Eu^3+	15	5.739	6.208	This study
Ca2La3(SiO4)3F:Eu^3+	1	0.621	1.190	47
PbNb2O6:Eu^3+	6	2.251	2.604	48
Ca3NbGa3Si2O14:Eu^3+	5	1.277	2.208	49
KCaBi(PO4)2:Eu^3+	21	1.773	2.043	50
K2Zr(PO4)2:Eu^3+	2	1.520	1.760	51
TiO2:Eu^3+	5	0.210	0.560	52
Li2Zr(PO4)2:Eu^3+	2	0.230	2.140	53
BaTa2O6:Eu^3+, B^3+	10	1.506	1.541	54
Ca2GdVO6:Eu^3+	15	10.263	2.385	55
Sr2GdVO6:Eu^3+	15	4.799	2.422	55
Sr2SiO4:Eu^3+	9	5.620	2.750	56
Sr2MgSi2O7:Eu^3+	5	4.240	1.040	57
Na2ZrO3:Eu^3+	2	5.117	1.593	58
KZr2(PO4)3:Eu^3+	2	4.650	1.044	59
BaWO4:Eu^3+	16	3.140	1.710	60

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The branching ratio (β) is an important optical parameter that provides insight into the radiative efficiency of Eu³⁺-doped phosphors and can be determined from the radiative transition probability (A_r or A(J, J′)) and the total radiative transition probability (ΣA(J, J′) using eqn (5):

In summary, the branching ratio (β) analysis provides clear evidence of the strong host-lattice dependence of the optical behavior in Ba₂LaVO₆:Eu³⁺ and Ba₂GdVO₆:Eu³⁺ phosphors. In Ba₂LaVO₆:Eu³⁺, relatively low β values (∼26–28%) suppress the ⁵D₀ → ⁷F₂ transition but still sustain red luminescence, accompanied by an unusual reinforcement of the ⁵D₀ → ⁷F₄ transition. In contrast, Ba₂GdVO₆:Eu³⁺ exhibits significantly higher β values (49.74–74.23%), ensuring efficient electric-dipole transitions and highlighting its potential as a promising candidate for solid-state laser applications (β ≥ 50).^44–46 When considered together with the monotonic decrease in Ω₂ and the concurrent reduction in the spectral asymmetry ratio, these results indicate that Eu³⁺ ions in the Gd-based host undergo a doping-induced evolution toward increasingly symmetric local environments. This evolution stabilizes the conventional ⁵D₀ → ⁷F₂ emission mechanism while simultaneously accelerating concentration quenching through enhanced Eu–Eu interactions. In contrast, the La-based host maintains higher lattice polarizability and dynamic flexibility, thereby promoting the unusual enhancement of the ⁵D₀ → ⁷F₄ transition and yielding distinct luminescence fingerprints.

The decay profiles for Ba₂LaVO₆:Eu³⁺ and Ba₂GdVO₆:Eu³⁺ phosphor series with excitation 395, 466 nm and emission at 696, 611 nm, are illustrated in Fig. 9a and b, respectively. The decay profiles of the phosphors can be fitted to a mono-exponential or double-exponential model as described by eqn (6):⁶¹

where, I_t is the PL intensity at time t after excitation, I₀ is the background intensity, and l_i, τ_i denote the amplitude and lifetime of the i-th decay component. For mono-exponential and double-exponential decay models are n = 1 and n = 2, respectively. In this case, τ₁ and τ₂ represent the long and short lifetime components, respectively, with I₁ and I₂ being their associated intensities. To provide a quantitative comparison, the observed or average lifetime τ_avg or τ can be found using eqn (7):⁶¹

Fig. 9 Luminescence decay curves of (a) Ba₂LaVO₆:Eu³⁺ and (b) Ba₂GdVO₆:Eu³⁺ phosphors, excited at 395 and 466 nm and monitored at 696 nm (⁵D₀ → ⁷F₄) and 611 nm (⁵D₀ → ⁷F₂), respectively.

