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Correlation of oxygen defects variations to band gap changes in luminescence properties of SrZnO2: 0.02 Eu3+ phosphors
Materials Letters 254 (2019) 21–23
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Correlation of oxygen defects variations to band gap changes in luminescence properties of SrZnO2: 0.02 Eu3+ phosphors Xiaojuan Zhu a, Yongping Pu a,⇑, Xinyang Pu b, Xin Xie a, Mingyuan Liu a, Qianle Li a a b
Shaanxi University of Science and Technology, Xi’an, Shaanxi 710021, China School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510006, China
a r t i c l e
i n f o
Article history: Received 8 April 2019 Received in revised form 5 July 2019 Accepted 7 July 2019 Available online 8 July 2019 Keywords: Eu3+ SrZnO2 Oxygen vacancies Phosphors Luminescence
a b s t r a c t The SrZnO2: 0.02 Eu3+ phosphors were prepared by conventional solid-state reactions in O2, air, N2 and Ar, respectively. X-ray photoelectron spectroscopy analyses show that the concentration of oxygen defects in samples change obviously under different atmospheres. From the data collected by UV–vis diffuse reflectance spectroscopy and the fluorescence spectra, it is found that Vӧ and oxygen interstitials can diminish band gaps of samples, promoting an increase in photoluminescence intensities of samples on the based of host-sensitized luminescence effect, in which the effect of oxygen vacancies is stronger than that of oxygen interstitials. The mechanism of band gap narrowing has been explained in detail. Ó 2019 Published by Elsevier B.V.
1. Introduction Rare-earth doped metal oxides have been extensively developed in recent years and applied to many optical materials fields, the spectroscopic properties of which with special repeat units (e.g. [SiO4], a-Al2O3, [PO4] /[LiO4] or [GeO4]/[ZnO4]) are governed by the optical transitions induced optical storage and selfactivation luminescence properties [1]. With the discovery of a large number of related oxides, band gap, as a critical parameter for an oxide insulator, directly affects electronic, magnetic and optical properties of the materials. Tunability of the band gap is very important for optimizing properties of materials and achieved through different preparation method [2], chemical doping [3], tuning crystallite sizes [4] and varying the oxygen stoichiometry [5], which is attributed to the introduction of defects in this systems. The type of defects are predominantly point defects (i.e. defect associated to one lattice point, such as cation or oxygen defects) [2,4–6]. As a promising optical material, since SrZnO2: Ba2+, Mn2+ phosphors were reported by Kubota et al. [7], a lot of research on SrZnO2 was carried out in the following years [8–12]. Up to now, however, few reports explore the effect of oxygen defects in the interior of SrZnO2. Europium is a popular activator due to its adjustable valence and sensitive luminescence emission to the crystal field. Usually, ⇑ Corresponding author. E-mail address: [email protected] (Y. Pu). https://doi.org/10.1016/j.matlet.2019.07.025 0167-577X/Ó 2019 Published by Elsevier B.V.
the reduction of Eu3+ ions occurs in reducing atmospheres. However, for some special compounds with stiff three dimensional (3D) enclosed structures (e.g., the tetrahedral anion groups) or Eu3+ ions substituted for the divalent cations, Eu2+ ions appear in samples prepared in air [13]. In our work, oxygen defects were introduced into SrZnO2: 0.02 Eu3+ phospors prepared at different atmospheres by conventional solid-state reactions, which lead to the band gap narrowing. The photoluminescence intensity of Eu3+ ions in sample was enhanced when the band gap reaches a minimum, the mechanism of which was given.
