Boron doping effect on the structural, spectral properties and charge transfer mechanism of orthorhombic tungsten bronze β-SrTa₂O₆:Eu³⁺ phosphor

Abstract

In the study, the effect of boron doping on spectral properties and CTB mechanism was investigated by using Eu³⁺ doped orthorhombic β-SrTa₂O₆. A phosphor series of Eu³⁺ doped SrTa₂O₆, and Eu³⁺ and B³⁺ co-doped SrTa₂O₆ polycrystals were fabricated by solid-state reaction at 1400 °C for 20 h in an air atmosphere. The X-ray diffractions of the main phase structure for all the ceramics maintained up to 10 mol% Eu³⁺ concentration, while the increase of XRD intensity for Eu³⁺ and B³⁺ co-doped samples was attributed to somewhat improvement of crystallization. SEM morphologies of grains showed that the presence of boron promotes agglomeration and grain growth. The doping of boron up to 20 mol% led to an increase in PL intensity, CTB energy slightly shifted to low energy, and also an increase occurred in the asymmetry ratio of the phosphor. Therefore, the low crystal field symmetry of the Eu³⁺ sites and some improvement in crystal structure properties for Eu³⁺, B³⁺ co-doped samples supported the PL increase. The trend of Judd–Ofelt parameters (Ω₂, Ω₄) is SrTa₂O₆:xEu³⁺, 0.1B³⁺ > SrTa₂O₆:xEu³⁺. The high Ω₂ parameter for boron co-doped samples showed a covalent Eu–O bond character with low symmetry of Eu³⁺ environment, while the high Ω₄ value indicated the reduction in electron density of the ligands. Some increase in the short decays of Eu³⁺, B³⁺ co-doped samples is probably due to the surface effect and low crystal field symmetry. The quantum efficiency of 0.05Eu³⁺, 0.1B³⁺ co-doped phosphor with the highest PL intensity increased by about 21% compared to that without boron.

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1. Introduction

The trivalent europium ion (Eu³⁺) is known for its strong luminescence in the red spectral region which exhibits interesting theoretical properties. The Eu³⁺ has a great advantage over other RE ions due to the initial levels of transitions in both the absorption and luminescence spectrum not degenerate (J = 0).^1,2 The charge transfer (CT) transitions of Eu³⁺ luminescence exhibit broad absorption bands called charge transfer (CT) bands or ligand-to-metal charge transfer (LMCT) bands in the ultraviolet region of electromagnetic spectrum. CT bands occur due to electron transfer where an electron is transferred from one or more neighboring atoms to the Eu³⁺ ion, and Eu³⁺ is formally reduced to Eu²⁺. The trivalent europium is the most oxidizing of the trivalent rare-earth ions, because Eu³⁺ lacks only one electron to reach a stable half-filled shell.¹ The CT bands are very intense compared to the f–f transitions due to the transitions are allowed by the Laporte selection rule, and the position of the CT bands strongly depends on the nature of the ligands. CT bands can be useful for sensitization of Eu³⁺ luminescence, because they can act as an antenna to absorb light and to transfer the excitation energy to Eu³⁺ ion.¹

Tungsten bronze (TB) crystal structures are good host matrix candidates for many luminescent applications due to their crystal structure with different coordinated cationic tunnels that allow doping of RE³⁺ ions.^3–7 SrTa₂O₆ occurs as three different polymorphs in SrO–Ta₂O₅ system.^6–9 Two polymorphs of SrTa₂O₆ are related to TB symmetry and occur at high temperatures. The β-SrTa₂O₆ crystallizes at temperatures between 1300–1500 °C, while the β′-SrTa₂O₆ forms at temperatures over 1500 °C.^6–9 The other polymorph is orthorhombic α-SrTa₂O₆ occurs at about 1100 °C and below. However, the synthesis of α-SrTa₂O₆ is difficult with the solid state reaction, so it can be produced by the flux method using strontium borate.¹⁰ Tungsten bronze polymorphs have similar lattice sizes but their crystal symmetries are different. This difference affects its physical properties, that β′-SrTa₂O₆ polymorph has better dielectric properties compared to β-SrTa₂O₆.^8,9 The β-SrTa₂O₆ is orthorhombic with space group Pnam and lattice parameters a = 12.37 Å, b = 12.43 Å, c = 7.72 Å, V = 1187.70 ų, Z = 10.^8,9 The β′-SrTa₂O₆ is tetragonal with space group P4/mbm and lattice parameters a = 12.47 Å, c = 3.90 Å, V = 606.27 ų, Z = 5.^8,9 The comparison of the two tungsten bronze polymorphs shows that the orthorhombic distortion of β-SrTa₂O₆ to β′-SrTa₂O₆ is accompanied by an expansion of the TaO₆ octahedral with narrowing of the (Sr₂)O₁₃ tricapped pentagonal prisms. Although the crystal lattice of β-SrTa₂O₆ is about 2.5% smaller than β′-SrTa₂O₆, Ta–O bond distances and corresponding octahedral volumes of β-SrTa₂O₆ are large, and it has a relaxed anion sublattice where electrostatic interactions are optimized via octahedral tilting distortions.⁸

