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Mild solvothermal synthesis of KZnF3 and KCdF3 nanocrystals
Materials Letters 59 (2005) 430 – 433 www.elsevier.com/locate/matlet
Mild solvothermal synthesis of KZnF3 and KCdF3 nanocrystals Bin Huanga, Jian-Ming Hongb, Xue-Tai Chena,c,*, Zhi Yua, Xiao-Zeng Youa a
State Key Laboratory of Coordination Chemistry and Coordination Chemistry Institute, Nanjing University, Nanjing 210093, P.R. China b Analytic Center of Nanjing University, Nanjing 210093, P.R. China c State Key Laboratory of Structural Chemistry, Fujian Institute of Structure of Matter, Fuzhou 350002, P.R. China Received 25 June 2004; accepted 21 September 2004 Available online 19 October 2004
Abstract KZnF3 and KCdF3 nanocrystals have been prepared via a mild solvothermal route, and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The reaction between M(CH3CO2)2d 2H2O (M=Zn, Cd) and KFd 2H2O in MeOH was investigated by variation of reaction temperature, reaction time and molar ratio of starting materials, which indicated that KZnF3 and KCdF3 nanocrystals could be controllably synthesized. D 2004 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; Peroskites; Metal fluorides; Solvothermal
1. Introduction In recent years, there have been much attention paid to complex metal fluorides because of their particular physical properties such as ferromagnetic [1], nonmagnetic insulator behavior [2], piezoelectric characteristics [3], and photoluminescence properties [4,5]. Among these important complex metal fluorides, KZnF3 and KCdF3 are particularly attractive because of their luminescent property [4,5] and the applications in radiation detection and laser medium [6]. AMF3 (A=Na, K, M=Zn and Cd) were normally synthesized by tradition high-temperature solid state reaction, which required complicated set-up under an atmosphere of F2 or HF mixed with an inert carrier gas to avoid possible contamination from oxygen [7,8]. A mechanochemical procedure using a planetary mill was also used to prepare micro-particles of these ternary metal fluorides [9]. Recently hydrothermal reaction proceeding at higher temperature and for long reaction time has been developed
* Corresponding author. State Key Laboratory of Coordination Chemistry and Coordination Chemistry Institute, Nanjing University, Hankou, Jiangsu, Nanjing 210093, P.R. China. 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.09.039
[10,11]. The products based on these procedures usually exhibit relatively large and varied grain sizes, and inhomogeneous morphologies. To our knowledge, there is no report of preparation of KZnF3 and KCdF3 nanocrystals. Here we report a mild solvothermal procedure to prepare KMF3 (M=Zn and Cd) nanocrystals using M(CH3CO2)2d 2H2O (M=Zn and Cd) and KFd 2H2O as precursors.
2. Experimental Chemical reagents used were Zn(CH3CO2)2d 2H2O, Cd(CH3CO2)2d 2H2O, and KFd 2H2O with 99% purity. KZnF3 and KCdF3 nanocrystals were prepared by the reaction between Zn(CH3CO2)2d 2H2O or Cd(CH3CO2)2d 2H2O and KFd 2H2O at different molar ratio of M2+: F (1:2, 1:4, 1:8) in autoclave at the set temperature. In a typical synthesis, Zn(CH3CO2)2d 2H2O (1.0 mmol) and KFd 2H2O (8.0 mmol) were each dissolved in 15.0 ml CH3OH, respectively, and then mixed together and stirred vigorously for 10 min at room temperature. The mixture was transferred into 50-ml Teflon-lined autoclave, sealed tight and maintained at the set temperature for different reaction time. After cooling down to room temperature naturally, the products were washed with ethanol for several
B. Huang et al. / Materials Letters 59 (2005) 430–433
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Table 1 The solvothermal synthesis conditions for KZnF3 and KCdF3 Sample no.