2. Experimental SrZnO2: 0.02 Eu3+ phosphors were prepared by conventional solid-state reactions. Stoichiometric mixtures of SrCO3 (99.9%), ZnO (99.9%), and Eu2O3 (99.9%) were calcined at 1165 °C for 3 h in Ar, N2, air and O2, respectively. The phase identification of all samples were carried by X-ray diffractometer (XRD, Rigaku D/MAX-2400, Tokyo, Japan) with Cu Ka radiation (k = 0.15406 nm) operating at 40 kV and 100 mA. The variation of oxygen defects and binding energy of Eu3+ ions in samples were measured by using X-ray photoelectron spectroscopy (XPS, AXIS SUPRA, UK) with Al Ka radiation (600 W). All the absolute binding energies of the photoelectron spectra were corrected with C 1s signal at 284.6 eV. Absorption spectra were measured in the range of 200–800 nm using UV–Vis spectrophotometer (Cary 5000,
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X. Zhu et al. / Materials Letters 254 (2019) 21–23
Agilent). The fluorescence spectra were recorded by using Fluorescence Spectrophotometer (FS5, UK) with the Xe lamp excitation source.
3. Results and discussion Fig. 1a shows the XRD patterns of SrZnO2: 0.02 Eu3+ powders prepared at different atmospheres, which were matched well with the standard card of SrZnO2 (JCPDS card No. 41-0551). To explore the variation of oxygen defects in samples prepared at different atmospheres by XPS analysis, the O1s spectra are shown in Fig. 1b–e and divided into three peaks by using a Gauss fitting method, which are associated with O2 ions in the host lattice (OL, 530.3 ± 0.3), oxygen defects (OV, 531.2 ± 0.3) and chemisorbed or dissociated oxygen atoms (OC, 532.6 ± 0.3), respectively [14–16]. As is known to all, oxygen defects include oxygen vacancies (containing three types of VxO, Vȯ and Vӧ) and oxygen interstitials, which can interconvert with each other. When the desorbed oxygen atoms leave intrinsical sites, some of which combine with each other to form gas O2 (OO ? (1/2)O2" + Vӧ + 2e), the other of which settle in interstitial sites to form VxO Oi pairs (OO ? VxO + Oi). As oxygen partial pressure increases and Oi M (1/2)O2", the absorption of oxygen atoms counteracts the fromation of VxO Oi pairs and pushes Oi back to VxO, causing a decrease in Oi atoms (VxO + Oi ? OO) and an increase in oxygen vacancies (OO ? (1/2) O2" + Vӧ + 2e, Vӧ + e ? Vȯ, Vȯ + e ? VxO). Thus, in Fig. 1b–e, the oxygen defects concentrations in samples change obviously: in oxygen atmosphere, oxygen defects are mainly oxygen interstitials; in N2 and Ar atmospheres, oxygen vacancies are dominant among all oxygen defects; in air, oxygen vacancies and oxygen interstitials are present in samples. The results are consistent with report [17]. The intensity of OC peak is related to the surface defects [18,19], as seen in Fig. 1b–e, which is proportional to the concentrations of oxygen vacancies. Moreover, the europium 3d XPS spectrum of samples prepared in air is shown in Fig. 1f, which only
exists one peak around 1135.1 eV, the shape and the binding energy of which agree well with that of Eu3+ 3d5/2 [20]. UV–vis absorption spectra of SrZnO2: 0.02 Eu3+ phosphors prepared at different atmospheres are shown in Fig. 2a. As a direct band gap semiconductor, optical absorption coefficient (a) and the band gap energies are correlated using formula [21,22]. As shown in Fig. 2b, for samples prepared in sequence of O2, air, N2 and Ar atmospheres, their band gap energies are estimated to 3.088, 3.042, 3.084 and 3.079 eV, respectively. Fig. 2a, b indicate that the absorption band edges have red-shift phenomenon and the band gaps become narrow as the oxygen vacancies increase. The results reveal that oxygen vacancies and Oi can diminish the band gap, the effect of oxygen vacancies for which is stronger than that of Oi atoms. Under the joint action of oxygen vacancies and Oi, the sample prepared in air has the lowest band gap of all the samples. Because oxygen defects can create defect levels, somewhere between the valence band top and the conduction band bottom. When an oxygen ion is removed, a defective host lattice with oxygen vacancies (including VxO, Vȯ and Vӧ) is created: for VxO, when two electrons occupy defect levels, owing to the attraction of the surrounded Zn2+ ions, these ions sites distortion leave defect levels set at the middle of the band gap; for Vӧ, when two electrons of defect levels move away, owing to the outward of surrounded Zn2+ ions, defect levels will move toward the conduction band, which make the conduction band bottom shift down. Thus, the outward relaxation behavior of Vӧ acts as narrowing band gap of this system [23–25]. In addition, for oxygen interstitials, extra levels are introduced close to the conduction band bottom with the density increase of ordered excess oxygen atoms, which lead to the band gap narrowing [5,25]. The excitation and emission spectra of SrZnO2: 0.02 Eu3+ phosphors prepared in different atmospheres are shown in Fig. 2c, d. Notably, the band gap energy (3.042 eV) of sample calcined in air is less than the excitation energy of Eu2+ ions, so the excited states of Eu2+ ions overlap the conduction band in favor of an electron capture [26,27]. The strong coupling of these levels
Fig. 1. (a) XRD patterns, Gaussian deconvoluted XPS spectra of (b-e) O1s and (f) Eu 3d for SrZnO2: 0.02 Eu3+ phosphors prepared at different atmospheres.