Boron is widely used as a dopant in glasses to improve its optical and mechanical properties which could promote vitrification of oxide.¹¹ However, the doping effect of boron has been studied in detail for certain structures such as TiO₂. In boron-doped TiO₂, interstitial B has the more stable form under high calcination temperature compared to substitutional B. An increase and decrease occurred in the lattice parameters are associated with interstitial B and substitutional B, respectively.^12,13 Boron types can take at least two different major positions in TiO₂ which are the oxygen–boron substitution (paramagnetic state), and the interstitial boron (diamagnetic state). In the paramagnetic B arrangement, electrons can be easily captured from other defect states, whereas in the diamagnetic B arrangement, the centers do not transfer charge to other defects in the system. In any case, the driving force for boron to occupy interstitial positions is the formation of very strong bonds with oxygen.¹⁴ The doping of boron to SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor improves the performance of trapping centers and afterglow times at the low temperature.¹⁵ The powders of Eu³⁺ doped CdNb₂O₆ and Eu³⁺, B³⁺ co-doped CdNb₂O₆ are prepared by molten salt synthesis method and the presence of orthorhombic single phase without any other interphase showed that boron is incorporated in the host structure and a significant increase occurred in PL.¹⁶

In the study, the effect of boron doping on the photoluminescence, charge transfer transition, and structural properties of the Eu³⁺ doped β-SrTa₂O₆ and Eu³⁺, B³⁺ co-doped β-SrTa₂O₆ phosphors were investigated. The structural and spectroscopic characterizations were carried out by XRD, SEM, XPS, PL, decay time analyses, and also the spectral properties were examined comparatively within the scope of Judd–Ofelt theory.

2. Experimental

SrTa₂O₆:xEu³⁺, yB³⁺ ceramic samples were fabricated in three groups by solid-state reaction. First group contains Eu³⁺ doped samples with x = 0.5, 1.5, 3, 5, 7, 10 mol% concentrations, second group has x = 5 mol% Eu³⁺ and y = 0, 5, 10, 20 mol% B³⁺ co-doped samples while the third group represents the y = 10 mol% B³⁺ and x = 0.5, 1.5, 3, 5, 7, 10 mol% Eu³⁺ doped samples. Each 1 mol% of x represents two atomic% values of the Eu₂O₃ compound. Strontium carbonate (SrCO₃: Sigma-Aldrich, 99%), and tantalum oxide (Ta₂O₅: Alpha Aesar, 99.9%) powders were used as starting materials in calculated stoichiometric amounts. Europium oxide (Eu₂O₃: Alpha Aesar, 99.9%) and boric acid (H₃BO₃: Kimyalab, %99.9) were used as dopant materials. The SrCO₃ and Ta₂O₅ were weighed according to the stoichiometry of SrTa₂O₆. For CO₂ extraction, SrCO₃ was kept in a furnace at 1200 °C for 1 h to obtain SrO. Then SrO and Ta₂O₅ powders were homogenized by mixing well with the addition of acetone in an agate mortar. The powder mixture of SrO and Ta₂O₅ was doped by the 0.5, 1.5, 3, 5, 7, 10 mol% Eu₂O₃ (for the 1st group), 5 mol% Eu₂O₃ and 0, 5, 10, 20 mol% H₃BO₃ (for the 2nd group), 0.5, 1.5, 3, 5, 7, 10 mol% Eu₂O₃ and 10 mol% H₃BO₃ (for the 3rd group), respectively. The final powder mixtures were mixed by acetone for further homogenization. The prepared powders were sintered in an electric furnace at 1400 °C for 20 h after pelleting.

The phase structure of the ceramic samples were examined by XRD (X-ray diffractometer; Bruker D-8 Advance, Bruker Corp., Germany) between 2θ = 20–65 °C using Cu-K_α (1.5406 Å) radiation and scan speed 2 °C min⁻¹. The grain morphology of the ceramic samples was investigated by SEM (scanning electron microscopy; FEG-SEM; XL 30S, Philips Corp., Netherlands). Elemental analysis was performed by the Thermo-Scientific Al-K_α model XPS device. Photoluminescence (PL) results were taken by a fluorescence spectrometer (FLS920, Edinburgh Instruments, UK) equipped with a 450 W ozone-free Xe lamp. Lifetime data were also obtained by fluorescence spectrometer (FLS920) using the time-correlated single-photon counting (TCSPC) system. PL analysis was performed at room temperature.

