Exploring the effect of boron on the grain morphology change and spectral properties of Eu³⁺ activated barium tantalate phosphor

Abstract

The effect of the grain morphology on the photoluminescence, charge transfer band, and decay properties was investigated by xEu³⁺, yB³⁺ (x = 10 mol%, y = 0, 5, 15, 30, 50, 70, and 100 mol%) co-doped BaTa₂O₆ ceramics fabricated by solid-state reaction. X-ray diffractions of the samples showed that the single-phase structure persisted up to 100 mol% and there was an improvement in crystallinity with increasing B³⁺ concentration. SEM micrographs of the Eu³⁺, B³⁺ co-doped grains showed that the flux effect of boron promotes grain growth and elongated grain shape. The PL emissions of the BaTa₂O₆:xEu³⁺, yB³⁺ co-doped phosphors increased up to 100 mol% B³⁺ concentration, and there was an increase in the intensities of the CTB energy ⁵D₀ → ⁷F₁ transition. The increase in PL may be attributed to the increased grain size leading to a decrease in the surface area (SA)/volume (vol) ratio with increasing B³⁺ concentration, as well as the improvement in crystallinity. However, the decrease in asymmetry ratio was related to the occupation of centrosymmetric (B) sites and the transformation from a rounded/irregular-like to an elongated/rod-like grain shape which has an increasing effect on the SA/vol ratio. The decreasing trend of the Judd–Ofelt parameters (Ω₂, and Ω₄) with the increase in boron was related to a high local symmetry of Eu³⁺ sites, and an increase in the electron density of the surrounding ligands, respectively. The increase in boron led to longer decays in the observed lifetime with bi-exponential characteristics. The CIE diagram and UV lamp photographs of the phosphors showed a color transition from red to orange associated with the increasing magnetic dipole transition. This study may provide an alternative perspective and new strategies to describe the control of grain morphology and luminescence concerning RE-doped phosphors.

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1. Introduction

Promising advances are being made in a variety of applications, including solid-state lasers, LEDs, optical communication, and temperature sensing, due to the activation of phosphors with trivalent rare earth ions (RE³⁺). Their unique optical properties such as extended luminescence times, narrow emission ranges, and excellent photostability are of great interest.^1–8 In the development of luminescent materials, the grain morphology of ceramic phosphors has an effect on the luminescence. Therefore, the grain size, and grain shape of phosphor may play an important role in the correlation between the material and spectral properties.^9–19 In the literature, researchers have drawn attention to the effects of morphological features such as grain size and grain shape on the luminescence mechanism, where the surface area (SA) to volume (vol) ratio is an important parameter.^9–19 Boron vitrification plays an effective role in grain morphology, increasing crystallization, reducing sintering temperature, and improving optical and structural properties.^18–22 The enhancement of luminescence with boron doping has been pointed out in many studies to be related to improved crystallinity.^23–25 However, there are also some studies showing the luminescence increase along with the decrease in crystallization with the doping of boron.^26,27 The photoluminescence of boron co-doped Ba_0.88Mg_0.93Al₁₀O_16.96:Eu³⁺ phosphor has been studied by Dulda et al.²⁸ where the grain size and crystallinity of phosphor increased as the grain shape changed from rod to hexagonal shape. Moreover, some studies have been performed by İlhan et al. on Eu³⁺, B³⁺ co-doped SrTa₂O₆,¹⁸ and Dy³⁺, B³⁺ co-doped CdNb₂O₆ (ref. 19) phosphors, where it is emphasized that the increased luminescence is closely related to grain morphology.

