Structure and luminescence properties of multicolor phosphor Ba₂La₃(GeO₄)₃F:Tb³⁺,Eu³⁺

Abstract

A new kind of multicolor phosphor Ba₂La₃(GeO₄)₃F:0.15Tb³⁺,xEu³⁺ (BLGOF:0.15Tb³⁺,xEu³⁺) has been acquired through the traditional high temperature solid phase synthesis method. The structural information of the phosphor was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Rietveld refinement. The optical properties of the phosphor have also been studied in detail, including its photoluminescence spectra (PL), photoluminescence excitation spectra (PLE), fluorescence decay curves, energy transfer mechanism and thermal quenching spectra. It has been found that the optimum concentration of Eu³⁺ in BLGOF:0.15Tb³⁺,xEu³⁺ is 0.24 mol and the energy transfer mechanism from Tb³⁺ to Eu³⁺ in BLGOF is quadrupole–quadrupole. The color of BLGOF:0.15Tb³⁺,xEu³⁺ phosphors can be changed from green to yellow/orange to red. Some details of the energy transfer are reviewed and the effect of complex anion regulation on thermal stability has also been studied. All the properties are good and can contribute to the promotion from the laboratory to practical application for the phosphor.

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1. Introduction

The lighting industry has a huge impact on the development of human society. In a long history, the lighting industry has undergone many updates. At present, the most popular lighting fixtures are light-emitting diodes (LEDs), among which white LEDs are known as the fourth-generation lighting source.^1–4 White LEDs (w-LEDs) have many advantages, such as environmental protection, pollution-free, energy saving, low carbon, long service life, low starting voltage, high brightness and so on. So many advantages make w-LEDs widely used, including in the exhibition industry, advertising industry, municipal lighting and even general lighting. Due to the reasons above, w-LEDs have also been a research hotspot in scientific research in recent years.^5–8

During the illumination of w-LEDs, chips would be used to excite phosphors to obtain white light with practical application value.^9–11 The method of obtaining white light via mixing multicolor phosphors was popular in recent years. Phosphors consist of host and activators and activators are usually rare earth ions or variable metal ions. One of the common methods for obtaining single-phase multicolor phosphors is energy transfer, i.e. fixing the concentration of energy transfer ion to the sensitizer and changing the concentration of activator to obtain multicolor phosphors, and Tb³⁺→Eu³⁺ ion pair is widely used.^12–15 As for the host, the apatite structure compounds have been extensively developed because of their advantages such as rich crystal field environment, structural stability, and so on. The chemical expression of natural apatite is Ca₅(PO₄)₃F and the apatite structure compounds can be expressed as M₁₀(XO₄)₆Y₂, in which M usually stands for Ca, Ba, Sr, Mn, etc.; X usually stands for Si, Ge, P, etc.; Y usually stands for F, Cl, Br, O, etc.^16–19 It can be inferred from the formula that the apatite structure compounds are available in many combinations based on the isomorphism and solid solution methods. Rich host structures could provide a large library of materials for the lighting industry. However, there is no report about the apatite structure phosphor of Ba₂La₃(GeO₄)₃F:Tb³⁺,Eu³⁺ to best of our knowledge.

In this report, the research on the new multicolor phosphor BLGOF:Tb³⁺,Eu³⁺ has been focused on the structure and luminescence properties. The structure information of the phosphor was studied by XRD, SEM and Rietveld refinement. The optical properties of the phosphor have also been studied in detail, including PL, PLE, fluorescence quenching mechanism, thermal spectra, fluorescence lifetime and so on. All the properties are good enough and could contribute to the practical application for this phosphor.