The corresponding average lifetimes (τ) are tabulated in Table 4. For the Ba₂LaVO₆:Eu³⁺ system, the lifetime values show only a minor decrease from 270 µs at 2.5 mol% to 233 µs at 30 mol%, resulting in closely overlapping decay profiles. This nearly stable decay behavior indicates that non-radiative cross-relaxation among Eu³⁺ ions remains limited even at high dopant concentrations, consistent with the PL results, which show the absence of severe concentration quenching up to 30 mol% and the sustained dominance of the ⁵D₀ → ⁷F₄ transition (Fig. 7b). By contrast, Ba₂GdVO₆:Eu³⁺ exhibits a markedly different decay behavior. At low Eu³⁺ concentrations (2.5–5 mol%), the τ values are exceptionally long (627–652 µs), reflecting efficient radiative relaxation. However, with increasing Eu³⁺ content, the lifetimes progressively shorten, reaching 196 µs at 30 mol%. This pronounced lifetime reduction correlates well with the PL spectra (Fig. 7d), where concentration quenching becomes evident beyond 15 mol% Eu³⁺. Importantly, this decay trend is also consistent with the Judd–Ofelt (J–O) analysis and the evolution of the spectral asymmetry ratio. Both the Ω₂ parameter and the I_{(⁵D0 → ⁷F2)}/I_{(⁵D0 → ⁷F1)} ratio decrease significantly with increasing Eu³⁺ concentration, indicating a gradual shift toward more symmetric and less covalent local environments around Eu³⁺ ions. Such a structural evolution weakens hypersensitive electric-dipole transitions and facilitates Eu–Eu multipolar interactions, thereby enhancing non-radiative energy transfer and cross-relaxation processes. In this context, the decay dynamics are fully consistent with the steady-state emission behavior: Ba₂LaVO₆:Eu³⁺ exhibits relatively stable lifetimes and a high tolerance to Eu³⁺ doping, supporting persistent abnormal ⁵D₀ → ⁷F₄-dominated emission, whereas Ba₂GdVO₆:Eu³⁺ shows initially long-lived emission followed by a rapid lifetime shortening at higher dopant levels, accounting for the earlier onset of concentration quenching and the conventional ⁵D₀ → ⁷F₂-dominated emission pattern. Thus, the decay analysis not only confirms the host-dependent quenching mechanisms but also provides dynamic evidence for distinct emission branching pathways in La-based and Gd-based double-perovskite hosts.

Table 4 Calculated radiative lifetimes (τ_r), observed lifetimes (τ), and calculated radiative quantum efficiencies (η_QE for Ba₂MVO₆:Eu³⁺ (M = La, Gd) phosphor series

Eu^3+ conc. (x mol%)	Ba2La1−xVO6:xEu^3+			Ba2Gd1−xVO6:xEu^3+
	τr (µs)	τ (µs)	ηQE (%)	τr (µs)	τ (µs)	ηQE (%)
2.5	1099	270	24.53	556	627	100<
5	1166	263	22.56	674	652	96.67
10	1161	256	22.02	727	614	84.39
15	1185	247	20.81	763	557	73.08
20	1186	247	20.80	830	362	43.66
30	1221	233	19.08	1042	196	18.77

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The radiative quantum efficiency (η_QE) calculated within the Judd–Ofelt (J–O) framework provides a theoretical estimate of the radiative contribution to the de-excitation of Eu³⁺ ions following optical excitation. According to eqn (8):

where A_r and A_nr are the radiative and non-radiative decay rates, respectively, τ_r is the radiative lifetime calculated from J–O analysis, and τ is the experimentally observed lifetime. As summarized in Table 4, the Ba₂LaVO₆:Eu³⁺ series exhibits calculated radiative lifetimes (τ_r) in the range of ∼1099–1221 µs, whereas the observed lifetimes (τ) vary between 270 and 233 µs. This results in relatively low calculated radiative quantum efficiencies (∼19–25%). The systematic decrease in η_QE with increasing Eu³⁺ concentration (from 24.5% at 2.5 mol% to 19.1% at 30 mol%) indicates enhanced non-radiative energy transfer among Eu³⁺ ions at higher dopant levels. This trend is consistent with the photoluminescence (PL) and decay results, where the Ba₂LaVO₆:Eu³⁺ host accommodates high Eu³⁺ concentrations without abrupt quenching, yet exhibits a gradual reduction in radiative efficiency due to cross-relaxation and defect-assisted non-radiative processes. Importantly, the relatively low Ω₂ values obtained from J–O analysis, together with the suppressed asymmetry ratio, confirm that Eu³⁺ ions in the La-based host predominantly occupy more symmetric local environments. Such environments favor stable emission dominated by the unusual ⁵D₀ → ⁷F₄ transition, albeit with modest calculated radiative efficiency.

