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Preparation and luminescence of Tb3+ doped glass ceramics containing SrWO4 crystals
Optik - International Journal for Light and Electron Optics 200 (2020) 163356
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Short note
Preparation and luminescence of Tb3+ doped glass ceramics containing SrWO4 crystals ⁎
T
⁎
Yulin Weia, Chunhui Sua,b, , Wentao Jiaa, Hongbo Zhanga, , Xiangyu Zoua,b a b
School of Material Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China Changchun Normal University, Changchun 130022, China
A R T IC LE I N F O
ABS TRA CT
Keywords: SrWO4 Tb3+ doped Ceramics Luminescence
Tb3+ doped glass ceramics containing SrWO4 crystals are prepared by melting crystallization method and their luminescence properties are studied. It is determined by DSC that the heat treatment temperature is 730℃ and the time is 2h. Through the XRD of the glass ceramics, it can determine the crystalize phase is SrWO4 after comparing with PDF normal card. In the emission spectrum of the Tb3+ doped SrWO4 glass ceramics samples, there are two obvious emission peaks at 490nm and 544nm corresponding to the transitions of 5D4→7F6 and 5D4→7F5 of Tb3+. Based on the fluorescence spectrum, it can be determined that the optimum concentration of Tb4O7 is 1.0 mol, and the fluorescence lifetime is 2.16 ms.
1. Introduction The tungstate has the properties of photoluminescence and wide application prospect as an important inorganic luminescence material [1]. The intrinsic spectral band of tungstate is very wide and accounts for most of the visible light region. The crystal phase is SrWO4 (scheelite tetragonal structure, which is a good self-activated fluorescent material). It can emit luminescence under a certain wavelength of light excitation. Its optical properties are stable, and it has excellent color purity and stable chemical properties [2–4]. Tb3+ has strong green emission due to the energy level transition from 5D4 to 7F5. Tb3+ doped SrWO4 glass ceramics (GCs) can be used in many fields such as fiber collimators, optical amplifiers and fiber couplers and so on, the reason of which is the similar absorption intensity of the WeO band and energy releasing from the energy level transition of Tb3+, so as to enhance its luminescence performance [5–8]. In recent years, there are amount of studies about strontium tungstate in the field of phosphor。Eu3+ doped SrWO4 nanocrystals synthesized by CTAB assisted hydrothermal method, which higher purity and more excellent grain development [9]. Eu3+ doped SrWO4 phosphor synthesized by sol-gel method, which easy stoichiometric control, good homogeneity and low calcination temperatures [10]. Sm3+ and Eu3+-Sm3+ doped SrWO4 phosphor synthesized by co-precipitation method, which exhibit a high luminous efficiency, purity and lower color temperature [11]. By contrary, there are rare studies on glass ceramics. In this paper Tb3+ doped SrWO4 glass ceramics are synthesized by melt-crystallization method and the affection of luminescence in different concentrations of Tb3+ are discussed.
⁎ Corresponding authors at: School of Material Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China. E-mail addresses: [email protected] (C. Su), [email protected] (H. Zhang).
https://doi.org/10.1016/j.ijleo.2019.163356 Received 4 June 2019; Accepted 3 September 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 200 (2020) 163356
Y. Wei, et al.
Fig. 1. (a) DSC curve of PG, (b) XRD patterns of GCs, (c) Crystal phase structure of SrWO4, (d) Transmittance Spectra of PG and GCs.