3. Results and discussions

3.1 Structural analyses

X-ray diffractions of SrTa₂O₆:xEu³⁺ samples for x = 0.5, 1.5, 3, 5, 7 10 mol% Eu³⁺ concentrations are shown in Fig. 1. The presence of single-phase continued from 0.5 to 10 mol%, and there was no impurity phase. The crystal symmetry of β-SrTa₂O₆ is identified by the first report as Pbam (no. 55) and JCPDS card no. 51-1683.¹⁷ However, Lee et al.⁷ reported that the crystal symmetry is suitable for the space group Pnam. Accordingly, the crystal symmetry of the orthorhombic TB samples was indexed with space group Pnam (no. 62).^8,9 Tetragonal tungsten bronze structure can be expressed by the A₄B₂C₄M₁₀O₃₀ general formula where the pentagonal (A), square (B), and triangular (C) tunnels have 15 CN (large ions such as alkaline earth ions and alkali), 12 CN (such as rare earth ions), 9 CN (small ions such as Li⁺), respectively. Also, the octahedral Ta sites with small and highly charged ions have 6 CN.^4–7 The Eu³⁺ ions (r = 1.12 Å, for CN 9) will be suitable for the A and B sites with 15 and 12 CN, respectively. Tungsten bronze symmetry at high concentrations may persist despite of some change occurring in the lattice, which can be attributed to the superiority of TB for RE³⁺ doping.^3–7 In addition, although the β-SrTa₂O₆ polymorph has preserved its single-phase structure, it is likely to suffer some increase in distortion due to its slightly longer Ta–O bond distances compared to β′-SrTa₂O₆. X-ray diffractions of SrTa₂O₆:xEu³⁺, yB³⁺ (x = 5 mol% Eu³⁺, y = 0, 5, 10, 20 mol% B³⁺), and SrTa₂O₆:xEu³⁺, yB³⁺ (x = 0.5, 1.5, 3, 5, 7, 10 mol% Eu³⁺, y = 10 mol% B³⁺) co-doped samples are shown in Fig. 2a, b and 3, respectively. As seen in the figures, the diffraction peaks of Eu³⁺, B³⁺ co-doped samples exhibited a slight narrowing compared to the Eu³⁺ doped samples, indicating some improvement in crystallization. The absence of secondary phases in XRD patterns can be also associated with the boron's successful incorporation into the structure. In Fig. 2b, the reflection (131) showed a slight shift towards the smaller two theta angles, which is likely related to the increase in B³⁺ concentration leading to the expansion of the lattice volume.

Fig. 1 XRD results of undoped, 0.5, 1.5, 3, 5, 7, and 10 mol% Eu³⁺ doped SrTa₂O₆ samples.

Fig. 2 XRD results of (a) 5 mol% Eu³⁺, and 0, 5, 10, 20 mol% B³⁺ co-doped SrTa₂O₆, (b) the shift of the two theta angle peaks (240) and (131) to lower angles.

Fig. 3 XRD results of 10 mol% B³⁺, and undoped, 0.5, 1.5, 3, 5, 7, 10 mol% Eu³⁺ co-doped SrTa₂O₆ samples.

SEM micrographs of SrTa₂O₆:xEu³⁺, yB³⁺ samples for x = 5 mol% Eu³⁺, y = 0, 5, 10, 20 mol% B³⁺ co-doped concentrations at 2000× and 10 000× magnifications under 15 kV accelerating voltage are shown in Fig. 4a–h, respectively. In SEM micrographs, it is seen that boron doping promotes agglomeration and growth, where grain boundaries are reduced or disappeared. In addition, grain size and agglomeration increase with increasing boron, as reported in the supporting study.¹⁵ The increase in grain size can be seen in Fig. 4a, c, e and g at 2000× magnifications. More detail is shown in Fig. 4b, d, f and h at 10 000× magnifications, which are shown in the inset figures of Fig. 4a, c, e and g as yellow windows. Fig. 5a and b shows the XPS analysis of the 5 mol% Eu³⁺, 20 mol% B³⁺ co-doped sample. In Fig. 5a, the binding energies of O1s, Sr3d, and Ta4f were identified at 529.26, 133.44, and 26.23 eV which belong to tantalum oxide structure, respectively. Also, the 1133 eV peak defines the europium dopant with Eu3d. In Fig. 5b, the binding energy of B1s is 189.59 eV. The broad spectrum in the range of about 187–192 eV shows that boron was incorporated into the structure. The binding energies of boron for the elemental and fully oxidized form (B₂O₃) are 187.2 eV (ref. 18) and 193.0 eV,^19,20 respectively. Accordingly, the broad B1s spectrum shows that interstitial B atoms can exist in different stoichiometric forms with oxygen in the structure.