2. Experimental details

2.1 Synthesis

Conventional solid-state method under high temperature was used to prepare the aimed phosphors BLGOF:Tb³⁺,Eu³⁺. The raw materials were BaCO₃ (Beijing Chemical Pharmaceutical Factory, A.R.), La₂O₃ (China Pharmaceutical Group Pharmaceutical Co., Ltd., 99.99%), GeO₂ (China Pharmaceutical Group Pharmaceutical Co., Ltd., 99.99%), NH₄F (Beijing Chemical Pharmaceutical Factory, A.R.), Tb₄O₇ (99.99%) and Eu₂O₃ (99.99%) and were used directly without processing. The selected starting materials were weighed by the electronic balance with 0.0001 g accuracy according to stoichiometric ratio. Afterwards they were mixed into an agate mortar and grounded thoroughly. Then the stoichiometric mixture was transferred into an alumina crucible and the crucible was sealed into a high temperature tube furnace and annealed at 1350 °C for 4 h. At last, the sample was cooled to room temperature gradually and ground into powder again for a series of tests followed.

2.2 Measurement

2.2.1 Structure and morphology. XRD patterns to determine the structure type were obtained via an X-ray powder diffractometer (D8 Advance, Bruker, Germany) with Cu-Kα radiation (λ = 0.15406 nm) operated at 40 kV and 30 mA. Powder diffraction data were fitted to obtain cell parameters by the Rietveld method using the computer software TOPAS 3.0 package.²⁰ Morphology, element distribution and composition of the compounds were observed by the method of SEM (JSM-6701F, Hitachi, Japan) with the accelerating voltage of 15 kV. The powder was attached to the conducting resin directly and sprayed with nano-platinum on the surface. The quantum efficiency of Ba₂La₃(GeO₄)₃F:0.15Tb³⁺,0.24Eu³⁺ sample was measured on a UV-vis-NIR spectrophotometer (UV-365).

2.2.2 Spectra. Excitation and emission spectra under room temperature were characterized on a Hitachi F-4600 fluorescence spectrophotometer. Photoluminescence (PL) system equipped with a xenon lamp (400 V, 150 W) acted as an excitation source. The 400 nm cut off filter was applied in the experiment to clear up the second-order emission of source radiation. Thermal spectra were measured on the same spectrophotometer, which was combined with a self-made heating attachment and a computer-controlled electric furnace. Decay curves were monitored by an Edinburgh FS5 Combined Fluorescence Lifetime & Steady State Spectrometer.

3. Results and discussions

The XRD patterns of the synthetic samples were shown in Fig. 1. It is obvious that the XRD patterns of BLGOF:0.15Tb³⁺,xEu³⁺ phosphors can be accurately assigned to the phase of Ba₂La₃(SiO₄)₃F (ICSD no. 170852, apatite, hexagonal, p6₃/m).²¹ Only when the isomorphic replacement of Tb³⁺ and Eu³⁺ to La³⁺ atoms was successful in the present sample structure can these positions and intensities of diffraction peaks match well with the reference ICSD file. In addition, the diffraction peaks of the samples shifted to the smaller 2θ angle side (bigger d-spacing, i.e., interplanar distances), which can be ascribed to the substitution of Si⁴⁺ by bigger Ge⁴⁺. Therefore, according to the theory of isomorphism, it could be inferred that the doping which was caused by Tb³⁺ and Eu³⁺ did not cause the structural change.^22–25

Fig. 1 XRD patterns of BLGOF:0.15Tb³⁺, BLGOF:0.15Eu³⁺, BLGOF:0.15Tb³⁺,xEu³⁺ (x = 0.06, 0.14, 0.18, 0.24, and 0.26) phosphors, and standard data of BLSOF (ICSD#170852) is shown as a reference.

To further understand the crystallographic information of the synthesized samples, powder diffraction data were fitted by the Rietveld method and the results are presented in Fig. 2, Tables 1 and 2. The observed (blue circles), calculated (red line), and difference (green line) XRD profiles shown in Fig. 2 are the Rietveld refinement of BLGOF at 298 K with λ = 1.5406 Å. Crystal structure of Ba₂La₃(SiO₄)₃F (apatite-type structure) was taken as starting model for Rietveld refinement. All the experimental peaks are well fitted by the refinement, indicating that all of those peaks are Bragg reflections from the BLGOF structure and converge to R_exp = 4.548%, R_wp = 6.259%, R_p = 4.830% and χ² = 1.375. As can be seen from Table 1, the refinement is stable and gives low R-factors. In the BLGOF compound, the lattice parameters are determined to be a = b = 1.0157 nm, c = 0.7464 nm and V = 0.6668 nm³, respectively. Moreover, the refined atomic positions and isotropic temperature factors for all atoms are listed in Table 2.^26,27