In contrast, the Ba₂GdVO₆:Eu³⁺ series shows significantly shorter calculated radiative lifetimes (τ_r ≈ 556–1042 µs), while the observed lifetimes (τ ≈ 627 µs at 2.5 mol% Eu³⁺) are initially comparable to or even exceed τ_r. Consequently, the calculated radiative quantum efficiency approaches unity at low Eu³⁺ concentrations (≈100% at 2.5 mol% and ≈97% at 5 mol%), indicating highly efficient radiative relaxation in the Gd-based host. However, with increasing Eu³⁺ content, η_QE decreases sharply (84% at 10 mol%, 74% at 15 mol%, 44% at 20 mol%, and ∼19% at 30 mol%), reflecting strong concentration quenching. This pronounced efficiency loss correlates with the marked decrease in Ω₂ and the spectral asymmetry ratio, demonstrating that Eu³⁺ ions progressively evolve toward more symmetric and rigid local environments at higher doping levels. Such structural evolution enhances Eu–Eu multipolar interactions and facilitates non-radiative energy migration, leading to a rapid decline in calculated radiative quantum efficiency. In summary, the calculated radiative quantum efficiency analysis highlights a clear host-dependent contrast: Ba₂LaVO₆:Eu³⁺ supports higher Eu³⁺ incorporation with moderate but relatively stable η_QE (∼20%), whereas Ba₂GdVO₆:Eu³⁺ exhibits near-unity η_QE at low doping but undergoes rapid efficiency degradation at higher concentrations due to symmetry-driven concentration quenching. This dual behavior is fully consistent with the PL, Judd–Ofelt, and decay analyses, confirming that lattice rigidity and local crystal-field evolution critically govern the balance between radiative and non-radiative energy dissipation in these double perovskite phosphors.

3.3 Thermal stability of photoluminescence

Fig. 10(a–d) illustrates the temperature-dependent photoluminescence (PL) behavior of the Eu³⁺ emission for Ba₂LaVO₆:Eu³⁺ (30 mol%) and Ba₂GdVO₆:Eu³⁺ (15 mol%) phosphors measured in the 300–550 K range. These specific compositions were selected because they correspond to the highest Eu³⁺ concentrations exhibiting maximum or near-maximum emission intensity before the onset of severe concentration quenching in each host lattice, thereby representing the optimal operating regime for thermal stability assessment. The normalized integrated intensities (I/I₃₀₀) were analyzed using the Arrhenius-type relation (eqn (9)):^62–71where C denotes the pre-exponential factor associated with the concentration of thermally activated non-radiative centers, E_a is the activation energy for thermal quenching, and k is the Boltzmann constant. For the Ba₂LaVO₆:Eu³⁺ (30 mol%) phosphor—where the ⁵D₀ → ⁷F₄ transition dominates—the PL intensity decreases gradually with increasing temperature, retaining about 41% of its room-temperature value at 550 K (Fig. 10a). The characteristic temperatures were interpolated from the normalized emission intensities indicated in the inset of Fig. 10a, yielding T_0.9 ≈ 332 K and T_0.5 ≈ 516 K. Arrhenius fitting (Fig. 10b) yields activation energy of E_a ≈ 0.177 eV and a C parameter of ≈53, indicating a moderate thermal barrier accompanied by a relatively high probability of non-radiative processes. The more pronounced intensity drop above ∼430–470 K (Fig. 10a) may be attributed to enhanced phonon coupling within the La–O sublattice and migration-assisted quenching at high Eu³⁺ concentrations. Nevertheless, the La-based host maintains efficient emission up to this temperature range, retaining ∼72% of its initial intensity at ∼430 K and ∼62% at 470 K, confirming that its flexible lattice supports radiative transitions over a wide temperature window. By comparison, the Ba₂GdVO₆:Eu³⁺ (15 mol%) phosphor exhibits superior thermal robustness (Fig. 10c). Its integrated PL intensity decreases more slowly with temperature, retaining approximately 57% of the initial emission at 550 K. Based on the normalized intensity values shown in the inset of Fig. 10c, the interpolated T_0.9 ≈ 364 K, while no half-intensity point is reached below 550 K, reflecting improved thermal endurance. Arrhenius analysis (Fig. 10d) yields a slightly higher activation energy (E_a ≈ 0.190 eV) and a lower C value (≈42), suggesting reduced involvement of thermally activated non-radiative centers compared with the La-based system. Consistently, the Gd-based phosphor retains ∼78% of its room-temperature intensity at ∼423 K (Fig. 10c), providing a comparable benchmark reference that highlights its gentler quenching slope and improved high-temperature endurance relative to the La-based system. This enhanced stability is attributed to the more compact lattice and stronger Gd–O bonding, which effectively suppress multiphonon relaxation and defect-assisted quenching. Notably, the moderate activation energy combined with a larger C factor in Ba₂LaVO₆:Eu³⁺ indicates that thermal quenching is governed not solely by barrier height but also by an increased probability of phonon-assisted non-radiative pathways. This interpretation is consistent with the Judd–Ofelt analysis, where higher Ω₄ values point to enhanced lattice polarizability and dynamic flexibility. Such an environment favors selective radiative relaxation into the ⁷F₄ level at room temperature, yet becomes increasingly susceptible to thermally activated multiphonon coupling at elevated temperatures.

