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The low temperature synthesis of Eu2+and Dy3+ activated Sr3Al2O6 nanophosphors by microwave method
ARTICLE IN PRESS Physica B 404 (2009) 4286–4289
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
The low temperature synthesis of Eu2+and Dy3+ activated Sr3Al2O6 nanophosphors by microwave method Ping Zhang a,, Li Lingxia a, Tian Yuming b a b
School of Electronic and Information Engineering, Tianjin University, Tianjin 300072, China School of Materials Science and Engineering, Taiyuan University of Science and Technology, Shanxi 030024, China
a r t i c l e in f o
a b s t r a c t
Article history: Received 17 March 2009 Received in revised form 27 July 2009 Accepted 3 August 2009
The Eu2+and Dy3+ activated Sr3Al2O6 (S3A2O-ED) nanophosphors were synthesized by a new microwave method. The S3A2O-ED sample calcined in microwave oven at around 650 1C for 20 min possesses a cubic Sr3Al2O6 single phase. The sample showed small size (80–100 nm) and spherical shape. The excitation and emission spectra indicated that excitation broad band chiefly sited in visible range and the nanophosphors emitted strong light at 611 nm under around 473 nm excitation. Comparing with conventional method, the microwave synthesis of S3A2O-ED greatly decreased the calcining temperature and time. However, the brightness of S3A2O-ED nanophosphors was reduced. The change of luminescent intensity in S3A2O-ED nanophosphors could be attributed to the effect of surface energy. & 2009 Elsevier B.V. All rights reserved.
PACS: 78.55.m Keywords: Sr3Al2O6 Microwave method Nanophosphors The low temperature synthesis
1. Introduction Long lasting phosphor is a kind of energy-storing materials. The materials can absorb the visible light, store the energy, and then release the energy as visible light which leads to long lasting afterglow in the darkness. Recently, strontium aluminate phosphors activated by europium have attracted much attention since they show excellent properties, such as high quantum efficiency [1], long persistence of phosphorescence and good stability [2,3] when compared with sulfide phosphorescent phosphors. These properties result in a wide application of the materials in many fields [4]. Strontium aluminates activated by Eu2+ with long afterglow, such as SrAl2O4 (Eu2+, Dy3+) and Sr4Al14O25 (Eu2+, Dy3+), have been studied extensively [5,6]. It is generally agreed that the phosphorescence of Eu2+ in most hosts originates from transitions between the 8S7/2 (4f7) ground state and the crystal field components of the 4f65d1 excited state configuration [7,8]. Although, the 4f electrons of Eu2+ are not sensitive to the changes of the crystals field strength due to the shielding function of outer shell, the 5d electrons are split easily by these changes. The peak positions in the emission spectra depend strongly on the nature of the Eu2+ surroundings, and Eu2+ ions can emit different visible lights in the
Corresponding author. Tel.: +86 22 27402838.
E-mail address: [email protected] (P. Zhang). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.08.068
various crystal fields [9]. By alternating the crystal structures of the matrix in which Eu2+ ions reside, visible light emitting with different wavelength was obtained. Examples that had been reported are emission at 510 nm for SrAl2O4 (Eu2+, Dy3+) and 490 nm for Sr4Al14O25 (Eu2+, Dy3+), respectively [10,11]. However, long lasting behavior of Eu2+ in Sr3Al2O6 has been little reported. Conventional synthesis of strontium aluminate phosphors demand controlled heating at high temperatures and long processing time, which often results in inhomogeneous products with low surface area [12]. Many investigations suggest that heating treatment is an important factor for controlling size and crystalline structure of the products. Therefore, it is necessary to search for new methods to prepare nanophosphors and control their size and morphology. Compared with conventional methods, microwave synthesis has the advantages of low calcining temperature, short time and saving energy. In this paper, S3A2O-ED nanophosphors were prepared by using the microwave method in a reducing atmosphere. The phase compositions, luminescent properties of sample prepared by the microwave method are investigated as a comparison with those of conventional method.
