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Synthesis and characterization of Co2+:LiGa5O8 nanocrystals by sol–gel method
Journal of Alloys and Compounds 461 (2008) 451–453
Synthesis and characterization of Co2+:LiGa5O8 nanocrystals by sol–gel method Xiulan Duan ∗ , DuorongYuan, Fapeng Yu, Xiqing Zhang State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China Received 2 April 2007; received in revised form 6 July 2007; accepted 6 July 2007 Available online 12 July 2007
Abstract Co2+ -doped LiGa5 O8 nanocrystalline powders were prepared by a sol–gel method. The gels and sintered samples were characterized by means of thermogravimetry and differential scanning calorimetry (TG/DSC), X-ray diffraction (XRD), Inductively coupled plasma-atomic emission spectrometry (ICP-AES), transmission electron microscope (TEM), Fourier transform infrared (FT-IR) spectra and near-infrared absorption spectrum. LiGa5 O8 nanocrystals were produced by calcining the gel above 500 ◦ C, with the crystallite size of 10–30 nm in the temperature range of 500–700 ◦ C. The absorption spectrum of Co2+ -doped LiGa5 O8 exhibited a broad absorption band near 1.5 m, which indicated that Co2+ ions substituted for the tetrahedrally coordinated Ga3+ ions in the LiGa5 O8 lattice. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanostructured materials; Sol–gel processes; X-ray diffraction; Optical properties
1. Introduction Spinel is a class of useful crystals which can be used as the laser host medium, the phosphor and magnetic materials [1–3]. When doped with transition metal ions, they possess useful properties for optical applications [4,5]. Some Co2+ -doped spinel crystals, such as MgAl2 O4 , LiGa5 O8 , have been used as passive Q-switches for neodymium and erbium-doped lasers operating at 1.3 and 1.5 m, respectively [6,7]. Lithium gallate (LiGa5 O8 ) crystal has an inverse spinel structure, and contains four formula units in the cubic cell [8]. One-half of the Ga3+ ions occupies tetrahedral sites while the other half of Ga3+ ions, together with the Li+ ions, occupies octahedral sites. Co2+ ions substitute for the tetrahedrally coordinated Ga3+ ions in the LiGa5 O8 lattice and occupy sites of C3 point group symmetry. Co2+ -doped LiGa5 O8 crystals were usually grown by flux method, which required very high temperature and rigid conditions. Recently, transparent polycrystalline ceramic materials have received much attention due to their improved qualities. Compared with single crystals, transparent ceramics have the advantages of low price, ease of fabrication and mass-
∗
Corresponding author. Tel.: +86 531 8362822; fax: +86 531 8564337. E-mail address: [email protected] (X. Duan).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.07.014
production. High quality Nd3+ :YAG ceramics have been developed by densifying the corresponding nanocrystalline powders by vacuum sintering [9]. If transparent Co2+ :LiGa5 O8 ceramics could be obtained by vacuum sintering, it might be used as a potential Q-switch material. The preparation of nanometer powders is the important step to obtain transparent polycrystalline ceramics. There are many methods to prepare mixed metal oxide powders, such as solid-state reaction, sol–gel method, hydrothermal synthesis. Compared with other techniques, the sol–gel method is a useful and attractive technique for the preparation of nanocrystalline powders because of its advantage of producing pure and ultrafine powders at low temperatures. In the present work, Co2+ -doped LiGa5 O8 nanoparticles were synthesized by the sol–gel method using citric acid as a chelating agent. The optical absorption spectrum of the sample was also studied. 2. Experimental Co2+ :LiGa5 O8 nanocrystals were synthesized via a citrate sol–gel method. All of the reagents and solvent are analytical grade and used without any further purification. Firstly, stoichiometric amounts of gallium nitrate (Ga(NO3 )3 ·xH2 O) and lithium acetate (Li(CH3 COO)·2H2 O) were dissolved in deionized water. Cobalt nitrate (Co(NO3 )2 ·6H2 O) was added to the above solution and the molar ratio of Co2+ ion and LiGa5 O8 is 2:100. Then an excess of citric acid (C6 H8 O7 ·H2 O), typically 3×, was added to the metal solution.