Fig. 10 Temperature-dependent photoluminescence behavior and Arrhenius analysis of PL intensity as a function of temperature between 300–550 K, (a) Ba₂LaVO₆:Eu³⁺ (30 mol%, 696 nm) and (c) Ba₂GdVO₆:Eu³⁺ (15 mol%, 611 nm) phosphors; (a), (c) the inset figures display the corresponding normalized intensity values (I/I₀) at selected temperatures, and (b), (d) Arrhenius plots of ln[(I/I₀)⁻¹ − 1] versus 1/T used to extract the activation energy (E_a) and pre-exponential factor (C), respectively.

Overall, Ba₂GdVO₆:Eu³⁺ exhibits a slower quenching rate than Ba₂LaVO₆:Eu³⁺, consistent with its higher E_a and smaller C parameter. Although both phosphors display comparable half-intensity temperatures in the range of ∼510–550 K, the Gd-based system shows a gentler high-temperature decay slope, confirming its superior thermal stability. These results demonstrate that the C parameter—reflecting the probability of defect-mediated non-radiative processes—is as critical as E_a in determining thermal quenching dynamics. The contrasting thermal behaviors correlate well with the structural and optical characteristics discussed in Sections 3.1–3.2. Rietveld refinement reveals that Ba₂LaVO₆ possesses a larger unit-cell volume (577.0 ų) than Ba₂GdVO₆ (563.3 ų), indicative of higher lattice polarizability and flexibility, which facilitate higher Eu³⁺ solubility but stronger phonon coupling. Conversely, the more compact Ba₂GdVO₆ lattice, characterized by stronger Gd–O bonding and reduced lattice relaxation, suppresses phonon-assisted quenching, yielding enhanced thermal stability (I₅₅₀/I₃₀₀ = 0.57). Judd–Ofelt parameters further support this trend: higher Ω₄ values in Ba₂LaVO₆:Eu³⁺ reflect a polarizable yet thermally sensitive environment, whereas lower Ω₄ in Ba₂GdVO₆:Eu³⁺ signifies a rigid lattice that preserves the conventional ⁵D₀ → ⁷F₂-dominated emission. To place these thermal stability results in a broader context, Table 5 compares the activation energies and thermal quenching behavior of the present Ba₂LaVO₆:Eu³⁺ and Ba₂GdVO₆:Eu³⁺ phosphors with those of representative Eu³⁺-activated red-emitting phosphors reported in the literature.^62–71 As summarized in Table 5, the present Ba₂LaVO₆:Eu³⁺ and Ba₂GdVO₆:Eu³⁺ phosphors exhibit activation energies of ∼0.18 and ∼0.19 eV, respectively, together with high emission retention at elevated temperature. Notably, the integrated intensity retained at 423 K reaches ∼72% for Ba₂LaVO₆:Eu³⁺ (30 mol%) and ∼78% for Ba₂GdVO₆:Eu³⁺ (15 mol%), values that are comparable to or exceed those reported for many oxide-based and phosphate-based Eu³⁺ phosphors, even though the present systems operate at relatively high Eu³⁺ concentrations. This comparison highlights that the balanced lattice rigidity and phonon coupling inherent to the double-perovskite framework enable competitive thermal robustness while preserving high dopant tolerance, positioning these materials favorably among reported Eu³⁺-activated red phosphors.