Fig. 2 Structure refinement of BLGOF (the red solid line, blue circles, short blue verticals and green solid lines represent the calculated pattern, the observed pattern obtained from XRD measurements, the Bragg positions, and the difference between the observed and calculated patterns, respectively).

Table 1 Refinement parameters for Ba₂La₃(GeO₄)₃F obtained from the Rietveld refinement using X-ray powder diffraction data at room temperature

Formula	Ba2La3(GeO4)3F
Space group	P63/m
Crystal system	Hexagonal
a/b (nm)	1.0157(18)
c (nm)	0.7464(14)
a/b	90°
γ	120°
V (nm^3)	0.6668(26)
R-Bragg	3.54169787
Rexp (%)	4.548
Rwp (%)	6.259
Rp (%)	4.830
χ^2	1.375

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Table 2 Fractional atomic coordinates, and occupancies parameters of BLGOF obtained from the Topas Rietveld Refinement using X-ray powder diffraction data at room temperature

Atom	x	y	z	occ	beq
La2	0.24324(22)	0.98660(28)	0.25	0.55	1
Ba2	0.24324(20)	0.98660(28)	0.25	0.45	1
La1	0.6666667	0.3333333	0.99794(55)	0.55	1
Ba1	0.6666667	0.3333333	0.99794(55)	0.45	1
Ge1	0.40454(36)	0.37250(34)	0.25	1	1
O1	0.5974	0.4496	0.25	1	1
O2	0.3442	0.4996	0.25	1	1
O3	0.3560	0.2721	0.0751	1	1
F	0	0	0.25	1	2.81

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In order to study the effect of structure regulation on crystal structure, we synthesized Ba₂La₃(SiO₄)₁(GeO₄)₂F (BLSi₁Ge₂F) and Ba₂La₃(SiO₄)₂(GeO₄)₁F (BLSi₂Ge₁F) hosts and refined their structures. Tables S1 and S2† are the atomic coordinate information of BLSi₁Ge₂F and BLSi₂Ge₁F. The cell parameters of several hosts are compared in Table S3.† Table S3† shows that the cell parameters increase with the increase of atomic radius. The structure control of complex anions can have a significant impact on the crystal structure, which may affect the properties of light emitting, energy transfer, thermal stability and so on.

To be more intuitive, the microstructure of BLGOF:0.15Tb³⁺,0.15Eu³⁺ was discovered by means of SEM and shown in the top right of Fig. 3. The sample composition seems to be irregular granules. The observation of Ba, La, Ge, O, F, Tb, Eu peaks in the EDX spectrum further implied the formation of the sample. In addition, the elemental mapping results as shown in the bottom of Fig. 3 demonstrated that all the elements in the sample were homogenously distributed over the granules.^28–30

Fig. 3 (Top right) SEM image of BLGOF:0.15Tb³⁺,0.15Eu³⁺ phosphor (Top left) EDX spectra of BLGOF:0.15Tb³⁺,0.15Eu³⁺ phosphors (Bottom) elemental distribution of BLGOF:0.15Tb³⁺,0.15Eu³⁺ phosphors.