Fig. 4 SEM micrographs of 5 mol% Eu³⁺, and different concentrations B³⁺ co-doped SrTa₂O₆ samples: (a and b) without B³⁺, (c and d) 5 mol% B³⁺, (e and f) 10 mol% B³⁺, (g and h) 20 mol% B³⁺, under 15 kV accelerating voltage at 2000× and 10 000× magnifications, respectively.

Fig. 5 XPS spectra of (a) 5 mol% Eu³⁺, 20 mol% B³⁺ co-doped SrTa₂O₆ sample, (b) B1s.

3.2 Photoluminescence of SrTa₂O₆:xEu³⁺ and SrTa₂O₆:xEu³⁺, yB³⁺ phosphors

Fig. 6a–c and 7a–c show PL excitations and emissions of orthorhombic tungsten bronze β-SrTa₂O₆ phosphors corresponding to the SrTa₂O₆:xEu³⁺ (x = 0.5, 1.5, 3, 5, 7, 10 mol% “without boron”), SrTa₂O₆:xEu³⁺, yB³⁺ (x = 5 mol% “Eu³⁺ fixed”, y = 0, 5, 10, 20 mol%), and SrTa₂O₆:xEu³⁺, yB³⁺ (x = 0.5, 1.5, 3, 5, 7, 10 mol%, y = 10 mol% “boron fixed”), respectively. In Fig. 6a–c, PL excitations under 618 nm were assigned with ⁷F₀ → ⁵D₄, ⁷F₀ → ⁵G_J, ⁷F₀ → ⁵L₆, ⁷F₀ → ⁵D₃, and ⁷F₀ → ⁵D₂ transitions, respectively. In Fig. 7a–c, PL emissions with 465 nm excitation were observed at ⁵D₀ → ⁷F_J, (J = 0, 1, 2, 3, 4, 5) corresponding to the Eu³⁺ transitions. Fig. 7a shows the PL spectra of SrTa₂O₆:xEu³⁺ (x = 0.5, 1.5, 3, 5, 7, 10) phosphors. The PL intensity reached its maximum value at 5 mol%, while the concentration quenching occurred at 7 and 10 mol%. Therefore, the concentration of the 5 mol% Eu³⁺ doped phosphor with the highest PL intensity was used to investigate the effect of different B³⁺ concentrations. The PL emissions of SrTa₂O₆:xEu³⁺, yB³⁺ (x = 5, y = 0, 5, 10, 20 mol%) phosphors are given in Fig. 7b. The PL emission intensity increased up to 20 mol% B³⁺ concentration, while the intensity increase slightly slowed down over 10 mol%. By choosing 10 mol% B³⁺ doping ratio, SrTa₂O₆:xEu³⁺, yB³⁺ (x = 0.5, 1.5, 3, 5, 7, 10 mol%, y = 10 mol%) phosphors were fabricated to compare with the SrTa₂O₆:xEu³⁺ (x = 0.5, 1.5, 3, 5, 7, 10 mol%) series. The PL emissions of SrTa₂O₆:xEu³⁺, 0.1B³⁺ phosphors are presented in Fig. 7c, respectively. The PL emission intensity of SrTa₂O₆:xEu³⁺, 0.1B³⁺ phosphors showed a nearly threefold increase compared to the SrTa₂O₆:xEu³⁺ series, which is compatible with the luminescence increase reported in the literature due to boron effect.

Fig. 6 PL excitation spectra and CT bands of (a) SrTa₂O₆:xEu³⁺, (b) SrTa₂O₆:0.05Eu³⁺, yB³⁺ (c) SrTa₂O₆:xEu³⁺, 0.1B³⁺ phosphors corresponding to 618 nm emission.

Fig. 7 PL emission spectra of (a) SrTa₂O₆:xEu³⁺, (b) SrTa₂O₆:0.05Eu³⁺, yB³⁺ (c) SrTa₂O₆:xEu³⁺, 0.1B³⁺ phosphors with excitation of 465 nm.