Starting materials
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12
Zn(CH3CO2)2d 2H2O Zn(CH3CO2)2d 2H2O Zn(CH3CO2)2d 2H2O Zn(CH3CO2)2d 2H2O Zn(CH3CO2)2d 2H2O Zn(CH3CO2)2d 2H2O Zn(CH3CO2)2d 2H2O Cd(CH3CO2)2d 2H2O Cd(CH3CO2)2d 2H2O Cd(CH3CO2)2d 2H2O Cd(CH3CO2)2d 2H2O Cd(CH3CO2)2d 2H2O
KFd 2H2O KFd 2H2O KFd 2H2O KFd 2H2O KFd 2H2O KFd 2H2O KFd 2H2O KFd 2H2O KFd 2H2O KFd 2H2O KFd 2H2O KFd 2H2O
a/b
Reaction temperature (8C)
Reaction time (h)
Products
1:2 1:4 1:8 1:8 1:8 1:8 1:8 1:2 1:4 1:8 1:8 1:8
150 150 150 150 150 120 80 150 150 150 120 80
6 6 6 2 2/3 6 6 6 6 6 6 6
KZnF3+ZnF2 KZnF3 KZnF3+K2ZnF4(trace) KZnF3 KZnF3 KZnF3 KZnF3 KCdF3+Cd(OH)F KCdF3 KCdF3 KCdF3 KCdF3
times and dried in vacuum. The influences of reaction time, temperature and molar ratio of starting materials on the products were also investigated. The products are denoted as S1–S12. The reaction conditions are summarized in Table 1. The products were characterized by a Shimadzu XD-3A X-ray powder diffractometor using Cu Ka (k=1.5418 2) radiation. The morphology and size of as-prepared samples were investigated by transmission electron microscopy (TEM, JEM-200CX) with an accelerating voltage of 120 KV. X-ray photoelectron spectroscopy (XPS) experiment was conducted with a VG ESCALB MK-II spectrometer using a Mg Ka (1253.6 eV) source.
Zn(CH3CO2)2d 6H2O/KFd 2H2O ratio at 150 8C for 6 h is shown in Fig. 1a. All the diffraction peaks can be indexed to that of JCPDS card No. 81-2097 with a calculated cell parameter a=4.068(3) 2. No peaks of any impurities can be found, indicating that pure KZnF3 has been prepared. According to the Scherrer equation, the size of KZnF3 particles was estimated to be ca. 25 nm. Similarly, KCdF3 nanocrystals (S9) were prepared via a similar route using Cd(CH 3 CO 2 ) 2 d 6H 2 O instead of Zn(CH3CO2)2d 6H2O under otherwise identical reaction conditions. The XRD patterns shown in Fig. 1b can be indexed to that of JCPDS card No. 22-802 with a calculated
3. Results and discussion In this current study, we adopted a solvothermal reaction employing M(CH3CO2)2d 2H2O and KF as starting materials in MeOH, from which nanocrystalline KZnF3 and KCdF3 were prepared. The powder X-ray diffraction (XRD) patterns of the product (S2) obtained with 1:4 of
Fig. 1. XRD patterns of (a) KZnF3 (S2) and (b) KCdF3 (S9).
Fig. 2. TEM images of (a) KZnF3 (S2) and (b) KCdF3 (S9).
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B. Huang et al. / Materials Letters 59 (2005) 430–433
cell parameter a=6.112(2), b=8.658(6), c=6.119(2) 2, indicating that the product is pure KCdF3. The Scherrer equation suggested that they have a size of ca. 80 nm. The TEM images of the as-prepared KZnF3 (S2) and KCdF3 (S9) shown in Fig. 2 indicated that both KZnF3 (S2) and KCdF3 (S9) are nanocubes. They have the average sizes of 25 and 75 nm, respectively, in good agreement with the XRD results. Both of them are not stable under electron beam radiation of TEM, therefore, it was difficult to obtain their selected area electron diffraction. XPS analysis was performed in order to further examine the composition of the as-prepared samples. The XPS spectra of KZnF3 (S2) and KCdF3 (S9) are displayed in Fig. 3. In the spectra of KZnF3 shown in Fig. 3a, there are a small adsorbed carbon C1s peak positioned at 284.5 eV, which was used to calibrate the acquired spectrum. A very weak O1s peak at 531.3 eV can be ascribed to the absorbed water on the surface of nanocrystals. The position of F1s (684.3 eV), K2p (292.2 eV) and Zn2p3/2 (1022.5 eV) agree well with those reported for ZnF2 and KF [12]. The analysis of the areas of these peaks gave atomic ratio K/Zn/F of 1.16:1.00:2.63, in good agreement with the expected atomic