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X. Duan et al. / Journal of Alloys and Compounds 461 (2008) 451–453
Fig. 1. DSC/TG curves of Co2+ -doped spinel precursor gel. Fig. 2. XRD patterns of sample heat-treated at different temperatures for 2 h. After mixing and thoroughly stirring, the solution was heated at 80 ◦ C, until a highly viscous gel was formed, then heated at 110 ◦ C. Pink fluffy product was obtained after the reaction. The precursor gels were calcined in the temperature range of 500–700 ◦ C. Thermogravimetry (TG) and differential scanning calorimetry (DSC) analyses of the precursor were performed using a DSC/TGA analyzer (Model Q600 SDT) in flowing nitrogen (20 ml/min). The heating rate was 5 ◦ C/min. X-ray powder diffraction (XRD) data were recorded on a Japan Rigaku D/Max-rA X-ray diffractometer system with graphite monochromatized Cu K␣ irradiation (λ = 0.15418 nm). The inductively coupled plasma-atomic emission spectrometry (ICP-AES) (Model IRIS Advantage) was used to measure the composition of the crystalline phase. The Fourier transform infrared (FT-IR) spectra of the samples were collected using a 5DX FT-IR spectrometer. Transmission electron micrograph (TEM) images were taken with a JEM-100CXII electron microscopy. The optical absorption spectrum was measured with a Hiachi model U-3500 recording spectrophotometer.
3. Results and discussion Fig. 1 shows the thermal behavior of the gel sample. The sharp exothermic peak at 175 ◦ C observed in the DSC curve can be contributed to the decomposition of nitrate, accompanied by significant weight loss. The exothermic peak at 500 ◦ C corresponds to the crystallization of the LiGa5 O8 spinel. No distinct weight loss is observed at temperature higher than 500 ◦ C because of the mass formation of spinel. The XRD patterns of the LiGa5 O8 gels heat-treated at 500–700 ◦ C for 2 h are shown in Fig. 2. The XRD pattern of the sample heat-treated at 500 ◦ C exhibits several weak peaks,
which shows that the sample has been crystallized at 500 ◦ C (also confirmed by DSC curve). With the heat-treatment temperature increasing, the intensity of the diffraction peaks increases, which is associated with an increase of crystallinity. All the peaks can be perfectly indexed to crystalline LiGa5 O8 (JCPDS 38-1371). These peaks at scattering angles (2θ) of 30.68, 36.20, 44.04, 54.86, 58.37, 64.13, correspond to the reflections from the 220, 311, 400, 422, 511 and 440 crystal planes, respectively, of the cubical phase of lithium gallium oxide. The compositional analysis by the ICP-AES results show that the percent of Li in the 700 ◦ C-sintered sample is 1.47% and the percent of Ga is 72.34%, respectively, which is consistent with the calculated values of Li (1.43%) and Ga (72.08%) percents in LiGa5 O8 phase. Therefore, we think that the crystalline phase is cubic LiGa5 O8 . The typical TEM images of as-synthesized nanocrystals are shown in Fig. 3. It is evident that most particles have a square or near spherical shape. The average grain sizes of the powders sintered at 500 ◦ C for 2 h (Fig. 3a) and 700 ◦ C for 2 h (Fig. 3b) were about 15 and 30 nm, respectively. Fig. 4 shows the FT-IR spectra collected on the Co2+ :LiGa5 O8 samples mulled in a KBr wafer as a function of temperature. The broad peak centered at 3410 cm−1 is attributed to the stretching vibration of H2 O, indicating the existence of water absorbed in the sample. The peaks at 1089 and 1384 cm−1
Fig. 3. Transmission electron microscope micrograph of powder sample annealed at 500 ◦ C (a) and 700 ◦ C (b).