X. Zhu et al. / Materials Letters 254 (2019) 21–23
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Fig. 2. (a) UV–vis absorption spectra, (b) (ahm)1/2 vs photon energy (hm) spectra, (c) PLE and (d) PL spectra of SrZnO2: 0.02 Eu3+ powders prepared at different atmospheres.
with the conduction band lead to the formation of the autoionization state, which indicates that Eu2+ ions do not exist stably in sample prepared in air (Fig. 1f). Attributed to band gap narrowing effect (Fig. 2a, b), the electrons of the conduction band and charge transfer band, shallow-trap electrons tend to relax directly or indirectly the excited state of Eu3+ ions, then return to the ground state level to participate in the luminescence behavior of Eu3+ ions on the based of host-sensitized luminescence effect [28], which lead to the strongest intensities of the excitation and emission spectra in sample calcined in air.
4. Conclusions In summary, by conventional solid-state reactions, SrZnO2: 0.02 Eu3+ phosphors were prepared in Ar, N2, air and O2, respectively, the oxygen defects concentrations of which change obviously in XPS spectra. The band gaps of samples were diminished and absorption spectra had red-shift phenomenon due to the appearance of Vӧ and oxygen interstitials, in which the effect of oxygen vacancies is stronger than that of oxygen interstitials. Eu2+ ions didn’t exist stably in samples prepared in air. Under the joint action of oxygen vacancies and Oi, the sample prepared in air had the lowest band-gap energy of 3.042 eV and the largest emission intensity on the based of host-sensitized luminescence effect. This research will help to developed a method to promote photoluminescence of Eu3+ ions by inducing oxygen defects.
Declaration of Competing Interest None.
Acknowledgements This work was financed by the National Natural Science Foundation of China (51872175), the International Cooperation Projects of Shaanxi Province (2018KW-027). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
H.W. Xu, L.L. Wang, M.H. Li, et al., Ceram. Int. 43 (2017) 10967–10974. G. Ou, D.K. Li, W. Pan, et al., Adv. Mater. 27 (2015) 2589–2594. Y.L. Yuan Zulfiqar et al., J. Mater. Sci. Mater. Electron. 27 (2016) 9541–9549. V.B. Kamble, A.M. Umarji, AIP Adv. 3 (2013) 082120–82125. H. Pan, J. Mater. Sci. 50 (2015) 4324–4329. S.A. Ansari, M.M. Khan, S. Kalathil, et al., Nanoscale 5 (2013) 9238–9246. S. Kubota, T. Oyama, H. Yamane, et al., Chem. Mater. 15 (2003) 3403–3405. Y.H. Wang, H. Gao, J. Solid State Chem. 179 (2006) 1870–1873. A. Manavbasi, J.C. LaCombe, J. Lumin. 128 (2008) 129–134. D.R. Taikar, C.P. Joshi, S.V. Moharil, et al., J. Lumin. 