ratio. Similar XPS result was obtained for KCdF3 with the ratio of K/Cd/F is 1.00:1.08:2.25. Further control experiments under different reaction conditions were carried out in order to reveal the factors affecting the reaction product. It was found that reaction temperature, reaction time and molar ratio of starting materials are important controlling factors. When the molar ratio of Zn(CH3CO2)2d 2H2O/KFd 2H2O is 1:2, the product (S1) was a mixture of KZnF3 and ZnF2 under the otherwise identical conditions (150 8C, 6 h). However, for the reaction of Cd(CH 3 CO 2 ) 2 d 2H 2 O/ KFd 2H2O=1:2, the product (S8) is the mixture of KCdF3 and Cd(OH)F. When the molar ratio of M(CH3CO2)2d 2H2O/KFd 2H2O is 1:4, the product is pure KZnF3 (S2) or KCdF3 (S9). Therefore, the molar ratio is an important controlling factor. When the molar ratio is 1:8 and reaction temperature is set at 150 8C, pure KZnF3 (S4, S5) can be obtained after relatively short time, for example, 2 h or even 40 min, however, the product (S3) is a mixture of KZnF3 and a trace of K2ZnF4 with reaction time of 6 h. Pure KZnF3 was obtained when the reaction temperature was lower than 150 8C (S6, S7). However, pure KCdF3 (S10–S12) were obtained from the reaction system using Cd(CH3CO2)2d 2H2O instead of Zn(CH3CO2)2d 2H2O. Therefore, reaction time and reaction temperature should be properly controlled for the preparation of pure KMF3. Shi et al. [11,12] have prepared KZnF3 and KCdF3 using MF2 as one of starting materials via the hydrothermal route, which required a relatively long reaction time (4–5 days). The particle size of the product is relatively large, 40–50 Am. In contrast, in our current study, we employ a simple solvothermal procedure using metal carboxylate and KFd 2H2O as starting materials and MeOH as solvent, from which pure KZnF3 and KCdF3 nanocrystals can be obtained within relatively short reaction time. Therefore, it could be concluded that the choice of starting materials and solvent is crucial in the control of the particle size of the product.
4. Conclusion A mild solvothermal procedure has been developed to give KZnF3 and KCdF3 nanocrystals at relatively low temperature within a short time and almost no oxycontaining impurities in the product. It had good reproducibility and simple maneuverability. We believe that our simple procedure can be extended to prepare nanocrystals of other metal fluorides.
Acknowledgements
Fig. 3. XPS spectra of KZnF3 (S2) and KCdF3 (S9).
This work was supported by Major State Basic Research Development Program (G200077500), Natural Science Grant of Jiangsu Province (BK 2004087), Nature Science
B. Huang et al. / Materials Letters 59 (2005) 430–433
Grant of China (20001004) and Grant of Instruments of Nanjing University.
References [1] A.H. Cooke, D.A. Jones, J.F.A. Silva, M.R. Weils, J. Phys. C.: Solid State Phys. 8 (1975) 4083. [2] R.A. Heaton, C. Lin, Phys. Rev., B 25 (1982) 3538. [3] M. Eibschutz, H.J. Guggenheim, Solid State Commun. 6 (1968) 737. [4] D.K. Sardar, W.A. Sibley, R. Aicala, J. Lumin. 27 (1982) 401.
433
[5] A. Gektin, I. Krasovitskaya, N. Shiran, J. Lumin. 72–74 (1997) 664. [6] M. Mortier, I. Gesland, M. Rousseau, Solid State Commun. 89 (1994) 369. [7] S. Somiys, S. Hirano, M. Yoshimura, K. Yanagisawa, J. Mater. Sci. 16 (1981) 813. [8] X.-M. Zhang, H.-Q. Su, C.-Y. Zang, F.-S. Wang, C.-S. Shi, Chem. J. Chin. Univ. 20 (1999) 1334. [9] J. Lee, Q.W. Zhang, F. Saito, Chem. Lett. (2001) 700. [10] R.N. Hua, Z.H. Jia, D.M. Xie, C.S. Shi, Mater. Res. Bull. 37 (2002) 1189. [11] H. Li, Z.H. Jia, C.S. Shi, Chem. Lett. 9 (2000) 1106. [12] J. Chastain, Handbook of X-ray Photoelectron Spectroscopy, PerkinElmer, Eden Prairie, MN, USA, 1992.