The optical properties of the phosphors were studied then. The PLE and PL spectra of BLGOF:0.15Tb³⁺, BLGOF:0.15Eu³⁺ and BLGOF:0.15Tb³⁺,0.24Eu³⁺ were depicted in Fig. 4. It could be seen that the Fig. 4(a) were the luminescence spectra of BLGOF:0.15Tb³⁺ phosphor. The PLE spectra of BLGOF:0.15Tb³⁺ showed typical 325–500 nm excitation peaks of Tb³⁺ with characteristic excitation peak at 375 nm (⁷F₆ → ⁵D₃). Under 375 nm excitation, Tb³⁺ emission peaks at 491, 549, 592 and 624 nm with ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) transition as the core were observed.³¹ Fig. 4(b) were the spectra of BLGOF:0.15Eu³⁺ phosphor. There were several peaks in the excitation spectra, which could be attributed to the transition of Eu³⁺ from the ground state ⁷F₀ to the excited state ⁵D₄ (362 nm), ⁵L₇ (381 nm), ⁵L₆ (395 nm), ⁵D₃ (414 nm) and ⁵D₂ (464 nm), respectively. Under 395 nm excitation, BLGOF:0.15Eu³⁺ phosphors emitted characteristic peaks of Eu³⁺ centered at 591 nm (⁵D₀ → ⁷F₁), 619 nm (⁵D₀ → ⁷F₂), and 654 and 700 nm originating from the ⁵D₀ → ⁷F_J (J = 3, 4) transition of Eu³⁺. The excitation spectra of Eu³⁺ and the emission spectra of Tb³⁺ had a overlap, indicating that energy transfer from Tb³⁺ to Eu³⁺ could be realized.

Fig. 4 (a) PLE spectrum (left) and PL spectrum (right) of BLGOF:0.15Tb³⁺ (λ_em = 549 nm and λ_ex = 375 nm) (b) PLE (left) and PL (right) spectra of BLGOF:0.15Eu³⁺ (λ_em = 619 nm and λ_ex = 395 nm) (c) PLE (left) and PL (right) spectra of BLGOF:0.15Tb³⁺,0.22Eu³⁺(λ_em = 619 nm and 549 nm; λ_ex = 375, and 395 nm).

Fig. 4(c) were the spectra of BLGOF:0.15Tb³⁺,0.24Eu³⁺ phosphors. In the same wavelength range, the PL spectra of 375 and 395 nm and the PLE spectra monitored by 549 and 619 nm were measured, and the relative intensities of Tb³⁺ and Eu³⁺ emission spectra and excitation spectra were compared. When 375 nm was used as excitation wavelength to excite the phosphor, the emission peaks of Eu³⁺ at 619 nm and Tb³⁺ at 549 nm could be observed, especially the emission peaks of Eu³⁺, which indicated that the BLGOF single-phase phosphor co-doped with Tb³⁺, Eu³⁺ could be adjusted from green to yellow, orange to red. When excited by 395 nm, only the characteristic peaks of Eu³⁺ could be observed. The 619 nm peak of the red spectrum is higher than that of the black one, which indicates that the emission intensity of Eu³⁺ decreases when the phosphor is excited by 375 nm wavelength. The emission spectra excited by 375 nm include not only the characteristic emission peaks of Eu³⁺ (⁵D₀–⁷F₂ at 619 nm), but also the characteristic emission peaks of Tb³⁺ (⁵D₄–⁷F_6,5). Therefore, in order to study the energy transfer from Tb³⁺ to Eu³⁺, we focus on the spectral characteristics of 375 nm excitation.³²

The overlapping part of emission spectrum (black, peak at 491 nm) of Tb³⁺ and excitation spectrum (red, peak at 464 nm) of Eu³⁺ was amplified as shown in Fig. 5. Using origin 9.0, the emission peaks of Tb³⁺ and the excitation peaks of Eu³⁺ and their overlaps are integrated respectively. The peak heights, half-width (FWHM) and peak areas are obtained as shown in the figure. Overlapping area accounts for 7.78% of Tb³⁺ emission peak area, 15.23% of Eu³⁺ excitation peak area, and the sum of Tb³⁺ emission peak area and Eu³⁺ excitation peak area decreases from 6.73% (BLSOF) to 5.43% (BLGOF). This overlap further proves the theoretical feasibility of energy transfer. The overlap area is smaller than that of BLSOF in terms of Tb³⁺ emission peak area and Eu³⁺ excitation peak area. This change may affect the energy transfer between Tb³⁺ and Eu³⁺.³³

Fig. 5 Area integral and related parameters for the emission spectra of BLGOF:0.15Tb³⁺ phosphors, the excitation spectra of BLGOF:0.15Eu³⁺ phosphors and their overlapping parts.