130 (2010) 1690–1693. L.Z. Yang, X.B. Yu, S.P. Yang, et al., Mater. Lett. 62 (2008) 907–910. H.K. Yang, J.H. Lee, W.T. Hong, et al., Ceram. Int. 41 (2015) 991–994. H. Li, Y. Wang, Inorg. Chem. 56 (2017) 10396–10403. V. Kumar, H.C. Swart, O.M. Ntwaeaborwa, et al., Mater. Lett. 101 (2013) 57–60. Y.S. Kim, W.P. Tai, S.J. Shu, et al., Thin Solid Films 491 (2005) 153–160. R. Al-Gaashani, S. Radiman, A.R. Daud, et al., Ceram. Int. 39 (2013) 2283–2292. W.J. Liu, W.W. Li, Z.G. Hu, et al., J. Appl. Phys. 110 (2011) 013901–13905. X.Y. Zhang, J.Q. Qin, Y.N. Xue, et al., Sci. Rep. 4 (2014) 4596. J.A. Varela, L.A. Perazolli, et al., Radiat. Effects Defect Solids 146 (1998) 131– 143. F.H. Le, L.X. Wang, W. Jia, et al., J. Alloy. Compd. 512 (2012) 323–327. D. Jiles, Introduction to the Electronic Properties of Materials, Chapman and Hall, New York, 1994. T. Prasada Rao, M.C. Santhosh Kumar, J. Cryst, Process. Technol. 2 (2012) 72– 79. F. Oba, A. Togo, I. Tanaka, et al., Phys. Rev. B 77 (2008) 245202–245206. L.P. Dong, R.X. Jia, B. Xin, et al., Sci. Rep. 7 (2017) 40160. K. Matsunaga, T. Tanaka, T. Yamamoto, et al., Phys. Rev. B 68 (2003) 085110– 85119. W.M. Yen, M. Raukas, S.A. Basun, et al., J. Lumin. 69 (1996) 287–294. A.N. Belsky, J.C. Krupa, et al., Displays 19 (1999) 185–196. P.A. Rodnyi, P. Dorenbos, G.B. Stryganyuk, et al., J. Phys. Condens. Matter 15 (2003) 719–729.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Correlation of oxygen defects variations to band gap changes in luminescence properties of SrZnO2: 0.02 Eu3+ phosphors Xiaojuan Zhu a, Yongping Pu a,⇑, Xinyang Pu b, Xin Xie a, Mingyuan Liu a, Qianle Li a a b
Shaanxi University of Science and Technology, Xi’an, Shaanxi 710021, China School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510006, China
a r t i c l e
i n f o
Article history: Received 8 April 2019 Received in revised form 5 July 2019 Accepted 7 July 2019 Available online 8 July 2019 Keywords: Eu3+ SrZnO2 Oxygen vacancies Phosphors Luminescence
a b s t r a c t The SrZnO2: 0.02 Eu3+ phosphors were prepared by conventional solid-state reactions in O2, air, N2 and Ar, respectively. X-ray photoelectron spectroscopy analyses show that the concentration of oxygen defects in samples change obviously under different atmospheres. From the data collected by UV–vis diffuse reflectance spectroscopy and the fluorescence spectra, it is found that Vӧ and oxygen interstitials can diminish band gaps of samples, promoting an increase in photoluminescence intensities of samples on the based of host-sensitized luminescence effect, in which the effect of oxygen vacancies is stronger than that of oxygen interstitials. The mechanism of band gap narrowing has been explained in detail. Ó 2019 Published by Elsevier B.V.