X. Duan et al. / Journal of Alloys and Compounds 461 (2008) 451–453
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band near 1.5 m is assigned to 4 A2 → 4 T1 (4 F) transition of the tetrahedrally coordinated Co2+ ion. This absorption band clearly indicated that Co2+ ions occupy C3 symmetry sites of spinelstructured LiGa5 O8 substituting for tetrahedrally coordinated Ga3+ ions. Previously, we have prepared Co2+ :MgGa2 O4 nanocrystals by the sol–gel method [13]. Both Co2+ :LiGa5 O8 and Co2+ :MgGa2 O4 nanocrystals exhibit broad absorption band near 1.5 m, which show that the corresponding ceramics obtained by densifying the corresponding nanocrystalline powders through vacuum sintering method might be used as potential candidates for saturable absorber materials. But Co2+ :LiGa5 O8 nano-scale powders have low crystalline temperature compared with Co2+ :MgGa2 O4 nano-scale powders. 4. Conclusion Fig. 4. IR spectra of sample heated at different temperatures: (a) 40 ◦ C; (b) 500 ◦ C; (c) 700 ◦ C.
Nano-scale powders of LiGa5 O8 -doped with Co2+ ions were prepared by a low temperature sol–gel method using citric acid as a chelating agent. LiGa5 O8 nanocrystals were formed above 500 ◦ C and the particle size was about 15–30 nm in the temperature range of 500–700 ◦ C. The absorption spectrum indicates that Co2+ ions locate in the tetrahedral sites in the LiGa5 O8 nanocrystallites. Acknowledgement This work is supported by the Research Fund of Key Lab for Nanomaterials, Ministry of Education (No. 2007-3). References
Fig. 5. Absorption spectrum for Co2+ :LiGa5 O8 nanocrystals. The inset shows the energy level of the tetrahedral Co2+ ion.
may be related to NO3 − ions. The peak at 1602 cm−1 is due to COO− group. With the temperature increasing, the abovementioned peaks became weaker or disappeared. When the samples were heated at 500 ◦ C, new absorption peaks (568, 486 and 434 cm−1 ) appeared, which indicates the formation of LiGa5 O8 nanocrystals, and the intensity of the absorption peaks increases with the temperature increasing. Many materials doped with tetrahedral Co2+ ion have been used as passive Q-switches because of their saturable absorption properties [10–12]. To confirm the existence of tetrahedral Co2+ ions in the LiGa5 O8 nanocrystals, the absorption spectrum of the nanocrystals was studied. A thin wafer was made from the as-prepared nanocrystalline powder heated at 700 ◦ C and was used to measure the optical absorption spectrum, the result is shown in Fig. 5. The absorption spectrum is similar to that of Co2+ -doped LiGa5 O8 crystal [7]. The broad absorption
[1] K.V. Yumashev, N.N. Posnov, V.P. Mikhailov, Appl. Phys. B 69 (1999) 41. [2] Y.E. Lee, D.P. Norton, C. Park, C.M. Rouleau, J. Appl. Phys. 89 (2001) 1653. [3] J. Ostorero, M. Guillot, M. Leblanc, D. Rouet, J. Appl. Phys. 69 (1991) 4571. [4] J.F. Donegan, F.J. Bergin, T.J. Glynn, G.F. Imbusch, J.P. Remeika, J. Lumin. 35 (1986) 57. [5] W. Zheng, Solid State Commum. 81 (1992) 135. [6] K.V. Yumashev, Appl. Opt. 38 (1999) 6343. [7] I.A. Denisov, M.I. Demchuk, N.V. Kuleshov, K.V. Yumashev, Appl. Phys. Lett. 77 (2000) 2455. [8] J.F. Donegan, F.G. Anderson, F.J. Bergin, T.J. Glynn, G.F. Imbusch, Phys. Rev. B 45 (1992) 563. [9] J. Lu, T. Murai, K. Takaichi, T. Uematsu, K. Misawa, M. Prabhu, J. Xu, K. Ueda, H. Yagi, T. Yanagitani, A.A. Kaminskii, A. Kudryashov, Appl. Phys. Lett. 78 (2001) 3586. [10] K.V. Yumashev, I.A. Denisov, N.N. Posnov, N.V. Kuleshov, R. Moncorge, J. Alloy Compd. 341 (2002) 366. [11] Z. Burshtein, Y. Shimony, R. Feldman, V. Krupkin, A. Glushko, E. Galun, Opt. Mater. 15 (2001) 285. [12] N.V. Kuleshov, V.P. Mikhailov, V.G. Scherbitsky, P.V. Prokoshiu, K.V. Yumashev, J. Lumin. 55 (1993) 265. [13] X.L. Duan, D.R. Yuan, X.F. Cheng, L.H. Wang, F.P. Yu, J. Alloy Compd. 439 (2007) 355.
