Excitation-Dependent Photoluminescence of BaZrO₃:Eu³⁺ Crystals

Abstract

The elucidation of local structure, excitation-dependent spectroscopy, and defect engineering in lanthanide ion-doped phosphors was a focal point of research. In this work, we have studied Eu³⁺-doped BaZrO₃ (BZOE) submicron crystals that were synthesized by a molten salt method. The BZOE crystals show orange–red emission tunability under the host and dopant excitations at 279 nm and 395 nm, respectively, and the difference is determined in terms of the asymmetry ratio, Stark splitting, and intensity of the uncommon ⁵D₀ → ⁷F₀ transition. These distinct spectral features remain unaltered under different excitations for the BZOE crystals with Eu³⁺ concentrations of 0–10.0%. The 2.0% Eu³⁺-doped BZOE crystals display the best optical performance in terms of excitation/emission intensity, lifetime, and quantum yield. The X-ray absorption near the edge structure spectral data suggest europium, barium, and zirconium ions to be stabilized in +3, +2, and +4 oxidation states, respectively. The extended X-ray absorption fine structure spectral analysis confirms that, below 2.0% doping, the Eu³⁺ ions occupy the six-coordinated Zr⁴⁺ sites. This work gives complete information about the BZOE phosphor in terms of the dopant oxidation state, the local structure, the excitation-dependent photoluminescence (PL), the concentration-dependent PL, and the origin of PL. Such a complete photophysical analysis opens up a new pathway in perovskite research in the area of phosphors and scintillators with tunable properties.

1. Introduction

The trivalent europium ion Eu³⁺ is considered to be one of the most sensitive lanthanide ions that displays environment- and symmetry-sensitive emissions owing to its pure magnetic dipole transition (MDT, ∆J = ±1), hypersensitive electric dipole transition (HEDT, ∆J = ±2), and neither magnetic nor electric ⁵D₀ → ⁷F₀ (∆J = 0) transition [1,2,3,4]. When Eu³⁺ is localized at a highly symmetric site with a center of inversion (C_i), its MDT predominates over its EDT. If Eu³⁺ is situated at a highly asymmetric site, its emission is the other way around [5]. In addition, the Eu³⁺ ion is one of the most fascinating dopant ions for quality red phosphors with a high quantum yield (QY), a decent thermal stability, and a long luminescence lifetime [6,7].

Perovskites with a generic formula ABO₃ are in high demand as luminescence hosts due to their structural flexibility, wide band gap, ease of doping, and ability to accommodate lanthanide ions at both A and B sites [8,9,10]. Among them, BaZrO₃ (BZO) is a unique material due to its wide tunable band gap (5.6 eV) [11], high refractive index [12], high proton conductivity, and high chemical and mechanical stability [10,13,14,15]. It has various applications in the areas of luminescence [16], catalysis [16,17], proton-conducting solid oxide fuel cells [18,19], and many others. Eu³⁺ ion-doped ABO₃ perovskites have attracted a lot of attention due to their high thermal and chemical stability, low environmental toxicity, and various applications in photocatalysis, white light generation [20], light emitting diodes (LEDs) [21], and bioimaging [22].

One can probe the local sites of Eu³⁺ ions in ABO₃ perovskites based on the ratio, spectral splitting, and appearance of ⁵D₀ → ⁷F_J (J = 0–4) emissions.¹ This kind of study is crucial to make materials with optimum light emitting properties. For example, Kunti et al. recently observed MDT and HEDT emissions with I_MDT >>> I_HEDT along with the host emission under the 275 nm excitation from their BaZrO₃:Eu samples synthesized by the solid state route [23]. There was a systematic host-to-dopant energy transfer with an increasing Eu³⁺ doping concentration. Based on the analysis of extended X-ray absorption fine structure (EXAFS) spectroscopic data, they concluded that Eu³⁺ ions were localized at Zr⁴⁺ sites [23]. Gupta, one of the co-authors of the current manuscript, and his coworkers observed spectral profiles with I_HEDT >>> I_MDT under various excitations from gel combustion-synthesized BZO:Eu samples [12], which was exactly opposite to what was observed by Kunti et al. [23]. Gupta et al. also proposed that a large fraction of Eu³⁺ ions occupied Zr⁴⁺ sites based on population analysis of lifetime spectra [12]. Kanie et al. synthesized BZO samples with different sizes and shapes and investigated their effects on luminescence [24].