1. Introduction Rare-earth doped metal oxides have been extensively developed in recent years and applied to many optical materials fields, the spectroscopic properties of which with special repeat units (e.g. [SiO4], a-Al2O3, [PO4] /[LiO4] or [GeO4]/[ZnO4]) are governed by the optical transitions induced optical storage and selfactivation luminescence properties [1]. With the discovery of a large number of related oxides, band gap, as a critical parameter for an oxide insulator, directly affects electronic, magnetic and optical properties of the materials. Tunability of the band gap is very important for optimizing properties of materials and achieved through different preparation method [2], chemical doping [3], tuning crystallite sizes [4] and varying the oxygen stoichiometry [5], which is attributed to the introduction of defects in this systems. The type of defects are predominantly point defects (i.e. defect associated to one lattice point, such as cation or oxygen defects) [2,4–6]. As a promising optical material, since SrZnO2: Ba2+, Mn2+ phosphors were reported by Kubota et al. [7], a lot of research on SrZnO2 was carried out in the following years [8–12]. Up to now, however, few reports explore the effect of oxygen defects in the interior of SrZnO2. Europium is a popular activator due to its adjustable valence and sensitive luminescence emission to the crystal field. Usually, ⇑ Corresponding author. E-mail address: [email protected] (Y. Pu). https://doi.org/10.1016/j.matlet.2019.07.025 0167-577X/Ó 2019 Published by Elsevier B.V.
the reduction of Eu3+ ions occurs in reducing atmospheres. However, for some special compounds with stiff three dimensional (3D) enclosed structures (e.g., the tetrahedral anion groups) or Eu3+ ions substituted for the divalent cations, Eu2+ ions appear in samples prepared in air [13]. In our work, oxygen defects were introduced into SrZnO2: 0.02 Eu3+ phospors prepared at different atmospheres by conventional solid-state reactions, which lead to the band gap narrowing. The photoluminescence intensity of Eu3+ ions in sample was enhanced when the band gap reaches a minimum, the mechanism of which was given.
2. Experimental SrZnO2: 0.02 Eu3+ phosphors were prepared by conventional solid-state reactions. Stoichiometric mixtures of SrCO3 (99.9%), ZnO (99.9%), and Eu2O3 (99.9%) were calcined at 1165 °C for 3 h in Ar, N2, air and O2, respectively. The phase identification of all samples were carried by X-ray diffractometer (XRD, Rigaku D/MAX-2400, Tokyo, Japan) with Cu Ka radiation (k = 0.15406 nm) operating at 40 kV and 100 mA. The variation of oxygen defects and binding energy of Eu3+ ions in samples were measured by using X-ray photoelectron spectroscopy (XPS, AXIS SUPRA, UK) with Al Ka radiation (600 W). All the absolute binding energies of the photoelectron spectra were corrected with C 1s signal at 284.6 eV. Absorption spectra were measured in the range of 200–800 nm using UV–Vis spectrophotometer (Cary 5000,
22
X. Zhu et al. / Materials Letters 254 (2019) 21–23
Agilent). The fluorescence spectra were recorded by using Fluorescence Spectrophotometer (FS5, UK) with the Xe lamp excitation source.
3. Results and discussion Fig. 1a shows the XRD patterns of SrZnO2: 0.02 Eu3+ powders prepared at different atmospheres, which were matched well with the standard card of SrZnO2 (JCPDS card No. 41-0551). To explore the variation of oxygen defects in samples prepared at different atmospheres by XPS analysis, the O1s spectra are shown in Fig. 1b–e and divided into three peaks by using a Gauss fitting method, which are associated with O2 ions in the host lattice (OL, 530.3 ± 0.3), oxygen defects (OV, 531.2 ± 0.3) and chemisorbed or dissociated oxygen atoms (OC, 532.6 ± 0.3), respectively [14–16]. As is known to all, oxygen defects include oxygen vacancies (containing three types of VxO, Vȯ and Vӧ) and oxygen interstitials, which can interconvert with each other. When the desorbed oxygen atoms leave intrinsical sites, some of which combine with each other to form gas O2 (OO ? (1/2)O2" + Vӧ + 2e), the other of which settle in interstitial sites to form VxO Oi pairs (OO ? VxO + Oi). As oxygen partial pressure