2. Experiment Formulation for precursor glasses (PG) in the light of the mole ratio as 4.5SrCO3-1.5WO3-6SiO2-7.5H3BO3-1NaF-0.2Sb2O3xTb4O7(x = 0.1, 0.4, 0.7, 1.0, 1.3 mol)20 g of raw materials is weighed, then uniformly mixed and placed in the platinum crucibles, melted at 1450 °C for 1 h in silicon molybdenum furnace. The liquid melt is poured onto the preheated iron plate and pressed by another plate to get glass samples. The glass samples are immediately transferred to a resistance furnace at 460 °C for 1 h to release internal stresses and naturally cool down to room temperature. To prepare the GCs, the PGs are subjected to heat treatment and crystallize. The obtained samples are polished optically for further characterization. Differential scanning calorimetry (DSC) of glass powder is carried out by a SDT2960 thermal analyzer with the heating rate of 10 °C/min. The measurements of the X-ray diffraction (XRD) are performed on a diffractometer (Rigaku Ultima IV, Japan) with CuKα1 radiation over the angular range 10°≤2θ≤80°.The optical transmittance of the glass ceramics is measured by UV-VIS spectrophotometer (SHIMADZU, UVmini-1240). The photoluminescence (PL) excitation and emission spectra is analyzed by FL-7000. The wavelength range of excitation spectrum is 200–500 nm, and the range of emission spectrum is 450–650 nm. 3. Results and discussion 3.1. Determination of heat treatment Fig.1 (a) is the DSC curve of PG. As can be seen, the crystallization starting temperature (Tx) of the glass is 730℃ and the crystallization temperature (Tp) is 750℃. From the viewpoint of energy saving, the glass ceramic samples are obtained at 730℃ as the crystallization temperature, and the heat treatment for 1 h, 1.5 h,2 h, and 2.5 h, respectively. Fig.1 (b) is the XRD spectra of precursor glasses (PG) and glass-ceramic (GCs) samples. It shows PG has only one peak, which is the typical feature of amorphous state. The GC samples prepared at 730 ℃ for 1 h have less spiculate diffraction peaks, but with the holding time increasing, the intensity of the diffraction peaks also increases, indicating that the grains grow up gradually in GCs. Compared with the standard card (PDF#08-0490), it can determined that the crystalize phase is SrWO4.The space group of SrWO4 is I41/a, the luminescence center is WO42−, in which W6+ locates at the center of tetrahedron and four O2− irons are at vertexes, as shown in Fig.1(c). The scherrer formula (1) is used to calculate the size of SrWO4 grains in the samples [12].
D=
kλ βcosθ
(1) 2
Optik - International Journal for Light and Electron Optics 200 (2020) 163356
Y. Wei, et al.
Fig. 2. (a) Excitation spectrum of GCs, (b) Emission spectrum of GCs, (c) Decay curves of GCs, (d) Chromaticity coordinates of GCs.
Where k is constant value 0.89, λ is X-ray wavelength 0.15405 nm, β is the half height width of diffraction peak, θ is diffraction angle. The calculated results of the grain size of SrWO4 at 730℃ for 1.5 h, 2 h and 2.5 h are about 77.7 nm, 80.2 nm, 98.6 nm, respectively. Fig. 1(d) shows the transmission spectra of GCs heated at 730 ℃ for 1 h, 1.5 h, 2 h and 2.5 h, besides PG. As can be seen, the transmittance of the sample decreases gradually with the prolongation of heat treatment time. The characteristic absorption peak at 544 nm is mainly due to the transition of 5D4→7F5 energy level of Tb3+. Meanwhile, combining with grain size and optical transmittance curve, it can be seen that the grain size increases and the transmittance decreases with the time increases, and the transmission is the lowest when the time increases to 2.5 h. In a word, the optimum heat treatment can be determined at 730℃ for 2 h. 3.2. Fluorescence spectrum analysis In order to study the effect of the concentration of Tb4O7 on the luminescence properties of GC samples, the concentrations of Tb4O7 are 0.1 mol,0.4 mol,0.7 mol,1.0 mol and 1.3 mol. Fig. 2(a) is the excitation spectra of Tb3+ different concentrations. The monitoring wavelength is 544 nm. Because of the shielding effect of the 4f-4f electrons in the outer layer of Tb3+, there are a series of characteristic bands at 350 nm, 369 nm and 376 nm corresponding to the energy levels transitions of 7F6→5D2, 7F6→5D6 and 7 F6→5D3, respectively. Nevertheless, the excitation band near 270 nm is caused by the charge transfer band (CTB) formed by the electron transfer from the 2p energy level of the O in the WO42− ion to the 5d energy level of the W. In summarize, the intensity is strongest at 376 nm, so 376 nm as the excitation wavelength of Tb3+. Fig. 2(b) is the emission spectrum of GCs containing SrWO4 phase with Tb3+ different concentrations. The excitation wavelength is 376 nm. In the range of 450–650 nm, there are four characteristic emission peaks that 490 nm,544 nm,587 nm and 622 nm corresponding to 5D4→7F6,5D4→7F5,5D4→7F4 and 5D4→7F3 energy levels of Tb3+, among which the green light at 544 nm is the strongest, and the blue light at 490 nm is the second. It can be seen from fluorescence spectrum that the intensity of the excitation peak increases firstly and then decreases with the increase of Tb4O7 concentration, ultimately more than 1.0 mol the intensity of emission peak decreases and concentration quenching occurs, correspondingly. Fig.2(c) shows the fluorescence decay curve of Tb3+ doped SrWO4 GCs. The average decay times (τ) of Tb3+ doped SrWO4 GCs are well fitted with a single exponential decay mode by the formula (2) [13].