Mild solvothermal synthesis of KZnF3 and KCdF3 nanocrystals Bin Huanga, Jian-Ming Hongb, Xue-Tai Chena,c,*, Zhi Yua, Xiao-Zeng Youa a
State Key Laboratory of Coordination Chemistry and Coordination Chemistry Institute, Nanjing University, Nanjing 210093, P.R. China b Analytic Center of Nanjing University, Nanjing 210093, P.R. China c State Key Laboratory of Structural Chemistry, Fujian Institute of Structure of Matter, Fuzhou 350002, P.R. China Received 25 June 2004; accepted 21 September 2004 Available online 19 October 2004
Abstract KZnF3 and KCdF3 nanocrystals have been prepared via a mild solvothermal route, and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The reaction between M(CH3CO2)2d 2H2O (M=Zn, Cd) and KFd 2H2O in MeOH was investigated by variation of reaction temperature, reaction time and molar ratio of starting materials, which indicated that KZnF3 and KCdF3 nanocrystals could be controllably synthesized. D 2004 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; Peroskites; Metal fluorides; Solvothermal
1. Introduction In recent years, there have been much attention paid to complex metal fluorides because of their particular physical properties such as ferromagnetic [1], nonmagnetic insulator behavior [2], piezoelectric characteristics [3], and photoluminescence properties [4,5]. Among these important complex metal fluorides, KZnF3 and KCdF3 are particularly attractive because of their luminescent property [4,5] and the applications in radiation detection and laser medium [6]. AMF3 (A=Na, K, M=Zn and Cd) were normally synthesized by tradition high-temperature solid state reaction, which required complicated set-up under an atmosphere of F2 or HF mixed with an inert carrier gas to avoid possible contamination from oxygen [7,8]. A mechanochemical procedure using a planetary mill was also used to prepare micro-particles of these ternary metal fluorides [9]. Recently hydrothermal reaction proceeding at higher temperature and for long reaction time has been developed
* Corresponding author. State Key Laboratory of Coordination Chemistry and Coordination Chemistry Institute, Nanjing University, Hankou, Jiangsu, Nanjing 210093, P.R. China. 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.09.039
[10,11]. The products based on these procedures usually exhibit relatively large and varied grain sizes, and inhomogeneous morphologies. To our knowledge, there is no report of preparation of KZnF3 and KCdF3 nanocrystals. Here we report a mild solvothermal procedure to prepare KMF3 (M=Zn and Cd) nanocrystals using M(CH3CO2)2d 2H2O (M=Zn and Cd) and KFd 2H2O as precursors.
2. Experimental Chemical reagents used were Zn(CH3CO2)2d 2H2O, Cd(CH3CO2)2d 2H2O, and KFd 2H2O with 99% purity. KZnF3 and KCdF3 nanocrystals were prepared by the reaction between Zn(CH3CO2)2d 2H2O or Cd(CH3CO2)2d 2H2O and KFd 2H2O at different molar ratio of M2+: F (1:2, 1:4, 1:8) in autoclave at the set temperature. In a typical synthesis, Zn(CH3CO2)2d 2H2O (1.0 mmol) and KFd 2H2O (8.0 mmol) were each dissolved in 15.0 ml CH3OH, respectively, and then mixed together and stirred vigorously for 10 min at room temperature. The mixture was transferred into 50-ml Teflon-lined autoclave, sealed tight and maintained at the set temperature for different reaction time. After cooling down to room temperature naturally, the products were washed with ethanol for several
B. Huang et al. / Materials Letters 59 (2005) 430–433
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Table 1 The solvothermal synthesis conditions for KZnF3 and KCdF3 Sample no.