Tetragonal is the middle-temperature polymorph of BaTa₂O₆ that occurs between 1150–1300 °C, and this polymorph related to tetragonal tungsten bronze (TTB) structure which can remain kinetically stable at high temperatures (e.g. 1550 °C).^29–31 TTB crystal structure belongs to a wide family of functional materials and offers a large scale of doping possibilities due to its suitable structure for different cationic dopants. TTB oxide structures having the A₂BC₂M₅O₁₅ formula (M = Ta, Nb) consist of three types of sites in which the pentagonal (A), square (B), and triangular (C) tunnels are represented by 15, 12, and 9 CN, respectively.^29,30

In this paper, we reported the effect of grain size and grain shape on spectral properties with a systematic study, which has been scarcely researched before in the literature. In our previous two studies,^18,19 effect of boron dopant has been examined up to 20 and 25 mol% concentrations, respectively. However, in this detailed study, the relationship between spectral properties and grain morphology was investigated by increasing the boron concentration up to 100 mol%, and interesting results were obtained. Depending on the structural and morphological changes with increasing boron concentration, the effects of varying grain size, grain shape, and different SA/vol ratios on luminescence, charge transfer band, decay properties, and Judd–Ofelt parameters were studied by applying XRD, SEM, PL, and decay analyses.

2. Experimental

BaTa₂O₆:xEu³⁺, yB³⁺ (x = 10 mol%, y = 0, 5, 15, 30, 50, 70, 100 mol%) samples were fabricated by solid-state method. The BaCO₃ (Sigma-Aldrich, 99%), Ta₂O₅ (Alfa Aesar, 99.9%), Eu₂O₃ (Alfa Aesar, 99.9%), and H₃BO₃ (Kimyalab, %99.9) were used in the synthesis process. The stoichiometric amount of the starting materials was mixed and ground well in an agate mortar to provide homogeneity. Then, by adding europium oxide (Eu₂O₃) and boric acid (H₃BO₃), the final mixture of the powders was thoroughly mixed and ground in an agate mortar for the last time to provide more homogeneity. For the sintering process, a sufficient amount of mixture was taken and sintered in an alumina crucible at 1425 °C for 20 h after pelleting.

The phase compositions of the samples were identified by XRD (X-ray diffractometer; PANanalytical Empyrean, U.K.) using Cu Kα radiation between 2θ = 20–70 °C with scan speed 2 °C min⁻¹. The grain morphologies of the samples were investigated by SEM (scanning electron microscopy; FE-SEM; Gemini 500, Zeiss Corp., Germany). 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 a fluorescence spectrometer (FLS920) using the time-correlated single-photon counting (TCSPC) system. All the analyses were performed at room temperature.

3. Results and discussions

3.1 XRD and SEM results

XRD results of 10 mol% Eu³⁺ and yB³⁺ (y = 0, 5, 15, 30, 50, 70, 100 mol%) co-doped BaTa₂O₆ ceramic samples are shown in Fig. 1. XRD patterns of BaTa₂O₆ showed a single tetragonal tungsten bronze structure (JCPDS card no: 17-0793) with space group P4/mbm (127). The absence of any minor phase can be explained by the successful incorporation of Eu³⁺ ions in the A and B sites of the TTB host and the dissolution of co-doped boron with different concentrations in the three tunnels of the structure. On the other hand, increasing B³⁺ concentration led to an increase in (001) and (002) XRD reflections, which would be attributed to a z-direction orientation. As mentioned in the SEM part, this phenomenon is related to the grain elongation of BaTa₂O₆ at high temperatures,^31,32 and also the boron vitrification may cause a similar effect in the structure. The shift of XRD peaks (311) towards the smaller two theta angles is shown in Fig. 2. The B³⁺ ions occupying the structure with a small ionic radius potentially caused an expansion in the lattice due to increased positive charge. Table 1 summarizes the variation of lattice data of all samples. The lattice data for 0 and 100 mol% B³⁺ samples are a = 12.6272 Å, c = 3.8916 Å, V = 620.51 ų, and a = 12.7436 Å, c = 3.9052 Å, V = 634.20 ų, respectively, in which the lattice volume increased as 2.2%. Additionally, in Fig. 2, the narrowing XRD peaks with increasing B³⁺ indicate an increase in crystallinity. The crystallite size of the samples determined using the Scherrer equation is given in Table 1, where the crystallite sizes from 0 to 100 mol% B³⁺ co-doped samples changed to 32.58 and 50.53 nm, respectively. The enhancement in crystal size may be attributed to the reduction in nucleation and lattice deformation resulting from increased boron concentration.