2. Experimental The powders (reagents) of SrCO3(AR), AlOH)3 (AR), Eu2O3 (99.99%) and Dy2O3 (99.99%) were used as raw materials for the
ARTICLE IN PRESS P. Zhang et al. / Physica B 404 (2009) 4286–4289
preparation of red phosphor samples of S3A2O-ED. The powders were weighted while the molar stoichiometry was 3(Sr0.97Eu0.02Dy0.01) Al2O3, and ground in an agate mortar for one hour for homogeneous mixing. The mixed powders were calcined in microwave oven at 650 1C for 20 min (heating rates at 30 1C/min) in a reducing atmosphere of active carbon to crystallize and form the luminescent centers. To facilitate the reaction of the materials, which do not absorb microwaves, dual crucible system was used. The power of microwave oven can adjust in linear between 0 and 2 kW, so the temperature and heating rate is under control. For comparison with conventional furnace heating method, the mixture was also treated in a mildly reducing atmosphere in a high temperature furnace at 1200 1C for 2 h (heating rate at 5 1C/min). Then, the red long afterglow S3A2O-ED phosphors were obtained. The crystalline structure of the phosphors was analyzed by X-ray diffractometer (Rigaka D/Max 2500 v/pc). The morphology and size of phosphors were observed using Scanning Electron Microscopy (SEM) (HITACHI X-650). The microstructure and surface morphology of the nanophosphors were observed by transmission electron microscopy (TEM, JEM-1200 EX II, JEOL). The excitation and emission spectra were recorded on the powder samples using a fluorescence spectrophotometer (SPEX F111 AI). The decay curve of afterglow was measured using the brightness meter (ST-86LA) after the samples were sufficiently excited for about 10 min. Prior to the afterglow measurements, samples were exposed to irradiation from a conventional tricolor fluorescent lamp. All the measurements were performed at room temperature.
4287
Fig. 2. TEM micrograph of S3A2O-ED prepared by microwave method (650 1C, 20 min).
3. Results and discussion Fig. 1 shows the XRD pattern of S3A2O-ED calcined at 650 1C for 20 min in microwave oven and at 1200 1C for 2 h in high temperature furnace. The XRD patterns of the pure cubic structure for Sr3Al2O6 essentially contrasted JCPDS date file no. 24-1187. No extra peaks of any other phase were detected. These results confirmed that the microwave method provided a satisfactory condition for the formation of single phase Sr3Al2O6 within a quite short time of 20 min at low calcining temperature of 600 1C. The average crystallite size of the phosphors synthesized through microwave method was calculated to be 92 nm according to the Debye–Scherrer equation [13]: D ¼ K l=ðb cos yÞ Fig. 3. SEM micrograph of S3A2O-ED prepared by conventional method (1200 1C for 2 h).
(a)
(b)
10
20
30
40
50
60
2θ / ° Fig. 1. X-ray diffraction patterns of S3A2O-ED synthesized by (a) microwave method (650 1C, 20 min) and (b) conventional method (1200 1C, 2 h).
where b is the width of the pure diffraction profile in radians, l ˚ 2y the diffraction angle the wavelength of the X-rays (1.54056 A), of the (4 4 0) peak (2y31.94), and D the average diameter of the crystallite. However, the average crystallite size of the sample synthesized through conventional method at 1200 1C for 2 h was calculated to be approximately 435 nm. These results were in good agreement with the particle sizes obtained by TEM (see Fig. 2) and SEM (Fig. 3). Fig. 2 shows the TEM micrograph of S3A2O-ED synthesized by the microwave method at 650 1C for 20 min. Fig. 3 shows the SEM micrograph of S3A2O-ED synthesized by the conventional method at 1200 1C for 2 h. In the case of the microwave method, the particles of the sample appear to be spherical shape and average particle size of 80–100 nm (see Fig. 2). This can be due to the reason that the S3A2O-ED phosphors were synthesized in a quite shorter time
ARTICLE IN PRESS P. Zhang et al. / Physica B 404 (2009) 4286–4289
with uniform heating in microwave oven. On the other hand, when the conventional method was used, the average particle size of S3A2O-ED phosphors was larger (400–500 nm) and had a needle-like shape (see Fig. 3). Fig. 4 presents the excitation and emission spectra of S3A2OED phosphor prepared by microwave method. It is observed that the excitation spectra of phosphor nanophosphors show a broad band from 400 to 550 nm under an emission of 611 nm. The nanophosphors can be excited by the visible light. The sample prepared by microwave method exhibited a broad band peak at 611 nm in emission spectra under the excitation of 473 nm, as a result of the 5d-4f transition of Eu2+ions in Sr3Al2O6. The reason is that the phosphorescence of Eu2+ in most of host is believed to be caused by the 4f-5d transition. Although, the 4f electrons of Eu2+ are not sensitive to the changes of the crystals field strength due to the shielding function of outer shell, the 5d electrons are split easily by these changes. The peak positions in the emission spectra depend strongly on the nature of the Eu2+ surroundings, and therefore, Eu2+ ions can emit different visible lights in the various crystal fields [9]. Consequently, the mixed states of 4f and 5d will be bigger split by the crystal field of Sr3Al2O6 than of SrAl2O4 [5], which lead to the red shift of emission peak of Sr3Al2O6. The schematic energy level of Eu2+ ion in host lattice as a function of the crystal field strength is shown in Fig. 5, which may be given a reasonable explanation to the changes of the emitting peaks with the different hosts. Fig. 6 shows the decay curve of S3A2O-ED prepared by microwave method and conventional method. The decay characteristics of S3A2O-ED phosphor indicated that the decay
Intensity (a.u.)