Starting materials
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12
Zn(CH3CO2)2d 2H2O Zn(CH3CO2)2d 2H2O Zn(CH3CO2)2d 2H2O Zn(CH3CO2)2d 2H2O Zn(CH3CO2)2d 2H2O Zn(CH3CO2)2d 2H2O Zn(CH3CO2)2d 2H2O Cd(CH3CO2)2d 2H2O Cd(CH3CO2)2d 2H2O Cd(CH3CO2)2d 2H2O Cd(CH3CO2)2d 2H2O Cd(CH3CO2)2d 2H2O
KFd 2H2O KFd 2H2O KFd 2H2O KFd 2H2O KFd 2H2O KFd 2H2O KFd 2H2O KFd 2H2O KFd 2H2O KFd 2H2O KFd 2H2O KFd 2H2O
a/b
Reaction temperature (8C)
Reaction time (h)
Products
1:2 1:4 1:8 1:8 1:8 1:8 1:8 1:2 1:4 1:8 1:8 1:8
150 150 150 150 150 120 80 150 150 150 120 80
6 6 6 2 2/3 6 6 6 6 6 6 6
KZnF3+ZnF2 KZnF3 KZnF3+K2ZnF4(trace) KZnF3 KZnF3 KZnF3 KZnF3 KCdF3+Cd(OH)F KCdF3 KCdF3 KCdF3 KCdF3
times and dried in vacuum. The influences of reaction time, temperature and molar ratio of starting materials on the products were also investigated. The products are denoted as S1–S12. The reaction conditions are summarized in Table 1. The products were characterized by a Shimadzu XD-3A X-ray powder diffractometor using Cu Ka (k=1.5418 2) radiation. The morphology and size of as-prepared samples were investigated by transmission electron microscopy (TEM, JEM-200CX) with an accelerating voltage of 120 KV. X-ray photoelectron spectroscopy (XPS) experiment was conducted with a VG ESCALB MK-II spectrometer using a Mg Ka (1253.6 eV) source.
Zn(CH3CO2)2d 6H2O/KFd 2H2O ratio at 150 8C for 6 h is shown in Fig. 1a. All the diffraction peaks can be indexed to that of JCPDS card No. 81-2097 with a calculated cell parameter a=4.068(3) 2. No peaks of any impurities can be found, indicating that pure KZnF3 has been prepared. According to the Scherrer equation, the size of KZnF3 particles was estimated to be ca. 25 nm. Similarly, KCdF3 nanocrystals (S9) were prepared via a similar route using Cd(CH 3 CO 2 ) 2 d 6H 2 O instead of Zn(CH3CO2)2d 6H2O under otherwise identical reaction conditions. The XRD patterns shown in Fig. 1b can be indexed to that of JCPDS card No. 22-802 with a calculated
3. Results and discussion In this current study, we adopted a solvothermal reaction employing M(CH3CO2)2d 2H2O and KF as starting materials in MeOH, from which nanocrystalline KZnF3 and KCdF3 were prepared. The powder X-ray diffraction (XRD) patterns of the product (S2) obtained with 1:4 of
Fig. 1. XRD patterns of (a) KZnF3 (S2) and (b) KCdF3 (S9).
Fig. 2. TEM images of (a) KZnF3 (S2) and (b) KCdF3 (S9).
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B. Huang et al. / Materials Letters 59 (2005) 430–433
cell parameter a=6.112(2), b=8.658(6), c=6.119(2) 2, indicating that the product is pure KCdF3. The Scherrer equation suggested that they have a size of ca. 80 nm. The TEM images of the as-prepared KZnF3 (S2) and KCdF3 (S9) shown in Fig. 2 indicated that both KZnF3 (S2) and KCdF3 (S9) are nanocubes. They have the average sizes of 25 and 75 nm, respectively, in good agreement with the XRD results. Both of them are not stable under electron beam radiation of TEM, therefore, it was difficult to obtain their selected area electron diffraction. XPS analysis was performed in order to further examine the composition of the as-prepared samples. The XPS spectra of KZnF3 (S2) and KCdF3 (S9) are displayed in Fig. 3. In the spectra of KZnF3 shown in Fig. 3a, there are a small adsorbed carbon C1s peak positioned at 284.5 eV, which was used to calibrate the acquired spectrum. A very weak O1s peak at 531.3 eV can be ascribed to the absorbed water on the surface of nanocrystals. The position of F1s (684.3 eV), K2p (292.2 eV) and Zn2p3/2 (1022.5 eV) agree well with those reported for ZnF2 and KF [12]. The analysis of the areas of these peaks gave atomic ratio K/Zn/F of 1.16:1.00:2.63, in good agreement with the expected atomic