Synthesis and characterization of Co2+:LiGa5O8 nanocrystals by sol–gel method Xiulan Duan ∗ , DuorongYuan, Fapeng Yu, Xiqing Zhang State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China Received 2 April 2007; received in revised form 6 July 2007; accepted 6 July 2007 Available online 12 July 2007
Abstract Co2+ -doped LiGa5 O8 nanocrystalline powders were prepared by a sol–gel method. The gels and sintered samples were characterized by means of thermogravimetry and differential scanning calorimetry (TG/DSC), X-ray diffraction (XRD), Inductively coupled plasma-atomic emission spectrometry (ICP-AES), transmission electron microscope (TEM), Fourier transform infrared (FT-IR) spectra and near-infrared absorption spectrum. LiGa5 O8 nanocrystals were produced by calcining the gel above 500 ◦ C, with the crystallite size of 10–30 nm in the temperature range of 500–700 ◦ C. The absorption spectrum of Co2+ -doped LiGa5 O8 exhibited a broad absorption band near 1.5 m, which indicated that Co2+ ions substituted for the tetrahedrally coordinated Ga3+ ions in the LiGa5 O8 lattice. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanostructured materials; Sol–gel processes; X-ray diffraction; Optical properties
1. Introduction Spinel is a class of useful crystals which can be used as the laser host medium, the phosphor and magnetic materials [1–3]. When doped with transition metal ions, they possess useful properties for optical applications [4,5]. Some Co2+ -doped spinel crystals, such as MgAl2 O4 , LiGa5 O8 , have been used as passive Q-switches for neodymium and erbium-doped lasers operating at 1.3 and 1.5 m, respectively [6,7]. Lithium gallate (LiGa5 O8 ) crystal has an inverse spinel structure, and contains four formula units in the cubic cell [8]. One-half of the Ga3+ ions occupies tetrahedral sites while the other half of Ga3+ ions, together with the Li+ ions, occupies octahedral sites. Co2+ ions substitute for the tetrahedrally coordinated Ga3+ ions in the LiGa5 O8 lattice and occupy sites of C3 point group symmetry. Co2+ -doped LiGa5 O8 crystals were usually grown by flux method, which required very high temperature and rigid conditions. Recently, transparent polycrystalline ceramic materials have received much attention due to their improved qualities. Compared with single crystals, transparent ceramics have the advantages of low price, ease of fabrication and mass-
∗
Corresponding author. Tel.: +86 531 8362822; fax: +86 531 8564337. E-mail address: [email protected] (X. Duan).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.07.014
production. High quality Nd3+ :YAG ceramics have been developed by densifying the corresponding nanocrystalline powders by vacuum sintering [9]. If transparent Co2+ :LiGa5 O8 ceramics could be obtained by vacuum sintering, it might be used as a potential Q-switch material. The preparation of nanometer powders is the important step to obtain transparent polycrystalline ceramics. There are many methods to prepare mixed metal oxide powders, such as solid-state reaction, sol–gel method, hydrothermal synthesis. Compared with other techniques, the sol–gel method is a useful and attractive technique for the preparation of nanocrystalline powders because of its advantage of producing pure and ultrafine powders at low temperatures. In the present work, Co2+ -doped LiGa5 O8 nanoparticles were synthesized by the sol–gel method using citric acid as a chelating agent. The optical absorption spectrum of the sample was also studied. 2. Experimental Co2+ :LiGa5 O8 nanocrystals were synthesized via a citrate sol–gel method. All of the reagents and solvent are analytical grade and used without any further purification. Firstly, stoichiometric amounts of gallium nitrate (Ga(NO3 )3 ·xH2 O) and lithium acetate (Li(CH3 COO)·2H2 O) were dissolved in deionized water. Cobalt nitrate (Co(NO3 )2 ·6H2 O) was added to the above solution and the molar ratio of Co2+ ion and LiGa5 O8 is 2:100. Then an excess of citric acid (C6 H8 O7 ·H2 O), typically 3×, was added to the metal solution.