t I (t ) = I0 exp ⎛ ⎞ ⎝τ ⎠
(2) 3
Optik - International Journal for Light and Electron Optics 200 (2020) 163356
Y. Wei, et al.
Table 1 Chromaticity coordinates of Tb3+ doped SrWO4 GCs. Tb4O7(mol)
λex(nm)
CIE x
CIE y
0.1 0.4 0.7 1.0 1.3
376 376 376 376 376
0.2330 0.2507 0.2535 0.2586 0.2578
0.4202 0.5171 0.5546 0.5717 0.5301
I(t) is the fluorescence intensity at the time t, I0 is the fluorescence intensity at t0. According to the calculated results, it can be clearly found that the fluorescence lifetime from 0.1 mol Tb4O7 to 1.3 mol Tb4O7 doped SrWO4 GCs are 1.93 ms, 1.97 ms, 2.04 ms, 2.16 ms and 2.09 ms, respectively. When the concentration of Tb3+ reached 1.0%, the fluorescence lifetime is the maximum, more than 1.0 mol it decreases which could prove the concentration quenching. Fig. 2(d) shows the chromaticity coordinates of Tb3+ doped SrWO4 GCs. It is clear that the color coordinates located in the green area, changing slowly from light green (0.2330, 0.4202) to dark green (0.2586, 0.5717), presents in Table 1. As a result, the optimal Tb3+ concentration can be determined to be 1.0 mol, and the best green emission light can be obtained. 4. Conclusion In this work, Tb3+ doped SrWO4 GCs are successfully prepared by melting crystallization method. The SrWO4 crystalize phase are determined by XRD, and the optimum heat treatment determined at 730℃ for 2 h.The emission peak of Tb3+ corresponded to the energy level transition of 5D4→7F5 (544 nm). The green luminescence is the strongest when the concentration reached 1.0 mol%, and the fluorescence lifetime is 2.16 ms. Therefore, Tb3+ doped SrWO4 GCs have potential applications in lighting, imaging, green laser and photoelectron. Acknowledgments This work is supported by the ‘111 ‘Project of China (D17017) and the Project of Jilin Provincial Science and Technology Department (20190802014ZG). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Z.X. Shi, Y. Lu, J. Wang, X. Guan, J. Shi, H. Jiang, Chinese J. Inorg. Chem. 34 (2018) 1975–1982. Z.H. Ju, R.P. Wei, J.X. Ma, C.R. Pang, W.S. Liu, J. Alloys. Compd. 507 (2010) 133–136. J.C. Sczancoski, L.S. Cavalcante, M.R. Joya, J.W.M. Espinosa, P.S. Pizani, J.A. Varela, E. Longo, J. Alloys. Compd. 330 (2009) 227–236. D.F. Zhou, Y.H. Chen, C.S. Shi, Y.G. Wei, H.T. Chen, M. Yin, J. Alloys. Compd. 322 (2001) 298–301. J. Zhang, Y.H. Wang, Z.Y. Zhai, G.B. Chen, Opt. Mater. (Amst) 38 (2014) 126–130. L.L. Zhang, M.Y. Peng, G.P. Dong, J.R. Qiu, Opt. Mater. (Amst) 34 (2012) 1202–1207. J.F.M. Dossantos, N.G.C. Astrath, M.L. Baesso, L.A.O. Nunes, T. Catunda, J. Lumin. 202 (2018) 363–369. E. Pavel, V. Marinescu, M. Lungulescu, B. Sbarcea, Mater. Lett. 210 (2018) 12–15. B.Y. Xu, G.F. Wang, Y. Li, S. Liu, L. Feng, J.S. Zhang, Chin. J. Lumin. 34 (2013) 1178–1182. P.F.S. Pereira, I.C. Nogueira, E. Longo, E.J. Nassar, I.L.V. Rosa, L.S. Cavalcante, J. Rare Earth. 33 (2015) 113–128. Y.D. Ren, Y.H. Liu, R. Yang, Superlattices Microstruct. 91 (2016) 138–147. Z.X. Zhang, Y.P. Zhang, C. Wang, Z.G. Feng, W.H. Zhang, H.P. Xia, J. Mater. Sci. Technol. 33 (2017) 432–437. Y.H. Xia, X.Y. Zou, H.B. Zhang, M.J. Zhao, S. Meng, C.H. Su, J. Shao, J. Non. Solids 500 (2018) 243–248.