Fig. 1 XRD results of 10 mol% Eu³⁺ and 0, 5, 15, 30, 50, 70, and 100 mol% B³⁺ co-doped BaTa₂O₆ samples.

Fig. 2 Shift of XRD two theta peaks (311) to lower angles.

Table 1 Cell parameters and average crystallite sizes for Eu³⁺, B³⁺ co-doped BaTa₂O₆

Sample	Lattice parameters			Crystallite size
	a (Å)	c (Å)	V (Å)^3	D (nm)
JPCDS card no. 17-0793	12.5600	3.9580	624.39	—
0 mol B^3+ 10 mol% Eu^3+	12.6272	3.8916	620.51	32.58
5 mol B^3+ 10 mol% Eu^3+	12.6452	3.8986	623.39	39.60
15 mol B^3+ 10 mol% Eu^3+	12.6740	3.8990	626.30	41.68
30 mol B^3+ 10 mol% Eu^3+	12.7024	3.9004	629.33	40.48
50 mol B^3+ 10 mol% Eu^3+	12.7088	3.9010	630.07	48.49
70 mol B^3+ 10 mol% Eu^3+	12.7364	3.9046	633.39	49.71
100 mol B^3+ 10 mol% Eu^3+	12.7436	3.9052	634.20	50.53

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Fig. 3(a–g) shows SEM micrographs of Eu³⁺, B³⁺ co-doped ceramic samples at 5000× magnification and 10 kV acceleration voltage. As seen from the SEM micrographs, boron doping promoted agglomeration, grain growth, and rod-like shape in grains. The elongation tendency of grains may be attributed to the vitrification effect improved by the presence of B³⁺, which is supported by (001) and (002) XRD reflections. In Fig. 3a, the undoped boron sample has a small grain size, and a round-like irregular grain-shaped morphology, while the grain sizes usually range from 0.5 to 1.5 μm. In Fig. 3b, the growing and elongating tendency of the grains started to occur in the 5 mol% B³⁺ co-doped sample, and it became more pronounced at 15 mol% B³⁺ (Fig. 3c). In Fig. 3d, the grain shape of 30 mol% B³⁺ doped sample had almost rod-like and thick-grained appearance while the growth and elongation trend in the grains continued for higher B³⁺ concentrations as seen in Fig. 3(e–g). In Fig. 3e, the amount of the grains with small size was considerably decreased at 50 mol% B³⁺, and it nearly disappeared at 70 (Fig. 3f) and 100 mol% (Fig. 3g) B³⁺ concentrations. The grain sizes for the without boron sample varied between 0.5–3.0 μm, while the thicknesses and lengths of the grains reached the range of 1.5–10 μm and 5–40 μm, for 100 mol% B³⁺ concentration, respectively.

Fig. 3 SEM micrographs of 10 mol% Eu³⁺, and different concentrations B³⁺ co-doped BaTa₂O₆ samples: (a) 0 mol%, (b) 5 mol% B³⁺, (c) 15 mol% B³⁺, (d) 30 mol% B³⁺, (e) 50 mol% B³⁺, (f) 70 mol% B³⁺, (g) 100 mol% B³⁺, under 10 kV accelerating voltage at 5.000× magnifications.