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X. Duan et al. / Journal of Alloys and Compounds 461 (2008) 451–453
Fig. 1. DSC/TG curves of Co2+ -doped spinel precursor gel. Fig. 2. XRD patterns of sample heat-treated at different temperatures for 2 h. After mixing and thoroughly stirring, the solution was heated at 80 ◦ C, until a highly viscous gel was formed, then heated at 110 ◦ C. Pink fluffy product was obtained after the reaction. The precursor gels were calcined in the temperature range of 500–700 ◦ C. Thermogravimetry (TG) and differential scanning calorimetry (DSC) analyses of the precursor were performed using a DSC/TGA analyzer (Model Q600 SDT) in flowing nitrogen (20 ml/min). The heating rate was 5 ◦ C/min. X-ray powder diffraction (XRD) data were recorded on a Japan Rigaku D/Max-rA X-ray diffractometer system with graphite monochromatized Cu K␣ irradiation (λ = 0.15418 nm). The inductively coupled plasma-atomic emission spectrometry (ICP-AES) (Model IRIS Advantage) was used to measure the composition of the crystalline phase. The Fourier transform infrared (FT-IR) spectra of the samples were collected using a 5DX FT-IR spectrometer. Transmission electron micrograph (TEM) images were taken with a JEM-100CXII electron microscopy. The optical absorption spectrum was measured with a Hiachi model U-3500 recording spectrophotometer.
3. Results and discussion Fig. 1 shows the thermal behavior of the gel sample. The sharp exothermic peak at 175 ◦ C observed in the DSC curve can be contributed to the decomposition of nitrate, accompanied by significant weight loss. The exothermic peak at 500 ◦ C corresponds to the crystallization of the LiGa5 O8 spinel. No distinct weight loss is observed at temperature higher than 500 ◦ C because of the mass formation of spinel. The XRD patterns of the LiGa5 O8 gels heat-treated at 500–700 ◦ C for 2 h are shown in Fig. 2. The XRD pattern of the sample heat-treated at 500 ◦ C exhibits several weak peaks,
which shows that the sample has been crystallized at 500 ◦ C (also confirmed by DSC curve). With the heat-treatment temperature increasing, the intensity of the diffraction peaks increases, which is associated with an increase of crystallinity. All the peaks can be perfectly indexed to crystalline LiGa5 O8 (JCPDS 38-1371). These peaks at scattering angles (2θ) of 30.68, 36.20, 44.04, 54.86, 58.37, 64.13, correspond to the reflections from the 220, 311, 400, 422, 511 and 440 crystal planes, respectively, of the cubical phase of lithium gallium oxide. The compositional analysis by the ICP-AES results show that the percent of Li in the 700 ◦ C-sintered sample is 1.47% and the percent of Ga is 72.34%, respectively, which is consistent with the calculated values of Li (1.43%) and Ga (72.08%) percents in LiGa5 O8 phase. Therefore, we think that the crystalline phase is cubic LiGa5 O8 . The typical TEM images of as-synthesized nanocrystals are shown in Fig. 3. It is evident that most particles have a square or near spherical shape. The average grain sizes of the powders sintered at 500 ◦ C for 2 h (Fig. 3a) and 700 ◦ C for 2 h (Fig. 3b) were about 15 and 30 nm, respectively. Fig. 4 shows the FT-IR spectra collected on the Co2+ :LiGa5 O8 samples mulled in a KBr wafer as a function of temperature. The broad peak centered at 3410 cm−1 is attributed to the stretching vibration of H2 O, indicating the existence of water absorbed in the sample. The peaks at 1089 and 1384 cm−1
Fig. 3. Transmission electron microscope micrograph of powder sample annealed at 500 ◦ C (a) and 700 ◦ C (b).
X. Duan et al. / Journal of Alloys and Compounds 461 (2008) 451–453
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band near 1.5 m is assigned to 4 A2 → 4 T1 (4 F) transition of the tetrahedrally coordinated Co2+ ion. This absorption band clearly indicated that Co2+ ions occupy C3 symmetry sites of spinelstructured LiGa5 O8 substituting for tetrahedrally coordinated Ga3+ ions. Previously, we have prepared Co2+ :MgGa2 O4 nanocrystals by the sol–gel method [13]. Both Co2+ :LiGa5 O8 and Co2+ :MgGa2 O4 nanocrystals exhibit broad absorption band near 1.5 m, which show that the corresponding ceramics obtained by densifying the corresponding nanocrystalline powders through vacuum sintering method might be used as potential candidates for saturable absorber materials. But Co2+ :LiGa5 O8 nano-scale powders have low crystalline temperature compared with Co2+ :MgGa2 O4 nano-scale powders. 4. Conclusion Fig. 4. IR spectra of sample heated at different temperatures: (a) 40 ◦ C; (b) 500 ◦ C; (c) 700 ◦ C.