4
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Short note
Preparation and luminescence of Tb3+ doped glass ceramics containing SrWO4 crystals ⁎
T
⁎
Yulin Weia, Chunhui Sua,b, , Wentao Jiaa, Hongbo Zhanga, , Xiangyu Zoua,b a b
School of Material Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China Changchun Normal University, Changchun 130022, China
A R T IC LE I N F O
ABS TRA CT
Keywords: SrWO4 Tb3+ doped Ceramics Luminescence
Tb3+ doped glass ceramics containing SrWO4 crystals are prepared by melting crystallization method and their luminescence properties are studied. It is determined by DSC that the heat treatment temperature is 730℃ and the time is 2h. Through the XRD of the glass ceramics, it can determine the crystalize phase is SrWO4 after comparing with PDF normal card. In the emission spectrum of the Tb3+ doped SrWO4 glass ceramics samples, there are two obvious emission peaks at 490nm and 544nm corresponding to the transitions of 5D4→7F6 and 5D4→7F5 of Tb3+. Based on the fluorescence spectrum, it can be determined that the optimum concentration of Tb4O7 is 1.0 mol, and the fluorescence lifetime is 2.16 ms.
1. Introduction The tungstate has the properties of photoluminescence and wide application prospect as an important inorganic luminescence material [1]. The intrinsic spectral band of tungstate is very wide and accounts for most of the visible light region. The crystal phase is SrWO4 (scheelite tetragonal structure, which is a good self-activated fluorescent material). It can emit luminescence under a certain wavelength of light excitation. Its optical properties are stable, and it has excellent color purity and stable chemical properties [2–4]. Tb3+ has strong green emission due to the energy level transition from 5D4 to 7F5. Tb3+ doped SrWO4 glass ceramics (GCs) can be used in many fields such as fiber collimators, optical amplifiers and fiber couplers and so on, the reason of which is the similar absorption intensity of the WeO band and energy releasing from the energy level transition of Tb3+, so as to enhance its luminescence performance [5–8]. In recent years, there are amount of studies about strontium tungstate in the field of phosphor。Eu3+ doped SrWO4 nanocrystals synthesized by CTAB assisted hydrothermal method, which higher purity and more excellent grain development [9]. Eu3+ doped SrWO4 phosphor synthesized by sol-gel method, which easy stoichiometric control, good homogeneity and low calcination temperatures [10]. Sm3+ and Eu3+-Sm3+ doped SrWO4 phosphor synthesized by co-precipitation method, which exhibit a high luminous efficiency, purity and lower color temperature [11]. By contrary, there are rare studies on glass ceramics. In this paper Tb3+ doped SrWO4 glass ceramics are synthesized by melt-crystallization method and the affection of luminescence in different concentrations of Tb3+ are discussed.
⁎ Corresponding authors at: School of Material Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China. E-mail addresses: [email protected] (C. Su), [email protected] (H. Zhang).
https://doi.org/10.1016/j.ijleo.2019.163356 Received 4 June 2019; Accepted 3 September 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 200 (2020) 163356
Y. Wei, et al.
Fig. 1. (a) DSC curve of PG, (b) XRD patterns of GCs, (c) Crystal phase structure of SrWO4, (d) Transmittance Spectra of PG and GCs.