3.2 Spectral properties for BaTa₂O₆:xEu³⁺, yB³⁺ (x = 10 mol%, y = 0, 5, 15, 30, 50, 70, and 100 mol%) phosphors

Fig. 4 shows PL excitation spectra of BaTa₂O₆:xEu³⁺, yB³⁺ (x = 10 mol%, y = 0, 5, 15, 30, 50, 70, 100 mol%) phosphors recorded at λ_em = 618 nm. PL excitations of all the phosphors were assigned by the ⁷F₀ → ⁵D₄, ⁷F₀ → ⁵G_J, ⁷F₀ → ⁵L₆, ⁷F₀ → ⁵D₃, and ⁷F₀ → ⁵D₂ transitions, which are related to the intra-configurational (f–f) transitions of Eu³⁺. In Fig. 4, the peaks associated with the CTB transition have a broadband and a maximum in the 340–350 nm range. The CTB peaks over 300 nm are related to the ligand nature of TTB structure due to the ligand environment with highly coordinated A and B tunnels.^33,34 As reported in similar studies,^18,29 the CTB transition intensity increased with increasing intensity of magnetic dipole transition or the formal reduction of Eu³⁺ to Eu²⁺ is increasing. Fig. 5 shows the PL emissions of BaTa₂O₆:xEu³⁺, yB³⁺ (x = 10 mol%, y = 0, 5, 15, 30, 50, 70, 100 mol%) phosphors monitored by the ⁵D₀ → ⁷F_J, (J = 0, 1, 2, 3, 4, 5) transitions under excitation of 394 nm, in which the phosphor luminescence increased up to 100 mol% B³⁺ concentration. The increase in luminescence can be evaluated in terms of crystallinity and morphology. As mentioned in XRD results, the improved crystallinity will contribute to the increase of the luminescence of BaTa₂O₆:Eu³⁺, B³⁺. On the other hand, literature results on the luminescence increase despite decreasing crystallinity^26,27 show that morphological parameters such as grain size and grain shape are very important in the increase in luminescence. The effects of the grain size and grain shape on luminescence are reported by different researchers. Kim and Kang⁹ highlighted that an increase in grain size led to an increase in luminescence due to the reduced presence of non-luminescent centers near the grain surface. Shan et al.¹⁰ investigated the effects of nanocrystals with different shapes on luminescence using surface area (SA)/volume (vol) ratios. Some researchers^11–13 suggested that phosphors with spherical grain morphology exhibit superior spectral characteristics, including reduced light scattering, enhanced luminescence, and higher definition. Accordingly, the effect of boron may be evaluated in terms of size and shape changes in the grains. Although the increase in boron concentration caused the grain size to increase, there was a deviation from the spherical grain shape due to grain elongation. The transformation of the grains from an irregular/round-like shape to an elongated/rod-like shape would be likely to support an increase in the SA/vol ratio. However, if the grain shape is preserved or there is no grain elongation, grain growth will contribute more to the decrease of the SA/vol ratio, and the activation of luminescence centers near the surface will increase further. In this process, since grain growth and grain elongation occur together, grain elongation will contribute to an increase in the SA/vol ratio. However, although there is an increase in luminescence, the difference in asymmetry ratio (⁵D₀ → ⁷F₂/⁵D₀ → ⁷F₁) is remarkable. The variation of the asymmetry ratio and B³⁺ concentration is shown in Fig. 6. The emission increases were about 11 and 3 fold from 0 to 100 mol% B³⁺ for the magnetic dipole transition (⁵D₀ → ⁷F₁) and the electric dipole transition (⁵D₀ → ⁷F₂), respectively. In TTB structure, the pentagonal (A) sites are non-centrosymmetric and associated with the ⁵D₀ → ⁷F₂ transition, where Eu³⁺ ions are sensitive to the crystal field. The square (B) sites are centrosymmetric and related to the ⁵D₀ → ⁷F₁ transition in which the behavior of Eu³⁺ ions will not be affected by the crystal field.³³ The Eu³⁺ ions inserted to TTB-host are more likely to occupy A sites (15 CN) rather than B sites (12 CN). The improving magnetic dipole transition indicated that the increased presence of boron increases the activation of Eu³⁺ ions in the B sites. On the other hand, based on the increase in ⁵D₀ → ⁷F₁ transition, a correlation between grain elongation and B site occupancy is noticeable. The magnetic dipole transitions of lanthanide ions are multiple transitions with strong magnetic dipole character and emission wavelengths throughout the near-infrared, visible, and ultraviolet (UV) spectral range, however, most of these transitions typically involve highly excited states that are rapidly decayed by non-radiative pathways such as multiphonon relaxation process.³⁵ Shan et al.¹⁰ highlighted that prism and rod nanocrystals with the same surface area (SA)/volume (vol) ratios can exhibit different lattice energies and multiphonon relaxation processes. Furthermore, the phosphor with low luminescence generally has a larger surface-to-volume (SA/vol) ratio and a higher quenching defect fraction than the phosphor with good luminescence. The low phonon energy modes are cut off at smaller particle sizes due to the phonon confinement effect significantly reducing the energy transfer and luminescence between a donor and an acceptor.^36–38 Consequently, based on the occupancy of B site, the magnetic dipole character supports the multiphonon relaxation process, and it is also likely that the elongated grain shape with increasing grain size of the phosphor promotes the multiphonon relaxation process due to the difference in phonon energy.