There were also reports on Eu³⁺-doped perovskites of SrZrO₃, SrSnO₃, BaTiO₃, BaSnO₃, and BaZr_xTi_1−xO₃. For example, Basu et al. proposed that Eu³⁺ ions resided at Sr²⁺ sites at low dopant concentrations and were distributed at both Sr²⁺ and Sn⁴⁺ sites at high doping levels in their polyol-synthesized SrSnO₃ nanoparticles based on EXAFS measurements [25]. The same group further proposed that Eu³⁺ ions occupied the centrosymmetric Sr²⁺ sites up to 1.5% Eu³⁺ doping and, beyond that, the synthesized SrSnO₃ nanoparticles formed a separate europium oxide phase based on time resolved emission spectroscopy (TRES) and electron paramagnetic resonance (EPR) studies. Similarly, based on EXAFS studies, Rabufetti et al. found that Eu³⁺ ions resided at Ba²⁺ sites at low dopant concentrations (up to 4% Eu³⁺ doping) but were distributed at both Ba²⁺ and Ti⁴⁺ sites at high doping levels in their vapor-diffusion sol-gel-synthesized BaTiO₃ nanocrystals [26]. Canu et al. have tuned the photoluminescence properties of Eu³⁺-doped BaZr_xTi_1-xO₃ perovskite by applying an electric field [27].

There are also reports that studied the effect of changing the A cation of AZrO₃:Eu on the luminescence emission intensities [28]. Katyayan et al. studied the impact of co-doping Tb³⁺ with Eu³⁺ on the optical and spectroscopic characteristics of BZO perovskite [29]. Another study investigated the effect of particle size and morphology on the fluorescence behaviors of these metal oxides [24].

Color tunability is achieved from samples with the same dopants and hosts by simply varying the excitation wavelength. For example, Gupta et al. showed different emission characteristics of SrZrO₃:Eu³⁺ nanoparticles in terms of the asymmetry ratio (A₂₁) under the excitations with host absorption, charge transfer, and the f-f band of Eu³⁺ [30]. Guo et al. synthesized a Bi³⁺ and Eu³⁺ ion co-doped Ba₉Lu₂Si₆O₂₄ single-phased phosphor via a conventional high-temperature solid-state reaction [31]. They demonstrated that the relative emission intensity of Bi³⁺ luminescent centers tightly depends on the incident excitation wavelength due to the complex energy transfer processes among these Bi³⁺ centers.

Furthermore, even though the induced electric dipole (ED) ⁵D₀ → ⁷F₀ transition is strictly forbidden by the ΔJ selection rule of the Judd–Ofelt theory, there are reported occurrences of it as a well-known example of the breakdown of the selection rules of the Judd–Ofelt theory [1]. For example, Guzmán-Olguín et al. showed an unusual great intensity of the ⁵D₀ → ⁷F₀ transition centered at 580 nm when they excited their Eu³⁺-doped BaHfO₃ perovskite ceramic under UV radiation with the wavelength associated with the charge transfer band (272 nm), while this transition was very weak when the sample was excited at 396 or 466 nm wavelengths [32]. One of the co-authors of this manuscript, Gupta, with his co-workers, reported the presence of two Stark components in the ⁵D₀ → ⁷F₀ transition from their Nd₂Zr₂O₇:Eu phosphor when excited at 256 nm [33].

It is clear that there is no systematic investigation of the luminescence of BZO:Eu nor studies on its ⁵D₀ → ⁷F₀ transition under host and Eu³⁺ excitations. In this work, we have first synthesized BZO:Eu submicron crystals using an environmentally friendly molten salt synthesis (MSS) method based on the report by Zhou et al. using barium oxalate and zirconium oxide as precursors and a KOH/NaOH salt mixture as the reaction medium [34]. We studied tuning the red to orange emission ratio from the BZO:Eu crystals by modulating the excitation wavelength and deciphered the local site occupancy of Eu³⁺ ions in BZO with Eu, Ba, and Zr-edge EXAFS analysis. More importantly, other than the weak ⁵D₀ → ⁷F₃ at 653 nm, we observed a strong ⁵D₀ → ⁷F₀ transition, which is known to be strictly forbidden by both EDT and MDT of Eu³⁺ ions, as based on the Judd–Ofelt theory. This observation suggests the deviation of luminescence properties of the Eu³⁺ dopant in the BZO host from the Judd–Ofelt theory. In other words, it indicates that Eu³⁺ ions are localized in highly asymmetric environments, e.g., C_n, C_nv, and C_s point group symmetry, so that the selection rules are relaxed to some extent by the mixing of a low-energy charge transfer state with the 4f⁶ configuration [1]. Moreover, designing functional materials that display excitation wavelength-dependent color tunability and understanding structure–property correlation is invaluable to materials scientists.

2. Experimental

The synthesis and instrumentation characterization of the BZO and BZOE submicron crystals are described in detail in the electronic supplementary information as S1. Briefly, six Ba_1-xZrO₃:x%Eu³⁺ (x = 0, 0.5, 1.0, 2.0, 5.0, 10.0) samples were synthesized using the MSS method following a procedure published previously with one of the co-authors of this manuscript. Based on the Eu³⁺ doping levels, the synthesized Ba_1-xZrO₃:x%Eu³⁺ samples with x = 0, 0.5, 1.0, 2.0, 5.0, 10.0 are designated as BZO, BZOE-0.5, BZOE-1, BZOE-2, BZOE-5, and BZOE-10, respectively.