Nano-scale powders of LiGa5 O8 -doped with Co2+ ions were prepared by a low temperature sol–gel method using citric acid as a chelating agent. LiGa5 O8 nanocrystals were formed above 500 ◦ C and the particle size was about 15–30 nm in the temperature range of 500–700 ◦ C. The absorption spectrum indicates that Co2+ ions locate in the tetrahedral sites in the LiGa5 O8 nanocrystallites. Acknowledgement This work is supported by the Research Fund of Key Lab for Nanomaterials, Ministry of Education (No. 2007-3). References
Fig. 5. Absorption spectrum for Co2+ :LiGa5 O8 nanocrystals. The inset shows the energy level of the tetrahedral Co2+ ion.
may be related to NO3 − ions. The peak at 1602 cm−1 is due to COO− group. With the temperature increasing, the abovementioned peaks became weaker or disappeared. When the samples were heated at 500 ◦ C, new absorption peaks (568, 486 and 434 cm−1 ) appeared, which indicates the formation of LiGa5 O8 nanocrystals, and the intensity of the absorption peaks increases with the temperature increasing. Many materials doped with tetrahedral Co2+ ion have been used as passive Q-switches because of their saturable absorption properties [10–12]. To confirm the existence of tetrahedral Co2+ ions in the LiGa5 O8 nanocrystals, the absorption spectrum of the nanocrystals was studied. A thin wafer was made from the as-prepared nanocrystalline powder heated at 700 ◦ C and was used to measure the optical absorption spectrum, the result is shown in Fig. 5. The absorption spectrum is similar to that of Co2+ -doped LiGa5 O8 crystal [7]. The broad absorption
[1] K.V. Yumashev, N.N. Posnov, V.P. Mikhailov, Appl. Phys. B 69 (1999) 41. [2] Y.E. Lee, D.P. Norton, C. Park, C.M. Rouleau, J. Appl. Phys. 89 (2001) 1653. [3] J. Ostorero, M. Guillot, M. Leblanc, D. Rouet, J. Appl. Phys. 69 (1991) 4571. [4] J.F. Donegan, F.J. Bergin, T.J. Glynn, G.F. Imbusch, J.P. Remeika, J. Lumin. 35 (1986) 57. [5] W. Zheng, Solid State Commum. 81 (1992) 135. [6] K.V. Yumashev, Appl. Opt. 38 (1999) 6343. [7] I.A. Denisov, M.I. Demchuk, N.V. Kuleshov, K.V. Yumashev, Appl. Phys. Lett. 77 (2000) 2455. [8] J.F. Donegan, F.G. Anderson, F.J. Bergin, T.J. Glynn, G.F. Imbusch, Phys. Rev. B 45 (1992) 563. [9] J. Lu, T. Murai, K. Takaichi, T. Uematsu, K. Misawa, M. Prabhu, J. Xu, K. Ueda, H. Yagi, T. Yanagitani, A.A. Kaminskii, A. Kudryashov, Appl. Phys. Lett. 78 (2001) 3586. [10] K.V. Yumashev, I.A. Denisov, N.N. Posnov, N.V. Kuleshov, R. Moncorge, J. Alloy Compd. 341 (2002) 366. [11] Z. Burshtein, Y. Shimony, R. Feldman, V. Krupkin, A. Glushko, E. Galun, Opt. Mater. 15 (2001) 285. [12] N.V. Kuleshov, V.P. Mikhailov, V.G. Scherbitsky, P.V. Prokoshiu, K.V. Yumashev, J. Lumin. 55 (1993) 265. [13] X.L. Duan, D.R. Yuan, X.F. Cheng, L.H. Wang, F.P. Yu, J. Alloy Compd. 439 (2007) 355.