Fig. 6(a) shows a series of emission spectra of BLGOF:0.15Tb³⁺,xEu³⁺ (x = 0, 0.06, 0.14, 0.18, 0.24 and 0.26) under near-ultraviolet excitation at 375 nm. The emission intensities of Tb³⁺ (549 nm) and Eu³⁺ (619 nm) at different concentrations (x) were shown in Fig. 6(b). It can be seen from the figure that the emission intensity of Tb³⁺ (549 nm) decreases with the increasing doping concentration of Eu³⁺ (619 nm). This may be due to the fact that Tb³⁺ transmits part of its energy to Eu³⁺. The emission intensity of Eu³⁺ (619 nm) increases first, and reaches the maximum when the doping concentration x = 0.24, then decreases. Concentration quenching may be responsible for the decrease of emission intensity of Eu³⁺ above x = 0.24. It can be inferred that energy transfer occurs from Tb³⁺ to Eu³⁺ in BLSOF.^34,35

Fig. 6 (a) A series of luminescence spectra under 375 nm near-ultraviolet excitation of BLSOF:0.15Tb³⁺,xEu³⁺ (x = 0, 0.06, 0.14, 0.18, 0.24 and 0.26) (b) the emission intensities of 549 nm (Tb³⁺ characteristic emission spectra) and 619 nm (Eu³⁺ characteristic emission spectra) at different Eu³⁺ doping concentrations (x) were measured under 375 nm excitation.

Fig. 7 mainly describes the decay curves of BLGOF:0.15Tb³⁺,xEu³⁺ excited by the light of 375 nm. The decay curve can be fitted perfectly based on the following decay equation as shown in the figure:³⁶

I(t) = I₀ + A exp(−t/τ) (1)

For the equation, I and I₀ stand the luminescence intensity at time t and 0, A is constant, t is the time, and τ is the decay time for the sample. Monitor wavelength of BLGOF:0.15Tb³⁺,xEu³⁺ decay curves is 549 nm, excitation wavelength is 375 nm. Obviously, the lifetime of Tb³⁺ decreases with the increase of Eu³⁺ concentration, which further confirms the energy transfer from Tb³⁺ to Eu³⁺ in BLGOF.

Fig. 7 The decay curves of BLGOF:0.15Tb³⁺,xEu³⁺ (x = 0, 0.06, 0.14, 0.18, 0.24, and 0.26) when excited by 375 nm and monitored by 549 nm.

The energy transfer efficiency from Tb³⁺ to Eu³⁺ can be calculated by formula (2):^37,38

η = 1 − (τ_s/τ_s0) (2)

As shown in Fig. 8, when Eu³⁺ concentration is 0.06, 0.14, 0.18, 0.24 and 0.26 mol, the energy transfer efficiency is 24%, 48.5%, 60%, 69.5% and 75%, respectively. As the concentration of Eu³⁺ increases, the energy transfer efficiency increases.

Fig. 8 The energy transfer efficiency from Tb³⁺ to Eu³⁺ of BLGOF:0.15Tb³⁺,xEu³⁺ (x = 0.06, 0.14, 0.18, 0.24, and 0.26) phosphors.

Based on BLASSE theory, the critical distance R_c can be calculated by formula (3)^39,40

R_c ≈ 2(3V/4πx_cN)^1/3 (3)

For Ba₂La₃(GeO₄)₃F matrix, N = 10, V = 0.666833 nm³ (cell volume obtained by structural refinement), the critical concentration of x_c is about 0.301 mol. Therefore, the critical distance of BLGOF:0.15Tb³⁺,xEu³⁺ is 0.7533 nm, so the energy transfer mechanism between Tb³⁺ and Eu³⁺ in BLGOF is electric dipole interaction.