The charge transfer bands (CTB) under 618 nm wavelength of ⁵D₀ → ⁷F₂ transition are shown in Fig. 6a–c. As seen in the figures, the CTB peaks are over 300 nm, which is related to the ligand nature of the host structure.^21,22 It has been reported by many studies that the CTB energy is found at low energies or over 300 nm due to the ligand environment with highly coordinated A and B tunnels of the tungsten bronze structure.⁶ Fig. 6b shows CT band position slightly affected with the increase in boron concentration, where the CTB peaks are at the wavelengths of 308.6, 310.4, 312.7, 314.2 nm for 0, 5, 10, 20 mol% B³⁺ concentrations, respectively. A shift to the lower energy in CTB position is also evident for the SrTa₂O₆:xEu³⁺, 0.1B³⁺ phosphors (Fig. 6c) compared to SrTa₂O₆:xEu³⁺ series (Fig. 6a), where CTB positions are by average about 313 and 309 nm, respectively. The shift in CTB energy is related to the binding strength of valence electrons and the site of rare earth.²² An atom with higher electronegativity will bind the oxygen 2p electron more strongly than an atom with lower electronegativity, where CT band will shift to a higher energy. The CTB energy will rise to a higher level, while the Eu³⁺ sites tend to be smaller. So, the CTB energy shift to the lower energy despite the high electronegativity of boron may be ascribed to the fact that the Eu³⁺ sites become larger. Another phenomenon is the relationship between CTB and hypersensitive transition. The inverse relationship between ⁵D₀ → ⁷F₂ transition and CTB is highlighted, where the more intensity of ⁵D₀ → ⁷F₂ transition indicates the lower energy of CTB.^22,23 In Fig. 7b, the asymmetry ratios for 0, 5, 10, and 20 mol% B³⁺ concentrations are 1.97, 2.34, 2.48, and 2.49, respectively. In Fig. 7a and c, the asymmetry values for Eu³⁺ doped and Eu³⁺, B³⁺ co-doped phosphors varied in the range of 1.83–2.07, and 2.21–2.58, respectively. The results show that boron doping causes a decrease in CTB energy and an increase in the ⁵D₀ → ⁷F₂ transition intensity. On the other hand, the inverse correlation between CTB intensity and ⁵D₀ → ⁷F₂ transition is previously observed in TTB-MTa₂O₆:Eu³⁺ (Sr, Ba, Pb) phosphors. The trend of the ⁵D₀ → ⁷F₂ transition is BaTa₂O₆:Eu³⁺ > SrTa₂O₆:Eu³⁺ > PbTa₂O₆:Eu³⁺ while trend of intensity of CTB and ⁵D₀ → ⁷F₁ transition is reverse.⁶ Hua et al.²⁴ compared the CINO:Eu³⁺, SINO:Eu³⁺, and BINO:Eu³⁺ phosphors. The CINO:Eu³⁺ phosphor has strong ⁵D₀ → ⁷F₂ transition and weak CTB intensity, while there is an opposite effect for SINO:Eu³⁺, BINO:Eu³⁺. A similar phenomenon was also observed in this study. In Fig. 6c, SrTa₂O₆:xEu³⁺, 0.1B³⁺ phosphors have a low CTB transition intensity and high asymmetry compared to SrTa₂O₆:xEu³⁺ phosphors (Fig. 6a). Since the formation of charge transfer bands or reduction of Eu³⁺ to Eu²⁺ is largely dependent on the nature of the ligands, the decrease in CTB density for the Eu³⁺, B³⁺ co-doped phosphors may be associated with the decreased electron transfer from neighboring atoms to Eu³⁺ ions in the tungsten bronze host. It has been reported in many studies that the addition of boron contributes to the improvement of crystallization and increases luminescence. Besides, it is generally believed that improved crystallinity properties and low crystal field symmetry are advantageous for the luminescence of rare earth activators.²⁵ Consequently, the increase in asymmetry associated with the low crystal field symmetry of the Eu³⁺ sites as well as some enhancement crystallinity in Eu³⁺, B³⁺ co-doped samples support the PL increase.

UV lamp photographs of the phosphors under 365 nm are given in Fig. 8a–c. As seen from the pictures, the Eu³⁺, B³⁺ co-doped phosphors in Fig. 8c are brighter than Eu³⁺ doped samples in Fig. 8a. The increase in brightness with the increasing B³⁺ concentration is more evident after 10 mol%, which is given in Fig. 8b.

Fig. 8 UV lamp photos of (a) SrTa₂O₆:xEu³⁺, (b) SrTa₂O₆:0.05Eu³⁺, yB³⁺ (c) SrTa₂O₆:xEu³⁺, 0.1B³⁺ phosphors at 365 nm.