increases and Oi M (1/2)O2", the absorption of oxygen atoms counteracts the fromation of VxO Oi pairs and pushes Oi back to VxO, causing a decrease in Oi atoms (VxO + Oi ? OO) and an increase in oxygen vacancies (OO ? (1/2) O2" + Vӧ + 2e, Vӧ + e ? Vȯ, Vȯ + e ? VxO). Thus, in Fig. 1b–e, the oxygen defects concentrations in samples change obviously: in oxygen atmosphere, oxygen defects are mainly oxygen interstitials; in N2 and Ar atmospheres, oxygen vacancies are dominant among all oxygen defects; in air, oxygen vacancies and oxygen interstitials are present in samples. The results are consistent with report [17]. The intensity of OC peak is related to the surface defects [18,19], as seen in Fig. 1b–e, which is proportional to the concentrations of oxygen vacancies. Moreover, the europium 3d XPS spectrum of samples prepared in air is shown in Fig. 1f, which only
exists one peak around 1135.1 eV, the shape and the binding energy of which agree well with that of Eu3+ 3d5/2 [20]. UV–vis absorption spectra of SrZnO2: 0.02 Eu3+ phosphors prepared at different atmospheres are shown in Fig. 2a. As a direct band gap semiconductor, optical absorption coefficient (a) and the band gap energies are correlated using formula [21,22]. As shown in Fig. 2b, for samples prepared in sequence of O2, air, N2 and Ar atmospheres, their band gap energies are estimated to 3.088, 3.042, 3.084 and 3.079 eV, respectively. Fig. 2a, b indicate that the absorption band edges have red-shift phenomenon and the band gaps become narrow as the oxygen vacancies increase. The results reveal that oxygen vacancies and Oi can diminish the band gap, the effect of oxygen vacancies for which is stronger than that of Oi atoms. Under the joint action of oxygen vacancies and Oi, the sample prepared in air has the lowest band gap of all the samples. Because oxygen defects can create defect levels, somewhere between the valence band top and the conduction band bottom. When an oxygen ion is removed, a defective host lattice with oxygen vacancies (including VxO, Vȯ and Vӧ) is created: for VxO, when two electrons occupy defect levels, owing to the attraction of the surrounded Zn2+ ions, these ions sites distortion leave defect levels set at the middle of the band gap; for Vӧ, when two electrons of defect levels move away, owing to the outward of surrounded Zn2+ ions, defect levels will move toward the conduction band, which make the conduction band bottom shift down. Thus, the outward relaxation behavior of Vӧ acts as narrowing band gap of this system [23–25]. In addition, for oxygen interstitials, extra levels are introduced close to the conduction band bottom with the density increase of ordered excess oxygen atoms, which lead to the band gap narrowing [5,25]. The excitation and emission spectra of SrZnO2: 0.02 Eu3+ phosphors prepared in different atmospheres are shown in Fig. 2c, d. Notably, the band gap energy (3.042 eV) of sample calcined in air is less than the excitation energy of Eu2+ ions, so the excited states of Eu2+ ions overlap the conduction band in favor of an electron capture [26,27]. The strong coupling of these levels
Fig. 1. (a) XRD patterns, Gaussian deconvoluted XPS spectra of (b-e) O1s and (f) Eu 3d for SrZnO2: 0.02 Eu3+ phosphors prepared at different atmospheres.
X. Zhu et al. / Materials Letters 254 (2019) 21–23
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Fig. 2. (a) UV–vis absorption spectra, (b) (ahm)1/2 vs photon energy (hm) spectra, (c) PLE and (d) PL spectra of SrZnO2: 0.02 Eu3+ powders prepared at different atmospheres.
with the conduction band lead to the formation of the autoionization state, which indicates that Eu2+ ions do not exist stably in sample prepared in air (Fig. 1f). Attributed to band gap narrowing effect (Fig. 2a, b), the electrons of the conduction band and charge transfer band, shallow-trap electrons tend to relax directly or indirectly the excited state of Eu3+ ions, then return to the ground state level to participate in the luminescence behavior of Eu3+ ions on the based of host-sensitized luminescence effect [28], which lead to the strongest intensities of the excitation and emission spectra in sample calcined in air.