The energy transfer mechanism can be further studied according to the following formula:⁴¹

(I_s0/I_s) ∝ C^n/3 (4)

I_S0 is the luminescence intensity of Tb³⁺ when doped with Tb³⁺ only, and I_S is the luminescence intensity of Tb³⁺ when doped with Tb³⁺ and Eu³⁺. C is the total concentration of Tb³⁺ and Eu³⁺ and n is the constant that can indicate the interaction between Tb³⁺ and Eu³⁺ (n = 6, dipole–dipole interaction; n = 8, dipole–quadrupole interaction; and n = 10, quadrupole–quadrupole interaction).

Linear fitting diagrams of I_S0/I_S and C^n/3 (n = 6, 8, or 10) were shown in Fig. 9, respectively. When n = 10, the linear fitting is the best. Based on Dexter equation, it can be concluded that the energy transfer mechanism of Tb³⁺–Eu³⁺ is quadrupole–quadrupole interaction. The energy transfer mechanism is as the same as that of BLSOF:Tb³⁺,Eu³⁺ phosphors.

Fig. 9 The energy transfer mechanism from Tb³⁺ to Eu³⁺ of BLGOF:0.15Tb³⁺,xEu³⁺ (x = 0.06, 0.14, 0.18, 0.24, and 0.26) phosphors.

In order to make the effect of complex anion structure regulation on energy transfer efficiency more intuitive, we compare the energy transfer efficiency of BLGOF with that of BLSOF phosphors, as shown in Fig. 10. Compared with BLSOF, the energy transfer efficiency decreases with the increase of anion radius. This may be due to the effect of complex anions on the environment around rare earth ions, which the radius of complex anions becomes larger and the energy transfer efficiency decreases as a whole.

Fig. 10 Comparation of the energy transfer efficiency between BLGOF phosphors and BLSOF phosphors at different Eu³⁺ doping concentrations.

In order to make the application performance of this series of phosphors more intuitive, we calculated the color coordinates of BLGOF:0.15Tb³⁺,xEu³⁺ phosphors excited by 375 nm (Table 3) and labeled them in CIE chromaticity diagram (Fig. 11). At the same time, the digital photos of BLGOF:0.15Tb³⁺, BLGOF:0.15Tb³⁺,0.06Eu³⁺, BLGOF:0.15Tb³⁺,0.14Eu³⁺, BLGOF:0.15Eu³⁺ under 365 nm near ultraviolet light excitation are given in the illustrations of Fig. 11. Thus, by adjusting the doping ratio of Tb³⁺ and Eu³⁺, the phosphors can be changed from green (0.37, 0.61) to yellow, orange-red and red (0.64, 0.36). In conclusion, thanks to energy transfer, the new series of single-phase phosphors BLGOF:0.15Tb³⁺,xEu³⁺ exhibit the advantages of tunable multicolor luminescence under near-ultraviolet (375 nm) excitation.^42,43

Table 3 CIE coordinate values of BLGOF:0.15Tb³⁺,xEu³⁺ (x = 0, 0.06, 0.14, 0.18, 0.24, and 0.26) and BLGOF:0.15Eu³⁺ at different Eu³⁺ doping concentrations

Number	Formula	CIE value
1	Ba2La2.85(GeO4)3F:0.15Tb^3+	(0.3684, 0.6093)
2	Ba2La2.79(GeO4)3F:0.15Tb^3+,0.06Eu^3+	(0.4807, 0.5055)
3	Ba2La2.71(GeO4)3F:0.15Tb^3+,0.14Eu^3+	(0.5689, 0.4239)
4	Ba2La2.67(GeO4)3F:0.15Tb^3+,0.18Eu^3+	(0.5957, 0.3990)
5	Ba2La2.61(GeO4)3F:0.15Tb^3+,0.24Eu^3+	(0.6158, 0.3804)
6	Ba2La2.59(GeO4)3F:0.15Tb^3+,0.26Eu^3+	(0.6258, 0.3711)
7	Ba2La2.85(GeO4)3F:0.15Eu^3+	(0.6424, 0.3555)

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Fig. 11 CIE of BLGOF:0.15Tb³⁺,xEu³⁺ (x = 0, 0.06, 0.14, 0.18, 0.24, and 0.26) when excited by 375 nm.