Fig. 4 PL excitation spectra and charge transfer bands of BaTa₂O₆:xEu³⁺, yB³⁺ phosphors corresponding to 618 nm emission.

Fig. 5 PL emission spectra of BaTa₂O₆:xEu³⁺, yB³⁺ phosphors with excitation of 394 nm.

Fig. 6 Asymmetry ratio and boron concentration variation of BaTa₂O₆:xEu³⁺, yB³⁺ 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.^39,40 The JO intensity parameters Ω_J (J = 2, 4, 6) from the emission spectrum for Eu³⁺ can be determined by eqn (1):

where 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, 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. 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 the infrared region. However, the effect of this transition on the calculation is negligible, which has been reported in some studies.^18,29 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), and h is Planck constant. The electric dipole line strengths (S_ED) from the JO parameters can be calculated by eqn (3):

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

where n is the refractive index of BaTa₂O₆ which was taken as 1.891 from ref. 29 and 41 The Judd–Ofelt intensity parameters (Ω₂, Ω₄) are given in Table 2. 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. The parameter Ω₄ is related to the electron density of the surrounding ligands, where an increase in the Ω₄ parameter shows a decreased electron density.^18,42 In Table 2, the Ω₂ parameter decreased from 5.299 to 1.506 × 10⁻²⁰ cm², and the Ω₄ parameter decreased from 3.794 and 1.541 × 10⁻²⁰ cm², with increasing B³⁺ concentration, respectively. The decreasing trend in the Ω₂ parameter showed weak covalent or strong ionic character of the Eu–O bond and high local symmetry of the Eu³⁺ sites, while the decrease in Ω₄ parameter indicated an increase in the electron density of the surrounding ligands. Moreover, the decreased Ω₄ parameter may be attributed to the increased charge transfer in the host which may be attributed to the reduction of Eu³⁺ to Eu²⁺ and a decrease in the electric dipole intensity. The branching ratio (β) of the Judd–Ofelt theory can be obtained from the radiated transition probability (A(J, J′)) and the total radiated transition probability (ΣA(J,J′)) using eqn (6), the results are given in Table 2:

Table 2 J–O parameters (Ω₂, Ω₄), radiative transition and total transition probabilities (A(J,J′), (A_r)), and branching ratios (β), of BaTa₂O₆:xEu³⁺, yB³⁺ (x = 10 mol%, y = 0, 5, 15, 30, 50, 70, and 100 mol%) phosphors

B^3+ conc. (mol%)	Eu^3+ transitions	Ω2 (10^−20 cm^2)	Ω4 (10^−20 cm^2)	A(J,J′) (s^−1)	Ar (s^−1)	β (%)
0	^5D0 → ^7F1	5.299	3.794	99.008	556.462	17.79
	^5D0 → ^7F2			339.338		60.98
	^5D0 → ^7F4			118.115		21.23
5	^5D0 → ^7F1	4.878	2.417	99.092	486.692	20.36
	^5D0 → ^7F2			312.352		64.18
	^5D0 → ^7F4			75.249		15.46
15	^5D0 → ^7F1	3.063	2.014	99.011	357.839	27.67
	^5D0 → ^7F2			196.130		54.81
	^5D0 → ^7F4			62.697		17.52
30	^5D0 → ^7F1	2.240	1.659	98.843	293.924	33.63
	^5D0 → ^7F2			143.416		48.79
	^5D0 → ^7F4			51.665		17.58
50	^5D0 → ^7F1	1.645	1.437	98.744	248.823	39.68
	^5D0 → ^7F2			105.324		42.33
	^5D0 → ^7F4			44.755		17.99
70	^5D0 → ^7F1	1.546	1.542	98.846	245.835	40.21
	^5D0 → ^7F2			98.978		40.26
	^5D0 → ^7F4			48.012		19.53
100	^5D0 → ^7F1	1.506	1.541	98.817	243.259	40.62
	^5D0 → ^7F2			96.461		39.65
	^5D0 → ^7F4			47.981		19.73

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The branching ratio of the phosphors for the ⁵D₀ → ⁷F₂ transition decreased due to the increasing magnetic dipole character. The β ratio of electric and magnetic transitions from 0 to 100 mol% B³⁺ changed to 60.98–39.65% and 17.79–40.62%, respectively.