3.3 Judd–Ofelt analysis of SrTa₂O₆:xEu³⁺and SrTa₂O₆:xEu³⁺, 0.1B³⁺ phosphors

Judd–Ofelt (JO) theory explains the intensity of electron transitions in 4f shell of rare-earth ions using three parameters and describes their spectral properties.^26,27 The JO intensity parameters Ω_J (J = 2, 4, 6) from emission spectrum for Eu³⁺ can be determined by eqn (1):^28,29where V is the transition frequency, I₁ and I_J for ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F_J transitions are the integrated intensities, respectively, S_MD = 9.6 × 10⁻⁴² (esu² cm²) is the magnetic dipole line strength,⁶ n is the refractive index, e = 4.803 × 10⁻¹⁰ (esu) is the elementary charge, J and J′ for the initial state and final state are the total angular momentum, respectively, |〈J‖U^J‖J〉|² is the symbol of double reduced matrix elements for unit tensor operators. For all electric dipole (ED) transitions originating from the ⁵D₀ level, the reduced matrix elements are zero except for the ⁵D₀ → ⁷F₂ (U² = 0.0032), ⁵D₀ → ⁷F₄ (U⁴ = 0.0023) and ⁵D₀ → ⁷F₆ (U⁶ = 0.0002) transitions.^28,29 The ⁵D₀ → ⁷F_J (J = 1, 2, 4, 6) transitions are used for the determination of the radiative transition probabilities while ⁵D₀ → ⁷F_J (J = 0, 3, 5) transitions are prohibited, and are not included JO calculation. The ⁵D₀ → ⁷F₆ transition related to Ω₆ parameter was not included in the calculation because it could not be detected by PL in infrared region. However, the effect of this transition on the calculation is negligible, which has been reported in some studies.^28,29 The spontaneous transition probability (A) is related to its dipole strength which can be expressed as eqn (2):where S_ED (electric dipole) and S_MD (magnetic dipole) are line strengths (in esu² cm²), h is Planck constant. The electric dipole line strengths (S_ED) from the JO parameters can be calculated by eqn (3):

The χ_ED and χ_MD are the local field corrections for the ED and MD transitions which can be found by eqn (4) and (5) respectively:

χ_MD = n³ (5)

where n is the refractive index of SrTa₂O₆ which was taken as 1.828 from ref. 6 and 7. The Judd–Ofelt intensity parameters (Ω₂, Ω₄) are given in Table 1. The parameter Ω₂, which is closely related to the hypersensitive electric dipole transition (⁵D₀ → ⁷F₂), indicates the covalency of the Eu–O bond character and the environmental changes of the Eu³⁺ ion.^6,7 The trend of the Ω₂ parameter is SrTa₂O₆:xEu³⁺, 0.1B³⁺ > SrTa₂O₆:xEu³⁺, where the slightly higher Ω₂ parameters of the boron doped phosphors indicate relatively high covalency of Eu–O and low local symmetry of Eu³⁺ sites. The parameter Ω₄ is related to the electron density of the surrounding ligands, where an increase in Ω₄ parameter indicates a decreased electron density.^28,29 The trend of the Ω₄ parameter is SrTa₂O₆:xEu³⁺, 0.1B³⁺ > SrTa₂O₆:xEu³⁺ wherein B³⁺ co-doped phosphors have relatively weak ligand density. The relatively high value of the Ω₄ parameter may be ascribed to the reduced charge transfer from the ligand ions to the Eu³⁺ ion, which supports the effect of boron to decrease the formation of CTB or the reduction of Eu³⁺ to Eu²⁺ and increase the ⁵D₀ → ⁷F₂ transition intensity.

Table 1 Judd–Ofelt intensity parameters (Ω₂, Ω₄) for SrTa₂O₆:xEu³⁺ (x = 0.5, 1.5, 3, 5, 7, 10 mol%) and SrTa₂O₆:xEu³⁺, yB³⁺ (x = 0.5, 1.5, 3, 5, 7, 10 mol%, y = 10 mol%) phosphors

Eu^3+ conc. (x mol%)	SrTa2O6:xEu^3+		SrTa2O6:xEu^3+, 0.1B^3+
	Ω2 (×10^−20 cm^2)	Ω4 (×10^−20 cm^2)	Ω2 (×10^−20 cm^2)	Ω4 (×10^−20 cm^2)
0.5	2.963	1.670	3.586	2.800
1.5	3.315	1.764	3.692	3.101
3	3.343	1.943	3.806	3.130
5	3.201	1.906	4.070	2.887
7	3.292	2.018	4.162	3.055
10	3.367	2.196	4.183	2.913