The thermal stability of BLGOF:0.15Tb³⁺,0.24Eu³⁺ phosphor was measured at last.

The temperature-dependent emission spectra and intensity–concentration relationship of Tb³⁺ (549 nm) for BLGOF:0.15Tb³⁺,0.24Eu³⁺ phosphor was shown in Fig. 12(a) and (b). It can be calculated that the luminescence intensity at 423 K remains 67.88% compared to the luminescence intensity at room temperature 298 K. The activation energy was studied in Fig. 12(c) to understand the temperature-dependent thermal quenching behavior according to the following formula:⁴⁴

ln(I₀/I − 1) = ln A − ΔE/kT4

In which I₀ means the initial emission intensity, I means the emission intensity of different temperatures, A is the coefficient, ΔE means the activation energy, k is the Boltzmann coefficient with a fixed value of 8.629 × 10⁻⁵ eV K⁻¹ and T is the temperature. Fig. 12(c) shows the linear fit of ln(I₀/I − 1) vs. 1/kT for the BLGOF:0.15Tb³⁺ sample and the slope is −0.1889, meaning the activation energy for thermal quenching of Tb³⁺ in BLGOF:0.15Tb³⁺,0.24Eu³⁺ sample is 0.1889 eV.³⁷ In addition, the quantum efficiency of BLGOF:0.15Tb³⁺,0.24Eu³⁺ sample is 12.36.

Fig. 12 (a) The luminescence spectra of BLGOF:0.15Tb³⁺,0.24Eu³⁺ at different temperatures (b) the intensity changes of BLGOF:0.15Tb³⁺,0.24Eu³⁺ phosphors 549 nm emission peak at different temperatures (c) ln(I₀/I − 1) versus 1/kT for the BLGOF:0.15Tb³⁺,0.24Eu³⁺.

4. Conclusions

Generally speaking, a novel multicolor phosphor Ba₂La₃(GeO₄)₃F:0.15Tb³⁺,xEu³⁺ has been synthesized by simple solid-state synthesis method. The effects of structure, luminescence behavior and complex anions on Tb³⁺/Eu³⁺ energy transfer were also studied. The main confusions are as follows: (1) the optimum concentration of Eu³⁺ in BLGOF:0.15Tb³⁺,xEu³⁺ is 0.24 mol. The energy transfer mechanism from Tb³⁺ to Eu³⁺ in BLGOF is quadrupole–quadrupole (q–q), and the complex anion regulation does not change the energy transfer mechanism compared to Ba₂La₃(SiO₄)₃F:0.15Tb³⁺,xEu³⁺. When the doping ratio of Tb³⁺/Eu³⁺ is different, the color of BLGOF:0.15Tb³⁺,xEu³⁺ phosphors can be changed from green, yellow/orange to red. (2) When [GeO₄]⁴⁻ was used to replace [SiO₄]⁴⁻, the cell parameters increased (BLSOF cell volume was 0.620402 (51) nm³, BLGOF cell volume was 0.666833 (26) nm³), the overlap area of spectrum became smaller, the energy transfer efficiency decreased. (3) Complex anion regulation has a certain effect on thermal stability. The luminescence intensity of 423 K remains 67.88%, and the thermal stability is good, but less than BLSOF:0.15Tb³⁺,0.22Eu³⁺ at 423 K (85.34%). With the increase of the anion radius of the complex, the luminescence retention rate of 423 K decreases and the activation energy of phosphor increases. All the features acquired above indicate that this phosphor might be a candidate for w-LEDs.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This present work is supported by the National Natural Science Foundations of China (Grant no. 51672257, 41672044 and 41172053), and the Fundamental Research Funds for the Central Universities (Grant no. 2652017091 and 2652017370).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra05915f

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