4. Conclusions In summary, by conventional solid-state reactions, SrZnO2: 0.02 Eu3+ phosphors were prepared in Ar, N2, air and O2, respectively, the oxygen defects concentrations of which change obviously in XPS spectra. The band gaps of samples were diminished and absorption spectra had red-shift phenomenon due to the appearance of Vӧ and oxygen interstitials, in which the effect of oxygen vacancies is stronger than that of oxygen interstitials. Eu2+ ions didn’t exist stably in samples prepared in air. Under the joint action of oxygen vacancies and Oi, the sample prepared in air had the lowest band-gap energy of 3.042 eV and the largest emission intensity on the based of host-sensitized luminescence effect. This research will help to developed a method to promote photoluminescence of Eu3+ ions by inducing oxygen defects.
Declaration of Competing Interest None.
Acknowledgements This work was financed by the National Natural Science Foundation of China (51872175), the International Cooperation Projects of Shaanxi Province (2018KW-027). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
H.W. Xu, L.L. Wang, M.H. Li, et al., Ceram. Int. 43 (2017) 10967–10974. G. Ou, D.K. Li, W. Pan, et al., Adv. Mater. 27 (2015) 2589–2594. Y.L. Yuan Zulfiqar et al., J. Mater. Sci. Mater. Electron. 27 (2016) 9541–9549. V.B. Kamble, A.M. Umarji, AIP Adv. 3 (2013) 082120–82125. H. Pan, J. Mater. Sci. 50 (2015) 4324–4329. S.A. Ansari, M.M. Khan, S. Kalathil, et al., Nanoscale 5 (2013) 9238–9246. S. Kubota, T. Oyama, H. Yamane, et al., Chem. Mater. 15 (2003) 3403–3405. Y.H. Wang, H. Gao, J. Solid State Chem. 179 (2006) 1870–1873. A. Manavbasi, J.C. LaCombe, J. Lumin. 128 (2008) 129–134. D.R. Taikar, C.P. Joshi, S.V. Moharil, et al., J. Lumin. 130 (2010) 1690–1693. L.Z. Yang, X.B. Yu, S.P. Yang, et al., Mater. Lett. 62 (2008) 907–910. H.K. Yang, J.H. Lee, W.T. Hong, et al., Ceram. Int. 41 (2015) 991–994. H. Li, Y. Wang, Inorg. Chem. 56 (2017) 10396–10403. V. Kumar, H.C. Swart, O.M. Ntwaeaborwa, et al., Mater. Lett. 101 (2013) 57–60. Y.S. Kim, W.P. Tai, S.J. Shu, et al., Thin Solid Films 491 (2005) 153–160. R. Al-Gaashani, S. Radiman, A.R. Daud, et al., Ceram. Int. 39 (2013) 2283–2292. W.J. Liu, W.W. Li, Z.G. Hu, et al., J. Appl. Phys. 110 (2011) 013901–13905. X.Y. Zhang, J.Q. Qin, Y.N. Xue, et al., Sci. Rep. 4 (2014) 4596. J.A. Varela, L.A. Perazolli, et al., Radiat. Effects Defect Solids 146 (1998) 131– 143. F.H. Le, L.X. Wang, W. Jia, et al., J. Alloy. Compd. 512 (2012) 323–327. D. Jiles, Introduction to the Electronic Properties of Materials, Chapman and Hall, New York, 1994. T. Prasada Rao, M.C. Santhosh Kumar, J. Cryst, Process. Technol. 2 (2012) 72– 79. F. Oba, A. Togo, I. Tanaka, et al., Phys. Rev. B 77 (2008) 245202–245206. L.P. Dong, R.X. Jia, B. Xin, et al., Sci. Rep. 7 (2017) 40160. K. Matsunaga, T. Tanaka, T. Yamamoto, et al., Phys. Rev. B 68 (2003) 085110– 85119. W.M. Yen, M. Raukas, S.A. Basun, et al., J. Lumin. 69 (1996) 287–294. A.N. Belsky, J.C. Krupa, et al., Displays 19 (1999) 185–196. P.A. Rodnyi, P. Dorenbos, G.B. Stryganyuk, et al., J. Phys. Condens. Matter 15 (2003) 719–729.