3. Results and Discussion

3.1. XRD Patterns

The XRD patterns of the BZO and BZO:Eu samples (Figure 1a) demonstrated that the diffraction peaks of all samples match with the cubic perovskite phase (Pm-3m) of BZO (JCPDS No. 74-1299) and no impurity peaks were observed. The substitution of Eu³⁺ for constituent ions is evidently aliovalent and may generate oxygen vacancies when resided at a Zr⁴⁺ site. In case if some fraction resides at a Ba²⁺ site, the charge compensation may invoke the creation of barium vacancies. As seen in

Table 1. Lattice constants and crystallite sizes of the BZOE obtained from Rietveld refinement of the XRD data shown in Figure 1a.

%Eu	0.0	0.5	1.0	2.0	5.0	10.0
a (Å)	4.1947 (2)	4.1954 (3)	4.1944 (3)	4.1952 (3)	4.1976 (3)	4.1971 (2)
Size (nm)	156 (5)	127 (4)	111 (3)	127 (3)	102 (2)	162 (5)

, the cell parameter variation is complex, which means different defect complex generations at different doping levels.

3.2. FTIR and Raman Spectroscopy

To further confirm the formation of the perovskite phase and rule out the formation of other phases, FTIR spectra of the samples were collected (Figure 1b). The only observed peak around 570 cm⁻¹ can be assigned to the anti-symmetric stretching Zr–O bond of the octahedral ZrO₆ unit of the BaZrO₃ lattice [35,36,37].

In the Raman spectra of the BZO and BZOE samples (Figure 1c), the peak around 600–900 cm⁻¹ is attributed to the symmetric stretch (ν) of the Zr–O bonds in BaZrO₃ [23]. With an increasing Eu³⁺ doping level, two extra peaks around 283 and 338 cm⁻¹ that correspond to symmetric A_g and degenerated F_g modes of the stretching vibrations of the C₂-octahedron (Eu₂-O) started to appear [38]. This means that Eu³⁺ ions stop going into the BZO lattice and precipitate as a separate phase of Eu₂O₃ at the doping concentration of 10.0%, which is similar to what Basu et al. observed from their polyol-synthesized SrSnO₃ nanoparticles based on EXAFS measurements [25]. The peaks between 100 and 230 cm⁻¹ can be assigned to BaCO₃ impurity, which did not show up in the XRD patterns and FTIR spectra due to a low percentage. The carbonate phase probably resulted from the chemisorption of atmospheric CO₂ on the surface of the BZO crystals upon its exposure to air. It was reported that the existence of such an impurity phase has no effect on the luminescence properties of the BZO perovskite [8].

3.3. SEM Images

The SEM images of the BZO and BZOE samples (Figure 1d) demonstrated that the particles were composed of a mixture of spheres and cubes with well-defined edges. In our earlier work, we found that cubic BZO microcrystals predominated when the synthesis was conducted at a higher annealing temperature. There was almost an equal number of spherical and cubical particles from these samples. No difference in the shape of the particles was noticed from these samples with different Eu³⁺ doping concentrations. However, the agglomeration of the particles increased with an increasing Eu³⁺ concentration. Based on the particle size distribution histograms of these samples obtained using the ImageJ software (Figure S1) and the crystallite sizes obtained from the XRD data (Table 1, Figure S2), no clear correlation between the average particle size and the Eu³⁺ doping concentration was established.

3.4. X-ray Absorption Spectroscopy

3.4.1. XANES

Figure 2a–c shows the normalized XANES spectra of three BZOE samples along with their standards (either the undoped BZO crystals or commercial Eu₂O₃ powder) at the Ba L₃ (Figure 2a), Zr K (Figure 2b), and Eu L₃ (Figure 2c) edges, respectively. The normalized XANES spectra at the Ba L₃-edge shown in Figure 2a are characterized by a sharp white line, which is the main absorption peak due to the transition 2p_3/2 → 5d. There is no appreciable difference in this edge upon Eu³⁺ doping.

It can be seen from Figure 2b that the Zr absorption edges of the BZOE samples coincide with that of the BZO sample, confirming that the oxidation state of Zr is 4+ in the doped samples. Two peaks, A (18,010 eV) and B (18,021 eV), observed in the BZO and BZOE samples just above the Zr absorption edge, are similar to those obtained by Fassbender et al. [39] and Giannici et al. [40] and are due to the octahedral oxygen coordination of Zr⁴⁺ in the samples. The overall shape of the Zr XANES spectra remains nearly unchanged upon Eu³⁺ doping, and it is suggested that the octahedral symmetry of the Zr atom does not break with doping. The slight increase in the A and B peaks of the doped samples compared to BZO suggest that the Eu³⁺ solubility is very limited (no more than 1–2%).