30
611nm
473nm
1.0 (b) Brightness (mcd/m2)
4288
0.8 0.6 0.4
(a)
0.2 0.0 0
100 200 300 400 500 600 700 800 900 Time (s)
Fig. 6. The decay curve of S3A2O-ED prepared by (a) microwave method and (b) conventional method.
process contained the rapid-decaying process and the slowdecaying one. The afterglow of nanophosphors, which allowed the time to be recognized by the brightness meter (Z1 mcd/m2), lasted for over 600 s after the excited source was cut off. The luminescent intensity of nanophosphors is much less than that of the phosphors prepared by conventional method after exciting for 10 min. This can be attributed to the surface energy. With regard to the mechanism of the long afterglow, it is the hole trappedtransported-detrapped process that results in the properties of long afterglow of S3A2O-ED phosphor, in which Dy ions play a role of a trapped energy level that can attract vacancies during the excitation and thus decrease the initial luminescence and prolong the luminescent duration. From the point of view of energy level, it may be suggested that the surface energy level is much deeper than the trap level of Dy and then can attract more vacancies than Dy, so that the initial luminescent intensity decreases.
20 (a)
4. Conclusions
(b)
10
0 400
450
500 550 600 Wavelength /nm
650
700
The S3A2O-ED nanophosphors were prepared by the microwave method at 650 1C for 20 min. The nanometer phosphors had pure cubic Sr3Al2O6 phase, with a spherical shape and 80–100 nm diameter. The excitation and emission spectra indicated that excitation broad band chiefly sited in visible range and the nanophosphors emitted strong light at 611 nm under around 473 nm excitation. Compared with the conventional method, the microwave synthesis of S3A2O-ED greatly decreased the calcining temperature, shored time and saved energy.
Fig. 4. (a) Excitation and (b) emission spectra of S3A2O-ED, prepared by microwave method.
Acknowledgements This work was financially supported by 863 program (2007AA03Z423) and China Postdoctoral Science Foundation (no. 20070410195).
4f65d 4f7 (6PJ)
References Blue UV
Green 7 8
4f ( S7/2)
Red
Fig. 5. Schematic energy level diagram of Eu2+ ions vs. the crystal field in the different hosts.
[1] [2] [3] [4]
B. Smets, J. Rutten, G. Hoeks, J. Electrochem. Soc. 136 (7) (1989) 2119. F.C. Palilla, et al., J. Electrochem. Soc. 115 (6) (1968) 642. W.Y. Jia, H.B. Yuan, W.M. Yen, J. Lumin. 76 (1998) 424. T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, J. Electrochem. Soc. 143 (1996) 2670. [5] M.Q. Wang, D. Wang, G.L. Lu, Mater. Sci. Eng. B 57 (1998) 18. [6] J. Holsa, H. Jungner, M. Lastusuari, J. Nittykoski, J. Alloys Compd. 323–324 (2001) 326.
ARTICLE IN PRESS P. Zhang et al. / Physica B 404 (2009) 4286–4289
[7] G. Blasse, W.L. Wanmaker, J.W. Tervrugt, A. Bril, Philips Res. Rep. 23 (1968) 189. [8] K. Yamazaki, H. Nakabayashi, Y. Kotera, A. Ueno, J. Electrochem. Soc. 133 (1986) 657. [9] G. Blasse, A. Bril, Philips Res. Rep. 23 (1968) 201. [10] Y. Lin, Z. Tang, J. Zhang, Z. Zhang, Mater. Lett. 51 (2001) 14.