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Fig. 9a–c shows the decay curves of SrTa₂O₆:xEu³⁺, SrTa₂O₆:0.05Eu³⁺, yB³⁺, and SrTa₂O₆:xEu³⁺, 0.1B³⁺ phosphors, respectively. The decay curves for ⁵D₀ → ⁷F₂ transition were detected by the emission of 618 nm and excitation of 465 nm. Fluorescence bi-exponential decay curves can be produced using eqn (6):^27,28

where I_t is PL intensity, t is the time after excitation, I₀ is the background, I₁ and I₂ are luminescence intensities corresponding to τ₁ (long) and τ₂ (short) lifetimes, respectively. Accordingly, the average lifetime (τ) can be calculated through eqn (7):^28,30

Fig. 9 Decay curves of (a) SrTa₂O₆:xEu³⁺, (b) SrTa₂O₆:0.05Eu³⁺, yB³⁺ (c) SrTa₂O₆:xEu³⁺, 0.1B³⁺ phosphors with emission of 618 nm and excitation of 465 nm.

The fitting parameters of bi-exponential decays for SrTa₂O₆:xEu³⁺, SrTa₂O₆:xEu³⁺, 0.1B³⁺, and SrTa₂O₆:0.05Eu³⁺, yB³⁺ are tabulated in Tables 2 and 3, where the average lifetimes varied between 1.178–0.904, 1.162–0.892, and 0.979–0.958 ms, respectively. In Table 2, for both phosphor series, a somewhat decrease was observed in the decay times with the increase of the Eu³⁺ concentration, and slightly low decay times were found in Eu³⁺, B³⁺ co-doped samples compared to Eu³⁺ doped samples. The increased asymmetry supports the relatively lower average lifetimes of Eu³⁺, B³⁺ co-doped samples.³¹ The τ₁ (long) and τ₂ (short) lifetimes of the phosphors are given in Tables 2 and 3. In the literature, the authors made definitions of the long (τ₁) and short (τ₂) lifetimes, where the short decay component is associated with Eu³⁺ ions close to the surfaces of the particles, while the long decay component is related to Eu³⁺ ions in inner part of the particles.^32–35 Padlyak et al.³⁶ emphasized that the long lifetimes are characteristic of luminescence centers and belong to isolated Eu³⁺ centers in oxide glasses and polycrystalline samples while the short components belong to the Eu³⁺–Eu³⁺ exchange-coupled pairs or small exchange-coupled Eu³⁺ clusters. On the other hand, Gupta et al.^37,38 highlighted that the long component and the short component are associated with high symmetry and low symmetry of Eu³⁺ ions, respectively. In Tables 2 and 3, there is an increase in τ₂ lifetimes and slight decrease in τ₁ lifetimes of Eu³⁺, B³⁺ co-doped samples compared to Eu³⁺ doped phosphors. Accordingly, the change in the τ₁ and τ₂ lifetimes may be originated from the activation of Eu³⁺ luminescence centers at the particle surface rather than in the particle core/isolated centers. The variation in τ₂ an τ₁ lifetimes can be explained by the lowering of local symmetry of Eu³⁺ due to the presence of boron, which was previously expressed as an increase in ⁵D₀ → ⁷F₂/⁵D₀ → ⁷F₁. The asymmetry variation of the Eu³⁺ doped samples is associated with the A, and B site occupation, charge balance, or some degree of crystal distortion in the tungsten bronze structure.^4,39 On the other hand, although there is some improvement in the crystal properties, the increase in the ratio of ⁵D₀ → ⁷F₂/⁵D₀ → ⁷F₁ or the decrease in the local symmetry of Eu³⁺ ions is high in Eu³⁺, B³⁺ co-doped samples. As mentioned in the XPS results, the fact that boron makes strong bonds with oxygen in the structure may be the reason for this difference (such as polarization). As another consequence, increase of grain size in SEM micrographs is associated with the vitrification effect that accelerates agglomeration in boron grains. By evaluating the SEM micrographs and the decay components, it is seen that there is a correlation between grain growth and the increase in the short components. Similar behavior has been observed in Sm³⁺ doped BaTa₂O₆ phosphor, where grain growth with increasing sintering temperature leads to an increase in short components.⁵ Although this requires further observation, it shows that grain growth may be related to the increase of Eu³⁺ luminescence centers on grain surfaces and a decrease in crystal field symmetry. The quantum efficiency (η_QE) of ⁵D₀ level can be found from the ratio of the average (observed) lifetime (τ) to the radiative lifetime (τ_r) by eqn (8):

where the radiative and nonradiative transitions of ⁵D₀ are A_r and A_nr, respectively. The radiative lifetimes (τ_r) and quantum efficiencies (η_QE) are tabulated in Table 2. The values of the η_QE% for SrTa₂O₆:xEu³⁺ and SrTa₂O₆:xEu³⁺, 0.1B³⁺ phosphors varied between 36.03–30.30% and 42.72–36.08%, respectively. The η_QE% for Eu³⁺ and B³⁺ co-doped phosphors corresponds to an approximate increase of 15–21%. The quantum efficiency of the highest luminescent (or 0.05Eu³⁺, 0.1B³⁺ co-doped) phosphor increased by about 21%. As mentioned in the previous sections, the low symmetry of the Eu³⁺ sites and some enhancement in crystal structure properties are likely to increase the luminescence efficiency.