The Eu L₃-edge XANES spectra of the samples (Figure 2c) show that the absorption edges coincide with that of the standard Eu₂O₃ sample, suggesting that the Eu dopant remains in the Eu³⁺ oxidation state in the BZOE samples. The increase in the white line at the edge indicates an increase in the empty Eu d-states at the Fermi level in the BZOE samples compared to Eu₂O₃, suggesting that the Eu³⁺ is at least partially in a different local environment than in Eu₂O₃.

3.4.2. EXAFS

Figure 3 presents the k²-weighted Fourier transformed spectra, |χ(R)|, of the BZO and BZOE samples (and Eu₂O₃ standard) at the Ba L₃ (Figure 3a), Zr K (Figure 3b), and Eu L₃ (Figure 3c) edges. For BZO in a cubic perovskite structure (ABO₃) with the space group Pm-3m, the Zr atoms are coordinated with 6 O atoms in a regular octahedral (6-fold (ZrO₆)) shape and the Ba atoms are coordinated with 12 O atoms in a cuboctahedral (12-fold (BaO₁₂)) shape in the first coordination shells. Theoretical EXAFS spectra have been generated using the above structure for the Ba (Figure S3), Zr (Figure S4), and Eu (Figure S5) edges of the BZO and BZOE samples and fitted to the experimental data.

Specifically, the Ba edge was fitted with a structural model including three paths, Ba–O (12-fold), Ba–Zr (8-fold), and Ba–Ba (6-fold). For the Ba edge fits, the path degeneracies were held constant and the σ² of the Ba–Zr and Ba–Ba paths were constrained to be identical. Windows of 2.0 Å⁻¹ < k < 8.5 Å⁻¹ with dk = 2.0 and 1.5 Å < R < 3.7 Å with dR = 0.2 were used for the Fourier transformation and fits, respectively. The fit results for the Ba–O paths of each sample are presented in

Table 2. Values of the amplitude reduction factor (S₀²) or path degeneracy (N), bond length, and disorder factor for the near neighbor paths obtained from EXAFS analysis of the BZO and BZOE samples at the Ba L₃, Zr K, and Eu L₃ edges.

Scattering Path	Parameter	BZO	BZOE-1	BZOE-2	BZOE-10
Ba–O N = 12	S02	0.74 ± 0.16	0.78 ± 0.18	0.80 ± 0.17	0.78 ± 0.17
	R (Å)	2.91 ± 0.02	2.91 ± 0.02	2.91 ± 0.02	2.91 ± 0.02
	σ2	0.011 ± 0.005	0.013 ± 0.005	0.013 ± 0.005	0.013 ± 0.005
Zr–O N = 6	S02	0.90 ± 0.10	1.0 ± 0.1	1.0 ± 0.1	1.1 ± 0.1
	R (Å)	2.10 ± 0.01	2.10 ± 0.01	2.10 ± 0.01	2.11 ± 0.01
	σ2	0.004 ± 0.001	0.005 ± 0.002	0.005 ± 0.002	0.006 ± 0.002
Scattering Path	Parameter	Eu2O3	BZOE-1	BZOE-2	BZOE-10
Eu–O S02 = 0.86	N	7	6.7 ± 1.7	9.8 ± 1.9	7.9 ± 0.7
	R (Å)	2.35 ± 0.01	2.27 ± 0.03	2.33 ± 0.02	2.39 ± 0.01
	σ2	0.012 ± 0.002	0.012 ± 0.002	0.012 ± 0.002	0.012 ± 0.002

and they indicate that the Ba environment is unchanged by Eu doping. The results for the other paths are in Table S1.

The Zr edges were modeled out to 4 Å using three single scattering paths, Zr–O (6-fold), Zr–Ba (8-fold), and Zr–Zr (6-fold), plus the three high amplitude linear multiple scattering paths. The path degeneracies were held constant for all paths and all the multiple scattering paths are constrained to have the same ΔR as the Zr–Zr path and a common σ² parameter. Windows of 2.0 Å⁻¹ < k < 13.0 Å⁻¹ with dk = 2.0 and 1.0 Å < R < 4.3 Å with dR = 0.2 were used for the Fourier transformation and fits, respectively. Table 2 reports the fit results for the Zr–O single scattering path with the other paths reported in Table S2. For all samples, the Ba–Zr and Zr–Ba paths are distances and disorder parameters and are consistent as are the Ba–Ba and Zr–Zr paths. The quality of the Zr edge fits (Figure S4) are limited by the constraints applied but are consistent across all samples, suggesting only limited Eu³⁺ doping at the Zr site.