4289
[11] Y. Lin, Z. Zhang, Z. Tang, J. Zhang, Z. Zhang, X. Lu, Mater. Chem. Phys. 70 (2001) 156. [12] R.P. Rao, J. Electrochem. Soc. 143 (1996) 189. [13] H. Klug, L. Alexander, X-ray Diffraction Procedures, Wiley, New York, 1962 p. 125.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
The low temperature synthesis of Eu2+and Dy3+ activated Sr3Al2O6 nanophosphors by microwave method Ping Zhang a,, Li Lingxia a, Tian Yuming b a b
School of Electronic and Information Engineering, Tianjin University, Tianjin 300072, China School of Materials Science and Engineering, Taiyuan University of Science and Technology, Shanxi 030024, China
a r t i c l e in f o
a b s t r a c t
Article history: Received 17 March 2009 Received in revised form 27 July 2009 Accepted 3 August 2009
The Eu2+and Dy3+ activated Sr3Al2O6 (S3A2O-ED) nanophosphors were synthesized by a new microwave method. The S3A2O-ED sample calcined in microwave oven at around 650 1C for 20 min possesses a cubic Sr3Al2O6 single phase. The sample showed small size (80–100 nm) and spherical shape. The excitation and emission spectra indicated that excitation broad band chiefly sited in visible range and the nanophosphors emitted strong light at 611 nm under around 473 nm excitation. Comparing with conventional method, the microwave synthesis of S3A2O-ED greatly decreased the calcining temperature and time. However, the brightness of S3A2O-ED nanophosphors was reduced. The change of luminescent intensity in S3A2O-ED nanophosphors could be attributed to the effect of surface energy. & 2009 Elsevier B.V. All rights reserved.
PACS: 78.55.m Keywords: Sr3Al2O6 Microwave method Nanophosphors The low temperature synthesis
1. Introduction Long lasting phosphor is a kind of energy-storing materials. The materials can absorb the visible light, store the energy, and then release the energy as visible light which leads to long lasting afterglow in the darkness. Recently, strontium aluminate phosphors activated by europium have attracted much attention since they show excellent properties, such as high quantum efficiency [1], long persistence of phosphorescence and good stability [2,3] when compared with sulfide phosphorescent phosphors. These properties result in a wide application of the materials in many fields [4]. Strontium aluminates activated by Eu2+ with long afterglow, such as SrAl2O4 (Eu2+, Dy3+) and Sr4Al14O25 (Eu2+, Dy3+), have been studied extensively [5,6]. It is generally agreed that the phosphorescence of Eu2+ in most hosts originates from transitions between the 8S7/2 (4f7) ground state and the crystal field components of the 4f65d1 excited state configuration [7,8]. Although, the 4f electrons of Eu2+ are not sensitive to the changes of the crystals field strength due to the shielding function of outer shell, the 5d electrons are split easily by these changes. The peak positions in the emission spectra depend strongly on the nature of the Eu2+ surroundings, and Eu2+ ions can emit different visible lights in the
Corresponding author. Tel.: +86 22 27402838.
E-mail address: [email protected] (P. Zhang). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.08.068
various crystal fields [9]. By alternating the crystal structures of the matrix in which Eu2+ ions reside, visible light emitting with different wavelength was obtained. Examples that had been reported are emission at 510 nm for SrAl2O4 (Eu2+, Dy3+) and 490 nm for Sr4Al14O25 (Eu2+, Dy3+), respectively [10,11]. However, long lasting behavior of Eu2+ in Sr3Al2O6 has been little reported. Conventional synthesis of strontium aluminate phosphors demand controlled heating at high temperatures and long processing time, which often results in inhomogeneous products with low surface area [12]. Many investigations suggest that heating treatment is an important factor for controlling size and crystalline structure of the products. Therefore, it is necessary to search for new methods to prepare nanophosphors and control their size and morphology. Compared with conventional methods, microwave synthesis has the advantages of low calcining temperature, short time and saving energy. In this paper, S3A2O-ED nanophosphors were prepared by using the microwave method in a reducing atmosphere. The phase compositions, luminescent properties of sample prepared by the microwave method are investigated as a comparison with those of conventional method.