ratio. Similar XPS result was obtained for KCdF3 with the ratio of K/Cd/F is 1.00:1.08:2.25. Further control experiments under different reaction conditions were carried out in order to reveal the factors affecting the reaction product. It was found that reaction temperature, reaction time and molar ratio of starting materials are important controlling factors. When the molar ratio of Zn(CH3CO2)2d 2H2O/KFd 2H2O is 1:2, the product (S1) was a mixture of KZnF3 and ZnF2 under the otherwise identical conditions (150 8C, 6 h). However, for the reaction of Cd(CH 3 CO 2 ) 2 d 2H 2 O/ KFd 2H2O=1:2, the product (S8) is the mixture of KCdF3 and Cd(OH)F. When the molar ratio of M(CH3CO2)2d 2H2O/KFd 2H2O is 1:4, the product is pure KZnF3 (S2) or KCdF3 (S9). Therefore, the molar ratio is an important controlling factor. When the molar ratio is 1:8 and reaction temperature is set at 150 8C, pure KZnF3 (S4, S5) can be obtained after relatively short time, for example, 2 h or even 40 min, however, the product (S3) is a mixture of KZnF3 and a trace of K2ZnF4 with reaction time of 6 h. Pure KZnF3 was obtained when the reaction temperature was lower than 150 8C (S6, S7). However, pure KCdF3 (S10–S12) were obtained from the reaction system using Cd(CH3CO2)2d 2H2O instead of Zn(CH3CO2)2d 2H2O. Therefore, reaction time and reaction temperature should be properly controlled for the preparation of pure KMF3. Shi et al. [11,12] have prepared KZnF3 and KCdF3 using MF2 as one of starting materials via the hydrothermal route, which required a relatively long reaction time (4–5 days). The particle size of the product is relatively large, 40–50 Am. In contrast, in our current study, we employ a simple solvothermal procedure using metal carboxylate and KFd 2H2O as starting materials and MeOH as solvent, from which pure KZnF3 and KCdF3 nanocrystals can be obtained within relatively short reaction time. Therefore, it could be concluded that the choice of starting materials and solvent is crucial in the control of the particle size of the product.
4. Conclusion A mild solvothermal procedure has been developed to give KZnF3 and KCdF3 nanocrystals at relatively low temperature within a short time and almost no oxycontaining impurities in the product. It had good reproducibility and simple maneuverability. We believe that our simple procedure can be extended to prepare nanocrystals of other metal fluorides.
Acknowledgements
Fig. 3. XPS spectra of KZnF3 (S2) and KCdF3 (S9).
This work was supported by Major State Basic Research Development Program (G200077500), Natural Science Grant of Jiangsu Province (BK 2004087), Nature Science
B. Huang et al. / Materials Letters 59 (2005) 430–433
Grant of China (20001004) and Grant of Instruments of Nanjing University.
References [1] A.H. Cooke, D.A. Jones, J.F.A. Silva, M.R. Weils, J. Phys. C.: Solid State Phys. 8 (1975) 4083. [2] R.A. Heaton, C. Lin, Phys. Rev., B 25 (1982) 3538. [3] M. Eibschutz, H.J. Guggenheim, Solid State Commun. 6 (1968) 737. [4] D.K. Sardar, W.A. Sibley, R. Aicala, J. Lumin. 27 (1982) 401.
433
[5] A. Gektin, I. Krasovitskaya, N. Shiran, J. Lumin. 72–74 (1997) 664. [6] M. Mortier, I. Gesland, M. Rousseau, Solid State Commun. 89 (1994) 369. [7] S. Somiys, S. Hirano, M. Yoshimura, K. Yanagisawa, J. Mater. Sci. 16 (1981) 813. [8] X.-M. Zhang, H.-Q. Su, C.-Y. Zang, F.-S. Wang, C.-S. Shi, Chem. J. Chin. Univ. 20 (1999) 1334. [9] J. Lee, Q.W. Zhang, F. Saito, Chem. Lett. (2001) 700. [10] R.N. Hua, Z.H. Jia, D.M. Xie, C.S. Shi, Mater. Res. Bull. 37 (2002) 1189. [11] H. Li, Z.H. Jia, C.S. Shi, Chem. Lett. 9 (2000) 1106. [12] J. Chastain, Handbook of X-ray Photoelectron Spectroscopy, PerkinElmer, Eden Prairie, MN, USA, 1992.