Fig. 7 shows the decay curves of ⁵D₀ → ⁷F₂ transition (618 nm) with excitation of 394 nm, for Eu³⁺, B³⁺ co-doped phosphors. The two exponential decay curves can be defined using eqn (7):

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 or observed lifetime (τ) can be found by eqn (8):^41–45

Fig. 7 Decay curves of BaTa₂O₆:xEu³⁺, yB³⁺ phosphors with the emission of 618 nm and excitation of 394 nm.

The variation of the bi-exponential decays depending on B³⁺ concentration is given in Table 3. The observed lifetimes increased from 0.934 to 1.362 μs in the range of 0–100 mol% B³⁺, respectively. Based on the phonon energy of RE ions for the same host, a relatively longer PL lifetime is attributed to a more symmetrical site as the f–f transition becomes more forbidden,⁴⁶ where the decrease in asymmetry ratio or increase in local environmental symmetry of Eu³⁺ ions supports the decay results. In Table 3, the long lifetime (τ₁) and short lifetime (τ₂) with increased B³⁺ changed from 1.109 and 0.436 μs to 1.445 and 0.654 μs, respectively. The long lifetime (τ₁) is associated with RE³⁺ ions on the inside of the particle/isolated centers, and the short lifetime (τ₂) is related to RE³⁺ ions close to the surface and/or on the surface of the particle.^45,47 Besides, the long lifetime and the short lifetime are associated with high symmetry and low symmetry of Eu³⁺ ions, respectively.⁴⁶ The variation of the long (τ₁ × I₁) components and short (τ₂ × I₂) components depending on B³⁺ concentration is displayed in Fig. 8. The increase in the long and short components shows that the presence of boron increases the activation of Eu³⁺ luminescence centers in both particle core/isolated centers and on the particle surface. However, the increase in τ₁ × I₁ trend is higher than the τ₂ × I₂, indicating that the activation of Eu³⁺ centers within the particle is increased more than Eu³⁺ luminescence centers at or near the particle surface. Moreover, the increase in the τ₁ × I₁ trend supports the more symmetrical or magnetic dipole character of Eu³⁺ sites and the occupation of Eu³⁺ ions in B-centrosymmetric sites.

Table 3 Long (τ₁) and short (τ₂) lifetimes, long (l₁) and short (l₂) luminescence intensities, average (observed) lifetimes (τ) and chi-square (x²) values for BaTa₂O₆:xEu³⁺, yB³⁺ (x = 10 mol%, y = 0, 5, 15, 30, 50, 70, and 100 mol%) phosphors

B^3+ conc. (mol%)	τ1 (ms)	l1	τ2 (ms)	l2	τ (ms)	x^2
0	1.109	2008	0.436	1801	0.934	1.123
5	1.049	5415	0.537	3837	0.913	1.104
15	1.227	5041	0.537	5828	0.995	1.232
30	1.250	8459	0.487	8344	1.038	1.249
50	1.366	10 280	0.439	7051	1.199	1.269
70	1.433	12 365	0.617	3821	1.337	1.073
100	1.445	15 783	0.654	4055	1.362	0.981

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Fig. 8 Variation of product curves for the long (τ₁ × I₁) and short (τ₂ × I₂) components depending on the boron concentration.