2. Experiment Formulation for precursor glasses (PG) in the light of the mole ratio as 4.5SrCO3-1.5WO3-6SiO2-7.5H3BO3-1NaF-0.2Sb2O3xTb4O7(x = 0.1, 0.4, 0.7, 1.0, 1.3 mol)20 g of raw materials is weighed, then uniformly mixed and placed in the platinum crucibles, melted at 1450 °C for 1 h in silicon molybdenum furnace. The liquid melt is poured onto the preheated iron plate and pressed by another plate to get glass samples. The glass samples are immediately transferred to a resistance furnace at 460 °C for 1 h to release internal stresses and naturally cool down to room temperature. To prepare the GCs, the PGs are subjected to heat treatment and crystallize. The obtained samples are polished optically for further characterization. Differential scanning calorimetry (DSC) of glass powder is carried out by a SDT2960 thermal analyzer with the heating rate of 10 °C/min. The measurements of the X-ray diffraction (XRD) are performed on a diffractometer (Rigaku Ultima IV, Japan) with CuKα1 radiation over the angular range 10°≤2θ≤80°.The optical transmittance of the glass ceramics is measured by UV-VIS spectrophotometer (SHIMADZU, UVmini-1240). The photoluminescence (PL) excitation and emission spectra is analyzed by FL-7000. The wavelength range of excitation spectrum is 200–500 nm, and the range of emission spectrum is 450–650 nm. 3. Results and discussion 3.1. Determination of heat treatment Fig.1 (a) is the DSC curve of PG. As can be seen, the crystallization starting temperature (Tx) of the glass is 730℃ and the crystallization temperature (Tp) is 750℃. From the viewpoint of energy saving, the glass ceramic samples are obtained at 730℃ as the crystallization temperature, and the heat treatment for 1 h, 1.5 h,2 h, and 2.5 h, respectively. Fig.1 (b) is the XRD spectra of precursor glasses (PG) and glass-ceramic (GCs) samples. It shows PG has only one peak, which is the typical feature of amorphous state. The GC samples prepared at 730 ℃ for 1 h have less spiculate diffraction peaks, but with the holding time increasing, the intensity of the diffraction peaks also increases, indicating that the grains grow up gradually in GCs. Compared with the standard card (PDF#08-0490), it can determined that the crystalize phase is SrWO4.The space group of SrWO4 is I41/a, the luminescence center is WO42−, in which W6+ locates at the center of tetrahedron and four O2− irons are at vertexes, as shown in Fig.1(c). The scherrer formula (1) is used to calculate the size of SrWO4 grains in the samples [12].
D=
kλ βcosθ
(1) 2
Optik - International Journal for Light and Electron Optics 200 (2020) 163356
Y. Wei, et al.
Fig. 2. (a) Excitation spectrum of GCs, (b) Emission spectrum of GCs, (c) Decay curves of GCs, (d) Chromaticity coordinates of GCs.
Where k is constant value 0.89, λ is X-ray wavelength 0.15405 nm, β is the half height width of diffraction peak, θ is diffraction angle. The calculated results of the grain size of SrWO4 at 730℃ for 1.5 h, 2 h and 2.5 h are about 77.7 nm, 80.2 nm, 98.6 nm, respectively. Fig. 1(d) shows the transmission spectra of GCs heated at 730 ℃ for 1 h, 1.5 h, 2 h and 2.5 h, besides PG. As can be seen, the transmittance of the sample decreases gradually with the prolongation of heat treatment time. The characteristic absorption peak at 544 nm is mainly due to the transition of 5D4→7F5 energy level of Tb3+. Meanwhile, combining with grain size and optical transmittance curve, it can be seen that the grain size increases and the transmittance decreases with the time increases, and the transmission is the lowest when the time increases to 2.5 h. In a word, the optimum heat treatment can be determined at 730℃ for 2 h. 3.2. Fluorescence spectrum analysis In order to study the effect of the concentration of Tb4O7 on the luminescence properties of GC samples, the concentrations of Tb4O7 are 0.1 mol,0.4 mol,0.7 mol,1.0 mol and 1.3 mol. Fig. 2(a) is the excitation spectra of Tb3+ different concentrations. The monitoring wavelength is 544 nm. Because of the shielding effect of the 4f-4f electrons in the outer layer of Tb3+, there are a series of characteristic bands at 350 nm, 369 nm and 376 nm corresponding to the energy levels transitions of 7F6→5D2, 7F6→5D6 and 7 F6→5D3, respectively. Nevertheless, the excitation band near 270 nm is caused by the charge transfer band (CTB) formed by the electron transfer from the 2p energy level of the O in the WO42− ion to the 5d energy level of the W. In summarize, the intensity is strongest at 376 nm, so 376 nm as the excitation wavelength of Tb3+. Fig. 2(b) is the emission spectrum of GCs containing SrWO4 phase with Tb3+ different concentrations. The excitation wavelength is 376 nm. In the range of 450–650 nm, there are four characteristic emission peaks that 490 nm,544 nm,587 nm and 622 nm corresponding to 5D4→7F6,5D4→7F5,5D4→7F4 and 5D4→7F3 energy levels of Tb3+, among which the green light at 544 nm is the strongest, and the blue light at 490 nm is the second. It can be seen from fluorescence spectrum that the intensity of the excitation peak increases firstly and then decreases with the increase of Tb4O7 concentration, ultimately more than 1.0 mol the intensity of emission peak decreases and concentration quenching occurs, correspondingly. Fig.2(c) shows the fluorescence decay curve of Tb3+ doped SrWO4 GCs. The average decay times (τ) of Tb3+ doped SrWO4 GCs are well fitted with a single exponential decay mode by the formula (2) [13].