The Eu L₃ EXAFS presented in Figure 3c clearly show that the 10.0% doped sample exhibits two peaks at ~3.1 Å and ~3.7 Å, which are characteristic of Eu₂O₃. As the doping level is reduced, these two peaks vanish to be replaced by two small peaks at ~3.0 Å and ~3.5 Å, similar to those observed in the Zr edge EXAFS. The Eu edges were modeled only to the first shell, as the attempts to model the longer paths were unsuccessful. The amplitude reduction factor S₀² was held constant at the value determined by fitting the Eu₂O₃ data. The path length, degeneracy, and disorder of the single Eu–O path being modeled were allowed to vary. Windows of 2.0 Å⁻¹ < k < 9.0 Å⁻¹ with dk = 2.0 and 1.0 Å < R < 2.4 Å with dR = 0.2 were used for the Fourier transformation and fits, respectively (Figure S5). The resulting fit parameters are reported in Table 2 and it is clear that the Eu–O distance and the path degeneracy increase with the doping level. At 1.0% doping, the Eu–O distance is 2.25 Å, which is longer than the Zr–O distance but significantly shorter than both the Ba–O and the Eu–O distances in Eu₂O₃. As the doping level increases, the Eu-O distance increases and then exceeds that found in Eu₂O₃. Similarly, the path degeneracy for the 1.0% sample is close to six, as would be expected for doping on the Zr site and increases to a value greater than that in Eu₂O₃. These results strongly suggest that Eu³⁺ at low doping levels sits at the Zr site and has a solubility limit between 1.0–2.0%. At higher concentrations, Eu³⁺ ions are found in a Eu₂O₃-like local environment. This result is consistent with the change in the white line of the Eu XANES, which increases for 1.0% and 2.0% but decreases for 10%.

3.5. PL Spectra

The concentration-dependent excitation spectra (λ_em = 625 nm, Figure 4a) and emission spectra (λ_ex = 279 nm and 395 nm, Figure 4b,c) of the BZOE samples demonstrated characteristic PL features of the Eu³⁺ dopant in solid-state hosts [41,42]. In general, there is no change of the excitation and emission spectral profiles, Stark splitting, and relative intensity of excitation and emission peaks under the same excitation wavelength among the BZOE samples with the tested Eu³⁺ doping concentrations. The spectra also clearly show that the 2.0% Eu³⁺-doped sample, BZOE-2, has the highest emission intensity among our samples using the MDT ⁵D₀→⁷F₁ as an example (Figure 4d).

The excitation spectra of the BZOE samples with λ_em = 625 nm corresponding to the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions consisted of two main features (Figure 4a): a broad band extending from 240–320 nm and several fine peaks in the range of 350–500 nm. The broad band peaking around 279 nm is attributed to the allowed charge transfer band (CTB) of electrons from the filled 2p orbital of O^2- to the vacant 4d-orbital of the Eu³⁺ ion. The fine peaks around 361, 375, 383, 387, 395, 405, 414, 456, 465, and 472 nm are attributed to the intra f-f transitions of Eu³⁺ ions. The main peaks located at 395 nm and 465 nm are attributed to ⁷F₀ → ⁵L₆ and ⁷F₀ → ⁵D₂, respectively.

Under the excitations of λ_ex = 279 and 395 nm, the emission spectra of the BZOE samples displayed the CTB and several fine peaks corresponding to ⁵D₀ → ⁷F_J (J = 0–4) transitions of Eu³⁺ in the spectral range of 550–750 nm (Figure 4b). Interestingly, several significant differences in terms of the appearance of ⁵D₀ → ⁷F₀ transition, the asymmetry ratio (A₂₁), and Stark splitting were observed from the emission spectra of the BZOE samples in the spectral range of 550–750 nm under these two excitation wavelengths.

Figure 5a shows a close look of the emission spectra using the BZOE-2 sample as an example. Specifically, under the 395 nm excitation, intense emission bands at 591 nm (⁵D₀ → ⁷F₁, MDT), 612 nm (⁵D₀ → F₂, HEDT), 701 nm (⁵D₀ → ⁷F₄), and 653 nm (⁵D₀ → ⁷F₃, weak peak) were observed [43]. There is no signature of ⁵D₀ → ⁷F₀ transition. The integral intensity of the HEDT at 612 nm is stronger than that of the MDT at 591 nm. The ⁵D₀ → ⁷F₃ transition is known to be allowed by neither MDT nor EDT. It is forbidden in nature according to the Judd–Ofelt (J–O) theory but could gain intensity via J-mixing. The ⁵D₀ → ⁷F₄ transition is also considered as an ED transition [1].

Under the CTB excitation at 279 nm, we observed several interesting emission features compared to the emission spectrum recorded under 395 nm excitation (Figure 5a): (a) an unusually intense 575 nm peak corresponding to ⁵D₀ → ⁷F₀ transition, which otherwise is forbidden by both ED and MD transitions [44], (b) increased Stark splitting, (c) enhanced intensity of the ⁵D₀ → ⁷F₃ peak, and (d) a significant change of the A₂₁ value. The possible reasons for these observations will be further discussed in the following sections.