2. Experimental The powders (reagents) of SrCO3(AR), AlOH)3 (AR), Eu2O3 (99.99%) and Dy2O3 (99.99%) were used as raw materials for the
ARTICLE IN PRESS P. Zhang et al. / Physica B 404 (2009) 4286–4289
preparation of red phosphor samples of S3A2O-ED. The powders were weighted while the molar stoichiometry was 3(Sr0.97Eu0.02Dy0.01) Al2O3, and ground in an agate mortar for one hour for homogeneous mixing. The mixed powders were calcined in microwave oven at 650 1C for 20 min (heating rates at 30 1C/min) in a reducing atmosphere of active carbon to crystallize and form the luminescent centers. To facilitate the reaction of the materials, which do not absorb microwaves, dual crucible system was used. The power of microwave oven can adjust in linear between 0 and 2 kW, so the temperature and heating rate is under control. For comparison with conventional furnace heating method, the mixture was also treated in a mildly reducing atmosphere in a high temperature furnace at 1200 1C for 2 h (heating rate at 5 1C/min). Then, the red long afterglow S3A2O-ED phosphors were obtained. The crystalline structure of the phosphors was analyzed by X-ray diffractometer (Rigaka D/Max 2500 v/pc). The morphology and size of phosphors were observed using Scanning Electron Microscopy (SEM) (HITACHI X-650). The microstructure and surface morphology of the nanophosphors were observed by transmission electron microscopy (TEM, JEM-1200 EX II, JEOL). The excitation and emission spectra were recorded on the powder samples using a fluorescence spectrophotometer (SPEX F111 AI). The decay curve of afterglow was measured using the brightness meter (ST-86LA) after the samples were sufficiently excited for about 10 min. Prior to the afterglow measurements, samples were exposed to irradiation from a conventional tricolor fluorescent lamp. All the measurements were performed at room temperature.
4287
Fig. 2. TEM micrograph of S3A2O-ED prepared by microwave method (650 1C, 20 min).
3. Results and discussion Fig. 1 shows the XRD pattern of S3A2O-ED calcined at 650 1C for 20 min in microwave oven and at 1200 1C for 2 h in high temperature furnace. The XRD patterns of the pure cubic structure for Sr3Al2O6 essentially contrasted JCPDS date file no. 24-1187. No extra peaks of any other phase were detected. These results confirmed that the microwave method provided a satisfactory condition for the formation of single phase Sr3Al2O6 within a quite short time of 20 min at low calcining temperature of 600 1C. The average crystallite size of the phosphors synthesized through microwave method was calculated to be 92 nm according to the Debye–Scherrer equation [13]: D ¼ K l=ðb cos yÞ Fig. 3. SEM micrograph of S3A2O-ED prepared by conventional method (1200 1C for 2 h).
(a)
(b)
10
20
30
40
50
60
2θ / ° Fig. 1. X-ray diffraction patterns of S3A2O-ED synthesized by (a) microwave method (650 1C, 20 min) and (b) conventional method (1200 1C, 2 h).
where b is the width of the pure diffraction profile in radians, l ˚ 2y the diffraction angle the wavelength of the X-rays (1.54056 A), of the (4 4 0) peak (2y31.94), and D the average diameter of the crystallite. However, the average crystallite size of the sample synthesized through conventional method at 1200 1C for 2 h was calculated to be approximately 435 nm. These results were in good agreement with the particle sizes obtained by TEM (see Fig. 2) and SEM (Fig. 3). Fig. 2 shows the TEM micrograph of S3A2O-ED synthesized by the microwave method at 650 1C for 20 min. Fig. 3 shows the SEM micrograph of S3A2O-ED synthesized by the conventional method at 1200 1C for 2 h. In the case of the microwave method, the particles of the sample appear to be spherical shape and average particle size of 80–100 nm (see Fig. 2). This can be due to the reason that the S3A2O-ED phosphors were synthesized in a quite shorter time
ARTICLE IN PRESS P. Zhang et al. / Physica B 404 (2009) 4286–4289
with uniform heating in microwave oven. On the other hand, when the conventional method was used, the average particle size of S3A2O-ED phosphors was larger (400–500 nm) and had a needle-like shape (see Fig. 3). Fig. 4 presents the excitation and emission spectra of S3A2OED phosphor prepared by microwave method. It is observed that the excitation spectra of phosphor nanophosphors show a broad band from 400 to 550 nm under an emission of 611 nm. The nanophosphors can be excited by the visible light. The sample prepared by microwave method exhibited a broad band peak at 611 nm in emission spectra under the excitation of 473 nm, as a result of the 5d-4f transition of Eu2+ions in Sr3Al2O6. The reason is that the phosphorescence of Eu2+ in most of host is believed to be caused by the 4f-5d transition. Although, the 4f electrons of Eu2+ are not sensitive to the changes of the crystals field strength due to the shielding function of outer shell, the 5d electrons are split easily by these changes. The peak positions in the emission spectra depend strongly on the nature of the Eu2+ surroundings, and therefore, Eu2+ ions can emit different visible lights in the various crystal fields [9]. Consequently, the mixed states of 4f and 5d will be bigger split by the crystal field of Sr3Al2O6 than of SrAl2O4 [5], which lead to the red shift of emission peak of Sr3Al2O6. The schematic energy level of Eu2+ ion in host lattice as a function of the crystal field strength is shown in Fig. 5, which may be given a reasonable explanation to the changes of the emitting peaks with the different hosts. Fig. 6 shows the decay curve of S3A2O-ED prepared by microwave method and conventional method. The decay characteristics of S3A2O-ED phosphor indicated that the decay
Intensity (a.u.)