Table 2 Long (τ₁) and short (τ₂) lifetimes, average (observed) lifetimes (τ), radiative lifetimes (τr), quantum efficiencies (η_QE) and chi-square (x²) values for SrTa₂O₆:xEu³⁺ (x = 0.5, 1.5, 3, 5, 7, 10 mol%) and SrTa₂O₆:xEu³⁺, yB³⁺ (x = 0.5, 1.5, 3, 5, 7, 10 mol%, y = 10 mol%) phosphors

Eu^3+ conc. (x mol%)	SrTa2O6:xEu^3+						SrTa2O6:xEu^3+, 0.1B^3+
	τ1 (ms)	τ2 (ms)	τ (ms)	τr (ms)	ηQE (%)	x^2	τ1 (ms)	τ2 (ms)	τ (ms)	τr (ms)	ηQE (%)	x^2
0.5	1.293	0.504	1.178	3.283	35.88	1.190	1.259	0.429	1.162	2.720	42.72	1.106
1.5	1.274	0.319	1.101	3.056	36.03	1.151	1.205	0.521	1.086	2.619	41.47	1.196
3	1.198	0.421	1.012	3.025	33.45	1.179	1.183	0.452	1.003	2.570	39.03	1.108
5	1.147	0.462	0.978	3.110	31.45	1.156	1.135	0.425	0.959	2.516	38.12	1.225
7	1.112	0.387	0.919	3.033	30.30	1.142	1.104	0.437	0.902	2.456	36.73	1.208
10	1.148	0.395	0.904	2.952	30.62	1.360	1.113	0.423	0.892	2.472	36.08	1.194
Avg	1.195	0.415	1.015	3.077	32.96	—	1.167	0.448	1.001	2.559	39.03	—

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Table 3 Long (τ₁) and short (τ₂) lifetimes, average (observed) lifetimes (τ) and chi-square (x²) values for SrTa₂O₆:xEu³⁺, yB³⁺ (x = 5 mol%, y = 0, 5, 10, 20 mol%) phosphors

B^3+ conc. (y mol%)	SrTa2O6:0.1Eu^3+
	τ1 (ms)	τ2 (ms)	τ (ms)	x^2
0	1.131	0.306	0.979	1.057
5	1.111	0.346	0.991	1.003
10	1.147	0.436	0.950	1.178
20	1.110	0.440	0.958	1.107

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4. Conclusion

The effect of boron on the structural and spectral properties was studied by using Eu³⁺ doped orthorhombic β-SrTa₂O₆ fabricated by solid-state method at 1400 °C for 20 h. The phase structure of all the ceramics preserved up to 10 mol%, and the Eu³⁺, B³⁺ co-doped samples have relatively slightly better crystallinity. SEM micrographs showed that the presence of boron promoted agglomeration and growth in grains. The increment in boron concentration led to an increase in PL up to 20 mol%, and the increase of PL may be related to the low symmetry of Eu³⁺ ions and the improvement of crystal properties. In Eu³⁺, B³⁺ co-doped samples, the charge transfer band slightly shifted to the lower energy and asymmetry of phosphor increased. The trend of Judd–Ofelt (Ω₂, Ω₄) parameters was SrTa₂O₆:xEu³⁺, 0.1B³⁺ > SrTa₂O₆:xEu³⁺. The high values of the Ω₂, and Ω₄ for Eu³⁺, B³⁺ co-doped phosphors related to a covalent Eu–O bond character with low symmetry of the Eu³⁺ ions and a reduced electron density in the ligands, respectively. While the increase of Eu³⁺ causes a slight decrease in the decay times, the decrease is slightly more in the Eu³⁺, B³⁺ co-doped samples. The increase in τ₂ lifetimes may be associated with the activation of the Eu³⁺ luminescence centers at the grain surface and near the surface. The quantum efficiencies of the Eu³⁺, B³⁺ co-doped phosphors increased between 15–21%.

Conflicts of interest

There are no conflicts to declare.

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