3.6. PL Lifetime Spectra and QY

Figure 6a,b show the results of the luminescence lifetime measurements of the BZOE-2 sample under the excitations at 279 and 395 nm with three different emission wavelengths of 575, 591, and 612 nm corresponding to ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₁, and ⁵D₀ → ⁷F₂ transitions, respectively. For the BZOE-2 sample, the luminescence lifetime curves recorded under the 279 nm excitation (Figure 6a) demonstrated a biexponential behavior with two slopes and they can be approximated using the following equation:

I = A₀ + A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂)

(1)

where A₁ and A₂ are the derived preexponential factors, and τ₁ and τ₂ are the lifetime values of the fast and slow decay components, respectively. The luminescence lifetime curves recorded under the 395 nm excitation (Figure 6b) could be fitted with monoexponential decay.

The population of Eu³⁺ ions with a particular lifetime is obtained by using the formula:

% of species n = (A_n ∗ τ_n)/(Σ_{n = 1,2}A_n ∗ τ_n)] ∗ 100

(2)

Under λ_ex = 279 nm, there were two lifetime values for all three emissions: short lifetime T_s (~360–460 µs, 15%) and long lifetime T_l (~1.0–1.5 ms, 85%). On the other hand, under λ_ex = 395 nm, only one short lifetime value was obtained as T_s (~370–580 µs).

The decay profiles of all other BZOE samples are mentioned in Figure S6. Under λ_ex = 279 nm and λ_em = 625 nm, the average lifetime values of the BZOE samples with Eu³⁺ doping levels of 0.5, 1.0, 2.0, 5.0, and 10.0% were 789, 820, 950, 853, and 813 µs, respectively. The effect of Eu³⁺ concentration on the average lifetime value of the BZOE samples (Figure 6c) indicated that the average lifetime value increased up to a 2.0% Eu³⁺ doping level. Beyond that doping concentration, there was a reduction due to concentration quenching, which is consistent with the phenomenon observed from the PL excitation and emission spectra shown in Figure 4.

Quantum yield (QY) is an important parameter to evaluate the properties and application potentials of phosphors. We measured and calculated the QY of our BZOE samples using the following equation:

$$\mathrm{QY}=\frac{\int F_{\mathrm{S}}}{\int L_{\mathrm{R}}+\int L_{\mathrm{S}}}$$

(3)

where F_S represents the emission spectrum of a sample, L_R is the excitation spectrum from an empty integrating sphere (without any sample), and L_S means the excitation spectrum of a sample. The effect of the Eu³⁺ concentration on the QY value of the BZOE samples (Figure 6d) indicated that the QY value of the BZOE samples increased from 2.2% to 14.0% as the Eu³⁺ doping level increased from 0.5% to 2.0%. After higher dopant concentrations, the QY value reduced to ~7.6–7.9%. This is again attributed to concentration quenching arising from non-radiative energy transfer among Eu³⁺ ions at high doping concentrations.

Concentration quenching is one of the most dominant phenomena that takes place at high dopant concentrations. It is attributed to increasing resonant energy transfer between Eu³⁺ ions at a high dopant concentration, which results in decreasing radiative emissions. To better understand the mechanism of the concentration quenching phenomenon of our BZOE samples, the critical distance (r_c) between Eu³⁺ dopant ions and quenching sites was calculated using the following equation:

$$r_{\mathrm{c}}=2{\left(\frac{3V}{4\pi X_{\mathrm{c}}N}\right)}^{\frac{1}{3}}$$

(4)

where V, X_c, and N are the volume of the unit cell, the critical concentration of Eu³⁺ and the number of cations per unit cell, respectively. The values of these three variables for our BZOE samples are 73.6575 ų, 0.02, and 8, respectively. Hence, the calculated critical distance r_c value was 9.58 Å. Since the Eu³⁺–Eu³⁺critical distance is more than 5 Å, multipolar interactions are responsible for the concentration quenching of our BZOE crystals. Therefore, various PL studies indicated that there is a close correlation between the doping concentration with the excitation and emission intensity, luminescence lifetime, and the QY of the BZOE crystals.

3.7. J–O Analysis

To explain the observed luminescence performance, J–O parameters were determined to provide empirical relations between the local site symmetry of Eu³⁺ ions in the BZO lattice, the crystal field strength of the BZO host lattice, and the Eu–O bond covalency and polarizability in the BZOE samples.⁴⁵ Based on various mathematical formulations, we have derived the radiative/non-radiative transition rates and the internal quantum efficiency of the BZOE-2 sample [45,46,47]. Various important optical parameters were calculated for the BZOE samples under the excitations at 279 and 395 nm (

Table 3. Calculated J–O parameters and radiative properties of the BZOE-2 sample (A_R = radiative Rate, A_NR = nonradiative rate, Ω_n = the Judd−Ofelt parameter, and β_n = branching ratio).