30
611nm
473nm
1.0 (b) Brightness (mcd/m2)
4288
0.8 0.6 0.4
(a)
0.2 0.0 0
100 200 300 400 500 600 700 800 900 Time (s)
Fig. 6. The decay curve of S3A2O-ED prepared by (a) microwave method and (b) conventional method.
process contained the rapid-decaying process and the slowdecaying one. The afterglow of nanophosphors, which allowed the time to be recognized by the brightness meter (Z1 mcd/m2), lasted for over 600 s after the excited source was cut off. The luminescent intensity of nanophosphors is much less than that of the phosphors prepared by conventional method after exciting for 10 min. This can be attributed to the surface energy. With regard to the mechanism of the long afterglow, it is the hole trappedtransported-detrapped process that results in the properties of long afterglow of S3A2O-ED phosphor, in which Dy ions play a role of a trapped energy level that can attract vacancies during the excitation and thus decrease the initial luminescence and prolong the luminescent duration. From the point of view of energy level, it may be suggested that the surface energy level is much deeper than the trap level of Dy and then can attract more vacancies than Dy, so that the initial luminescent intensity decreases.
20 (a)
4. Conclusions
(b)
10
0 400
450
500 550 600 Wavelength /nm
650
700
The S3A2O-ED nanophosphors were prepared by the microwave method at 650 1C for 20 min. The nanometer phosphors had pure cubic Sr3Al2O6 phase, with a spherical shape and 80–100 nm diameter. The excitation and emission spectra indicated that excitation broad band chiefly sited in visible range and the nanophosphors emitted strong light at 611 nm under around 473 nm excitation. Compared with the conventional method, the microwave synthesis of S3A2O-ED greatly decreased the calcining temperature, shored time and saved energy.
Fig. 4. (a) Excitation and (b) emission spectra of S3A2O-ED, prepared by microwave method.
Acknowledgements This work was financially supported by 863 program (2007AA03Z423) and China Postdoctoral Science Foundation (no. 20070410195).
4f65d 4f7 (6PJ)
References Blue UV
Green 7 8
4f ( S7/2)
Red
Fig. 5. Schematic energy level diagram of Eu2+ ions vs. the crystal field in the different hosts.
[1] [2] [3] [4]
B. Smets, J. Rutten, G. Hoeks, J. Electrochem. Soc. 136 (7) (1989) 2119. F.C. Palilla, et al., J. Electrochem. Soc. 115 (6) (1968) 642. W.Y. Jia, H.B. Yuan, W.M. Yen, J. Lumin. 76 (1998) 424. T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, J. Electrochem. Soc. 143 (1996) 2670. [5] M.Q. Wang, D. Wang, G.L. Lu, Mater. Sci. Eng. B 57 (1998) 18. [6] J. Holsa, H. Jungner, M. Lastusuari, J. Nittykoski, J. Alloys Compd. 323–324 (2001) 326.
ARTICLE IN PRESS P. Zhang et al. / Physica B 404 (2009) 4286–4289
[7] G. Blasse, W.L. Wanmaker, J.W. Tervrugt, A. Bril, Philips Res. Rep. 23 (1968) 189. [8] K. Yamazaki, H. Nakabayashi, Y. Kotera, A. Ueno, J. Electrochem. Soc. 133 (1986) 657. [9] G. Blasse, A. Bril, Philips Res. Rep. 23 (1968) 201. [10] Y. Lin, Z. Tang, J. Zhang, Z. Zhang, Mater. Lett. 51 (2001) 14.
4289
[11] Y. Lin, Z. Zhang, Z. Tang, J. Zhang, Z. Zhang, X. Lu, Mater. Chem. Phys. 70 (2001) 156. [12] R.P. Rao, J. Electrochem. Soc. 143 (1996) 189. [13] H. Klug, L. Alexander, X-ray Diffraction Procedures, Wiley, New York, 1962 p. 125.