Table 5 Comparison of thermal stability parameters (activation energy and intensity retention at 423 K) for Eu³⁺-activated red-emitting phosphors

Phosphor	Eu^3+ conc. (mol%)	λex (nm)	Ea (eV)	I/I0 (%) at 423 K	Reference
Ba2LaVO6:Eu^3+	30	397	∼0.18	∼72	This work
Ba2GdVO6:Eu^3+	15	397	∼0.19	∼78	This work
Na2SrMg(PO4)2:Eu^3+	9	395	0.20	71	62
LiSrVO4:Eu^3+	9	326	0.28	46	63
LiSrVO4:Eu^3+, 0.09Na^+	9	326	0.29	69	63
Ba2CaZn2Si6O17:Eu^3+	9	395	0.17	69	64
Ca3Al2Ge3O12:Eu^3+	40	394	0.16	∼79	65
Na3Sc2(PO4)3:Eu^3+	35	394	∼0.23	∼73	66
Y2(MoO4)3:Eu^3+	7	395	0.31	∼58	67
Ba3Lu4O9:Eu^3+	25	396	0.21	64	68
Ca2GdSbO6:Eu^3+	50	464	∼0.17	73	69
Sr2LaTaO6:Eu^3+	20	394	0.26	73	70
Ca2GdNbO6:Eu^3+	40	465	∼0.17	∼72	71
Ca2GdTaO6:Eu^3+	40	465	∼0.17	∼76	72

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3.4 Colorimetric analysis

The colorimetric characteristics of Ba₂LaVO₆:Eu³⁺ and Ba₂GdVO₆:Eu³⁺ phosphors were analyzed using the CIE 1931 chromaticity diagram (Fig. 11a and b). The corresponding chromaticity coordinates (x, y), color purity (CP), and correlated color temperature (CCT) values are summarized in Table 6. These parameters provide quantitative insight into the emission hue, spectral saturation, and perceived warmth of the emitted light—key factors for lighting and display applications. The color purity, which expresses the degree of spectral saturation relative to ideal monochromatic emission, was calculated using eqn (10):^71–74where (x_i, y_i) = (0.313, 0.329) are the coordinates of the standard white point, and (x_d, y_d) denote those of the dominant wavelength. The correlated color temperature (CCT), representing the perceptual warmth or coolness of the emission, was estimated using McCamy's empirical eqn (11):⁷⁵

CCT = −449n³ + 3525n² − 6823n + 5520.33 (11)

where n = (x − x_i)/(y − y_i). The colorimetric analysis reveals distinct host-dependent behaviors (Fig. 11 and Table 6). For Ba₂LaVO₆:Eu³⁺, the chromaticity coordinates (x ≈ 0.625–0.636, y ≈ 0.363–0.375) fall into the orange-red region, with color purity values around 75–78%. This is somewhat unexpected considering the abnormal reinforcement of the ⁵D₀ → ⁷F₄ transition in this host, which would suggest a deeper red output. The discrepancy arises because the 700 nm emission band lies in a region of low photopic eye sensitivity, thus contributing less to the CIE chromaticity than the stronger 611 nm ⁵D₀ → ⁷F₂ transition. As a result, despite the unusual spectral dominance of ⁵D₀ → ⁷F₄, the La-host series appears shifted toward orange-red in the CIE diagram. In contrast, Ba₂GdVO₆:Eu³⁺ phosphors exhibit CIE coordinates that move further into the red region (x ≈ 0.634–0.668, y ≈ 0.332–0.365) with substantially higher color purity values (90–99%) across most doping levels. This behavior is consistent with the conventional dominance of the ⁵D₀ → ⁷F₂ transition, which falls in the eye's region of high sensitivity, thereby yielding more saturated red coordinates in the CIE diagram even though the ⁵D₀ → ⁷F₄ contribution is weaker. At higher Eu³⁺ concentrations, multipole–multipole interactions and concentration quenching broaden the emission, leading to a slight reduction in color purity (e.g., 90.35% at 30 mol% Eu³⁺). The CCT values for both hosts remain below 3000 K, confirming their classification as warm-light phosphors. Ba₂LaVO₆:Eu³⁺ exhibits lower CCT values (1745–1926 K), reflecting a warmer but less saturated orange-red emission. By contrast, Ba₂GdVO₆:Eu³⁺ yields somewhat higher CCT values (1886–3012 K) due to the stronger ⁵D₀ → ⁷F₂ contribution, yet still remains within the warm-red category. Overall, these findings demonstrate that while Ba₂LaVO₆:Eu³⁺ provides a unique example of spectral abnormality (⁵D₀ → ⁷F₄ enhancement) that is not fully captured in CIE space, Ba₂GdVO₆:Eu³⁺ delivers visually deeper red coordinates with higher color purity, underlining the complementary optical advantages of La-based and Gd-based double perovskite hosts. Furthermore, the photographs taken under 365 nm UV excitation (the insets of Fig. 11a and b) provide visual confirmation of the colorimetric analysis: Ba₂LaVO₆:Eu³⁺ exhibits a bright orange-red glow, whereas Ba₂GdVO₆:Eu³⁺ displays a deeper and more saturated red emission, consistent with the chromaticity coordinates and color purity trends summarized in Table 6. The complementary optical behavior of these La-based and Gd-based dual perovskites highlights how lattice rigidity and spectral selectivity co-shape their color performance under near-ultraviolet excitation.