t I (t ) = I0 exp ⎛ ⎞ ⎝τ ⎠
(2) 3
Optik - International Journal for Light and Electron Optics 200 (2020) 163356
Y. Wei, et al.
Table 1 Chromaticity coordinates of Tb3+ doped SrWO4 GCs. Tb4O7(mol)
λex(nm)
CIE x
CIE y
0.1 0.4 0.7 1.0 1.3
376 376 376 376 376
0.2330 0.2507 0.2535 0.2586 0.2578
0.4202 0.5171 0.5546 0.5717 0.5301
I(t) is the fluorescence intensity at the time t, I0 is the fluorescence intensity at t0. According to the calculated results, it can be clearly found that the fluorescence lifetime from 0.1 mol Tb4O7 to 1.3 mol Tb4O7 doped SrWO4 GCs are 1.93 ms, 1.97 ms, 2.04 ms, 2.16 ms and 2.09 ms, respectively. When the concentration of Tb3+ reached 1.0%, the fluorescence lifetime is the maximum, more than 1.0 mol it decreases which could prove the concentration quenching. Fig. 2(d) shows the chromaticity coordinates of Tb3+ doped SrWO4 GCs. It is clear that the color coordinates located in the green area, changing slowly from light green (0.2330, 0.4202) to dark green (0.2586, 0.5717), presents in Table 1. As a result, the optimal Tb3+ concentration can be determined to be 1.0 mol, and the best green emission light can be obtained. 4. Conclusion In this work, Tb3+ doped SrWO4 GCs are successfully prepared by melting crystallization method. The SrWO4 crystalize phase are determined by XRD, and the optimum heat treatment determined at 730℃ for 2 h.The emission peak of Tb3+ corresponded to the energy level transition of 5D4→7F5 (544 nm). The green luminescence is the strongest when the concentration reached 1.0 mol%, and the fluorescence lifetime is 2.16 ms. Therefore, Tb3+ doped SrWO4 GCs have potential applications in lighting, imaging, green laser and photoelectron. Acknowledgments This work is supported by the ‘111 ‘Project of China (D17017) and the Project of Jilin Provincial Science and Technology Department (20190802014ZG). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Z.X. Shi, Y. Lu, J. Wang, X. Guan, J. Shi, H. Jiang, Chinese J. Inorg. Chem. 34 (2018) 1975–1982. Z.H. Ju, R.P. Wei, J.X. Ma, C.R. Pang, W.S. Liu, J. Alloys. Compd. 507 (2010) 133–136. J.C. Sczancoski, L.S. Cavalcante, M.R. Joya, J.W.M. Espinosa, P.S. Pizani, J.A. Varela, E. Longo, J. Alloys. Compd. 330 (2009) 227–236. D.F. Zhou, Y.H. Chen, C.S. Shi, Y.G. Wei, H.T. Chen, M. Yin, J. Alloys. Compd. 322 (2001) 298–301. J. Zhang, Y.H. Wang, Z.Y. Zhai, G.B. Chen, Opt. Mater. (Amst) 38 (2014) 126–130. L.L. Zhang, M.Y. Peng, G.P. Dong, J.R. Qiu, Opt. Mater. (Amst) 34 (2012) 1202–1207. J.F.M. Dossantos, N.G.C. Astrath, M.L. Baesso, L.A.O. Nunes, T. Catunda, J. Lumin. 202 (2018) 363–369. E. Pavel, V. Marinescu, M. Lungulescu, B. Sbarcea, Mater. Lett. 210 (2018) 12–15. B.Y. Xu, G.F. Wang, Y. Li, S. Liu, L. Feng, J.S. Zhang, Chin. J. Lumin. 34 (2013) 1178–1182. P.F.S. Pereira, I.C. Nogueira, E. Longo, E.J. Nassar, I.L.V. Rosa, L.S. Cavalcante, J. Rare Earth. 33 (2015) 113–128. Y.D. Ren, Y.H. Liu, R. Yang, Superlattices Microstruct. 91 (2016) 138–147. Z.X. Zhang, Y.P. Zhang, C. Wang, Z.G. Feng, W.H. Zhang, H.P. Xia, J. Mater. Sci. Technol. 33 (2017) 432–437. Y.H. Xia, X.Y. Zou, H.B. Zhang, M.J. Zhao, S. Meng, C.H. Su, J. Shao, J. Non. Solids 500 (2018) 243–248.
4