BZOE-2	AR (s−1)	ANR (s−1)	η(%)	Ω2 (×10−20)	Ω4 (×10−20)	β1(%)	β2(%)	β4(%)	Ω2/Ω4
λex = 279 nm	212.77	787.4	21.3	1.04	0.917	23.5	42.4	18.6	1.13
λex = 395 nm	369	2331	13.7	2.27	2.78	13.6	53.7	32.5	0.82

). The BZOE-2 sample had a higher internal quantum efficiency (IQE) under the 279 nm excitation compared to 395 nm excitation. Its non-radiative transition (A_NR, 787.4 s⁻¹) and radiative transition (A_R, 212.77 s⁻¹) values under 279 nm were lower compared to those under 395 nm excitation (A_NR = 2331 s⁻¹ and A_R = 369 s⁻¹). When changing λ_ex from 279 nm to 395 nm, the increase in the A_NR value was higher than that of the A_R value.

For the J–O parameters, Ω₂ (the short range parameter) gives information related to the covalent character, local symmetry, and structural distortion in the vicinity of Eu³⁺ ions, whereas Ω₄ intensity parameters (the long range parameter) provides bulk information such as the viscosity and rigidity of the host lattice [48]. Under the 279 nm excitation, the observed trend of the J–O parameters (Ω₄ < Ω₂) suggested that excited Eu³⁺ ions were mostly localized in a highly asymmetric and distorted environment. On the other hand, under the 395 nm excitation, the J–O parameter trend reversed with Ω₄ > Ω₂, which confirmed that a large fraction of excited Eu³⁺ ions occupies relatively less distorted and asymmetric sites. The value of the J–O ratio (Ω₂/Ω₄) of lower than one suggests a high asymmetry of the Eu³⁺ environment where its value higher than one suggests a low asymmetry. The fractional distribution of branching ratios suggests that, under the 279 and 395 nm excitations, photon parts emitted via MDT are 23.5% and 13.6%, whereas those emitted via HEDT are 42.4% and 53.7%, respectively.

3.8. Discussion

As marked by numbers one and two on the color coordinated diagram (Figure 5b), the intense peaks around 575 nm and 612 nm impart orange emissions under λ_ex = 279 nm and red emissions under λ_ex = 395 nm, respectively. It demonstrated that one can achieve orange–red color tunability by selectively exciting the same material with dopant or host excitations. The different photophysical processes happening under these two excitations are schematically depicted in Figure 5d. The different spectral features of the BZOE samples observed under the 279 and 395 nm excitations suggest that the excited Eu³⁺ ions are relaxed through different channels to the ground states.

Some authors have proposed theoretical models for the observed ⁵D₀ → ⁷F₀ transition, including the breakdown of the closure approximation in the Judd–Ofelt theory and third order perturbation theory [1,32,33]. The most obvious explanation assumes that this transition is due to J-mixing or to the mixing of low-lying charge-transfer states into the wave functions of the 4f⁶ configuration. Experimentally, the number of Stark components of the ⁵D₀ → ⁷F₀ transition indicates the number of local sites of Eu³⁺ ions in host lattices. It is normally allowed when Eu³⁺ ions are situated at sites lacking inversion symmetry [1,49]. The presence of an unsplitted single band of the ⁵D₀ → ⁷F₀ transition under λ_ex = 279 nm suggests that a large fraction of Eu³⁺ ions are located at the non-inversion symmetric sites in the BZOE submicron crystals. This hypothesis is further supported by the appearance of forbidden ⁵D₀ → ⁷F₃ peaks with large Stark splitting [30,44]. On the other hand, under λ_ex = 395 nm, the observed phenomena, including the absence of ⁵D₀ → ⁷F₀ transition, weak ⁵D₀ → ⁷F₃ transition, and a low extent of Stark splitting of ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions, suggest that a large fraction of Eu³⁺ ions at doping sites, which are less asymmetric or distorted, are selectively excited.

Based on the selection rules of point group symmetry, the ⁵D₀ → ⁷F₀ transition appears when Eu³⁺ dopants are located at sites lacking an inversion center with 10 designated non-cubic point groups, including C_6v, C₆, C_3v, C₃, C_4v, C₄, C_2v, C₂, C_s, and C₁ [50]. The ⁵D₀ → ⁷F₀ transition is not allowed in cubic groups with inversion symmetry such as T, T_d, and O or non-cubic point groups without inversion symmetry such as D₂, D₃, D_3h, C_3h, D₃, D₄, S₄, D_2d, D_4d, and D₆ [49].