Table 6 CIE color coordinates, color purities and CCT parameters for Ba₂LaVO₆:Eu³⁺ (λ_ex = 396 nm, λ_em = 596 nm), Ba₂GdVO₆:Eu³⁺ (λ_ex = 466 nm, λ_em = 611 nm) phosphor series

Eu^3+ con. (x mol%)	Ba2La1−xVO6:xEu^3+				Ba2Gd1−xVO6:xEu^3+
	CIE coordinates		Color purity (%)	CCT (K)	CIE coordinates		Color purity (%)	CCT (K)
	x	y			x	y
2.5	0.6305	0.3690	76.34	1821	0.6670	0.3328	98.92	2971
5	0.6250	0.3746	75.22	1745	0.6579	0.3418	96.44	2562
10	0.6341	0.3655	77.10	1881	0.6628	0.3369	97.77	2773
15	0.6337	0.3659	77.01	1874	0.6679	0.3320	99.17	3012
20	0.6362	0.3634	77.54	1922	0.6548	0.3449	95.61	2443
30	0.6364	0.3632	77.58	1926	0.6343	0.3652	90.35	1886

---

Fig. 11 CIE chromaticity diagrams and UV lamp photographs under 365 nm excitation for (a) Ba₂LaVO₆:Eu³⁺ and (b) Ba₂GdVO₆:Eu³⁺ phosphors.

4. Conclusion

This work presents a systematic comparison of Ba₂LaVO₆:Eu³⁺ and Ba₂GdVO₆:Eu³⁺ double perovskite phosphors, clarifying how host-lattice rigidity governs Eu³⁺ emission behavior, dopant tolerance, and thermal stability. X-ray diffraction and Rietveld refinement confirm that both systems crystallize in the orthorhombic Pnma structure, while SEM reveals distinct host-dependent microstructural characteristics, consistent with the relatively flexible La-based lattice and the more rigid Gd-based framework. XPS analysis further verifies the exclusive presence of Eu³⁺ in representative high-doping compositions, confirming successful lattice incorporation without secondary phases. Owing to its larger unit-cell volume and higher lattice polarizability, Ba₂LaVO₆:Eu³⁺ stabilizes an abnormal emission process dominated by the ⁵D₀ → ⁷F₄ transition under near-UV excitation, accompanied by high Eu³⁺ solubility up to 30 mol% without severe concentration quenching. This behavior is supported by Ω₄ > Ω₂ Judd–Ofelt parameters, excitation-dependent branching ratios, nearly invariant decay lifetimes, and moderate radiative quantum efficiency. Despite its unconventional emission behavior, the La-based phosphor retains ∼41% of its room-temperature intensity at 550 K, indicating robust thermal stability. In contrast, Ba₂GdVO₆:Eu³⁺ follows the conventional Eu³⁺ emission scheme dominated by the hypersensitive ⁵D₀ → ⁷F₂ transition. The rigid lattice enables high radiative efficiency and excellent color purity at low dopant concentrations, but enhanced Eu–Eu interactions lead to concentration quenching beyond ∼15 mol%. The higher activation energy and lower pre-exponential factor derived from thermal quenching analysis indicate reduced phonon-assisted non-radiative losses and superior intrinsic lattice stability relative to the La-based host. CIE chromaticity analysis confirms that the La-based host yields a warm orange-red emission with moderate color purity despite abnormal ⁵D₀ → ⁷F₄ reinforcement, while the Gd-based host delivers visually deeper red emission with higher color purity owing to the conventional dominance of the ⁵D₀ → ⁷F₂ transition. Overall, this comparative study establishes host-lattice rigidity as a decisive parameter controlling emission branching, dopant tolerance, and thermal quenching in Eu³⁺-activated Ba₂MVO₆ (M = La, Gd) phosphors. The La-based hosts favor structural flexibility and unconventional ⁵D₀ → ⁷F₄ reinforcement, whereas Gd-based hosts provide rigid-lattice stabilization, high radiative efficiency, and saturated red emission within a narrower doping window. These insights provide fundamental guidelines for host-controlled emission tuning in double perovskite phosphors, rather than focusing on direct device-oriented optimization.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that supports the current research can be made available upon reasonable request to the corresponding author.

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