The quantum efficiency (η_QE) of the phosphors can be found from the ratio of the average (observed) lifetime (τ) to the radiative lifetime (τ_r) by eqn (9):

where the radiative and nonradiative transitions of ⁵D₀ level are A_r and A_nr, respectively. The radiative lifetimes (τ_r) and quantum efficiencies (η_QE) are tabulated in Table 4. The values of the η_QE% from 0 to 100 mol% B³⁺ varied to 51.95% and 33.14%, respectively, which corresponds to a decrease of approximately 36%, which may be attributed to the multiphonon relaxation process associated with the improving magnetic dipole character. Moreover, the energy transfer possibility between Eu³⁺–Eu³⁺ ions can be predicted by eqn (10):⁴¹where τ₀ is the observed lifetime for low concentration. Table 4 summarizes the energy transfer probabilities decreased by approximately 46%, in which the increase in B³⁺ concentration caused the local environment of Eu³⁺ to become more symmetrical and the energy transfer process to become more inhibiting.

Table 4 Radiative lifetimes (τ_r), observed lifetimes (τ), quantum efficiencies (η_QE) and energy transfer possibilities (η_Eu→Eu) of BaTa₂O₆:xEu³⁺, yB³⁺ (x = 10 mol%, y = 0, 5, 15, 30, 50, 70, and 100 mol%) phosphors

B^3+ conc. (mol%)	τr (ms)	τ (ms)	ηQE (%)	ηEu→Eu (%)
0	1.797	0.934	51.95	0.00
5	2.055	0.913	44.43	2.23
15	2.795	0.995	35.61	−6.60
30	3.402	1.038	30.51	−11.17
50	4.019	1.199	29.83	−28.43
70	4.068	1.337	32.86	−43.19
100	4.111	1.362	33.14	−45.92

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Fig. 9 displays the chromaticity coordinates of BaTa₂O₆:xEu³⁺, yB³⁺ phosphors in the CIE diagram, illustrating how they vary depending on the concentration of Eu³⁺. CIE coordinates (x, y) of the phosphors from 0 to 100 mol% B³⁺ changed to (0.6616, 0.3382) and (0.6339, 0.3657), respectively. The phosphor's emission shows a progression from red to orange as B³⁺ concentration increases. Thus, by augmenting the B³⁺ concentration in the CIE diagram, the emission of Eu³⁺ can be adjusted from the red region to the orange region. The UV lamp photographs of the phosphors under 365 nm are given in Fig. 9, in the inset figure. The UV lamp photo of the undoped boron sample has a red appearance, showing a transition towards an orange appearance with increasing B³⁺ concentration.

Fig. 9 CIE coordinates (x, y) and UV lamp photos at 365 nm of BaTa₂O₆:xEu³⁺, yB³⁺ phosphors.

4. Conclusion

In the study, the effect of the grain size and grain shape on the spectral properties was investigated by using BaTa₂O₆:xEu³⁺, yB³⁺ (x = 10 mol%, y = 0, 5, 15, 30, 50, 70, 100 mol%) co-doped phosphor ceramics. XRD results showed a single-phase structure up to 100 mol%, while the increase in B³⁺ concentration led to an improvement in crystallinity and an increase in reflections (001) and (002) associated with elongation in the z-direction. SEM examinations revealed the elongation and growth in grains with increasing B³⁺ concentration. The increase in PL emission was associated with improved crystallinity and grain growth or decreasing SA/vol ratio, while the increased magnetic dipole character of phosphor was attributed to centrosymmetric B-site occupation and grain elongation. The decrease in the Ω₂ and Ω₄ parameters indicated the high local symmetry of the Eu³⁺ sites and increased charge transfer in the host, respectively. The increased B³⁺ presence led to an increase in the observed lifetime, which is attributed to a more symmetrical site as the f–f transition becomes more forbidden. The increase in the long and short components showed the activation of Eu³⁺ centers within the grains and at/near the grain surfaces increased, in addition, the relatively high increase in long components supported the increased site symmetry and magnetic dipole character of Eu³⁺ ions. The decrease in quantum efficiency and energy transfer efficiency was 36% and 46% from 0 to 100 mol% B³⁺, respectively. The decrease in quantum efficiency and energy transfer efficiency was associated with the increased multiphonon relaxation process resulting from the magnetic dipole transition. The variation of luminescence properties concerning grain morphology also demonstrates that they can be adjusted by controlling the reaction conditions.

Conflicts of interest

There are no conflicts to declare.

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