The ideal BZO is a perfect cubic perovskite with O_h point group symmetry (space group: Pm-3m), which has 12-coordinated Ba²⁺ sites and 6-coordinated Zr⁴⁺ sites in cuboctahedra and octahedral geometries, respectively [51]. The observed emission spectra are in line with Eu³⁺ ions occupying the Zr⁴⁺ sites in the BZOE samples even with the following ionic radii values of Ba²⁺ (r_ion = 161 pm @ CN = 12), Zr⁴⁺ (r_ion = 72 pm @ CN = 6), and Eu³⁺ ions (r_ion = 95 pm @ CN = 6). Substituting Eu³⁺ ions at the Zr⁴⁺ sites distorts the symmetric ideal perovskite structure of BZO and invokes charge compensation by oxygen vacancies, which reduce the point group symmetry from O_h to further lower symmetry. This is consistent with our EXAFS analysis (Figure 3), especially at a low Eu³⁺ doping level before the low amount of Eu₂O₃ phase forms.

It has been reported that the emission of Eu³⁺ dopant in a cubic structure with the O_h point group should only have a single unsplitted ⁵D₀ → ⁷F₁ transition peak [52]. By considering the most sensitive peaks for ⁵D₀ → ⁷F₀ and ⁵D₀ → ⁷F₂ transitions, there are 0 and 2 Stark components under λ_ex = 395 nm and 1 and 3 Stark components under λ_ex = 279 nm, respectively (Figure 5a). This observation suggested D₃ and C_3V point group symmetry around Eu³⁺ ions in our BZOE samples [52].

The Kroger–Vink notation for the substitution, wherein trivalent Eu³⁺ ions occupy tetravalent Zr⁴⁺ sites, is formulated below [53]:

$$2E{u}^{\dot{}\dot{}\dot{}}+{Zr}_{Zr}^{\dot{}\dot{}\dot{}\dot{}}\leftrightarrow E{u}_{Zr}^{\prime }+V_O^{\dot{}\dot{}}$$

(5)

Defects such as $V_O^{\dot{}\dot{}}$ and $E{u}_{Zr}^{\prime }$in the BZOE crystals provide additional pathways for non-radiative relaxation. They tend to quench PL by absorbing emitted photon energy from Eu³⁺ ion centers ($E{u}_{Ba}^{\dot{}}$) [47]. Hence, although Eu³⁺ ions occupy Zr⁴⁺ sites ($E{u}_{Zr}^{\prime })$, we assume that there are enough oxygen vacancies surrounding them with random distribution. There would be two scenarios: one with enough $E{u}_{Zr}^{\prime }$ surrounded by oxygen vacancies in a close vicinity (x), designated as x$E{u}_{Zr}^{\prime }$, and another with $E{u}_{Zr}^{\prime }$ surrounded by oxygen vacancies at a much farther-off distance (y), designated as y$E{u}_{Zr}^{\prime },$ such as y >> x. The point group symmetry of x$E{u}_{Zr}^{\prime }$, as discussed above, is C_3v, and that of y$E{u}_{Zr}^{\prime }$ is D₃. As schematically shown in Figure 5d, upon the excitation with the Eu³⁺ f-f band at 395 nm, the prevalent excited species is y$E{u}_{Zr}^{\prime }$, whereas upon excitation with the host CTB selectively, a large fraction excited species is x$E{u}_{Zr}^{\prime }$.

4. Conclusions

In this work, BZOE submicron crystals with varied Eu³⁺ doping concentrations were synthesized using the molten salt method. XANES and EXAFS spectroscopies confirm that Eu is stabilized in a +3 oxidation state at Zr⁴⁺ s at a low doping concentration, while a separate Eu₂O₃ phase forms at the highest 10% doping level. Based on the PL measurement, it was established that europium is localized at Zr⁴⁺ sites in two different environments: one close to zirconium vacancies with C_3v symmetry and one far off from zirconium vacancies with D₃ symmetry. Interestingly, when excited at the charge transfer band of the BZO host at 279 nm, a large fraction of Eu³⁺ ions at non-symmetric C_3v sites were excited to give a highly intense ⁵D₀ → ⁷F₀ transition, large spectral splitting, and intense MDT peaks compared to HEDT peaks. On the other hand, when excited at a dopant transition wavelength of 395 nm, a relatively large fraction of Eu³⁺ dopants, which are far off from zirconium vacancies with D₃ symmetry, were excited to give no ⁵D₀ → ⁷F₀ transition, highly intense HEDT peaks compared to MDT peaks, and fewer Stark components. This excitation wavelength dependence induces emission light tunability of orange light at λ_ex = 279 nm and red light at λ_ex = 395 nm from the BZOE samples. This observation is further justified by the trend of the J–O parameters, especially with Ω₄ < Ω₂ at λ_ex = 279 nm and Ω₄ > Ω₂ at λ_ex = 395 nm. This work demonstrates the role of local dopant sites, defects, excitation wavelengths, and doping concentrations on optimizing the optical properties of lanthanide-doped perovskite phosphors for efficient optoelectronics and scintillator applications.