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Structural and dielectric properties of Ba5LnSn3Nb7O30 (Ln=La, Nd) ceramics
Materials Letters 58 (2004) 2654 – 2657 www.elsevier.com/locate/matlet
Structural and dielectric properties of Ba5LnSn3Nb7O30 (Ln=La, Nd) ceramics Liang Fang *, L. Chen, H. Zhang, C.L. Diao, R.Z. Yuan State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, People’s Republic of China Received 10 November 2003; received in revised form 18 March 2004; accepted 19 March 2004 Available online 28 May 2004
Abstract Two novel Ba5LnSn3Nb7O30 (Ln = La, Nd) ceramics were prepared and characterized. Both compounds are paraelectric phases adopting the filled tetragonal tungsten bronze (TB) structure at room temperature. Ba5LaSn3Nb7O30 and Ba5NdSn3Nb7O30 ceramics show high dielectric constant of 171 and 182 together with low dielectric loss 1.2 10-3 and 1.8 10-3 at 1 MHz, respectively. In comparison with dielectric ceramics with TB structure in BaO – Ln2O3 – TiO2 – Ta2O5 (Ln = La, Nd) system, the temperature coefficients of the dielectric constant (se) are significantly reduced. D 2004 Elsevier B.V. All rights reserved. Keywords: X-ray diffraction; Dielectrics; Tungsten bronze; Niobates; Microstructure
1. Introduction Electronic materials with high dielectric constants, high Q-values and good stability of temperature coefficient of resonant frequency have been extensively studied because of their applications in discrete and multilayer capacitors (MLC), microwave telecommunication applications and low loss substrates for microwave integrated circuits [1 – 4]. Dielectric materials for temperature stable and temperature compensating capacitor applications should have reasonable dielectric constants with good thermal stability and low dielectric loss [5]. So far, many dielectric materials have been studied such as BaO – TiO2, BaO – Nd2O3 – TiO2 [4]. However, these materials (e < 100) are insufficient for miniaturization of microelectronic devices. Hence, the development of new materials with dielectric constants in excess of 100 has become very important. Rao et al. [6] and Koshy et al. [7] proposed some promising compounds with tungsten bronze (TB) structure under the formula Ba 3 (RE) 3 Ti5Nb5O30 (RE = La, Y, Sm, Nd) for their high dielectric
* Corresponding author. Tel.: +86-27-87884448; fax: +86-2787879468. E-mail address: [email protected] (L. Fang). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.03.042
constants of above 130. Chen and Yang [8,10] and Fang et al. [9,12] and Zhang et al. [11] also reported the presence of dielectric materials with filled TB structure in the BaO – Ln 2 O 3 – TiO 2 – Ta 2 O 5 and SrO – Ln 2 O 3 – TiO 2 – Ta 2 O 5 (Ln = La, Sm, Nd) system with high dielectric constant above 103 and low dielectric loss in the order of 10-3 –10-4. The problem for ceramics with TB structure in the above systems is the relatively large negative temperature coefficient of dielectric constant [8 –10]. The TB structure consists of a complex array of distorted BO6 octahedra sharing corners in such a way that three different types of interstices (A, B and C) are available for a wide variety of cations occupying in the general formula (A1)2(A2)4(C)4(B1)2(B2)8O30. It has been found that different ionic substitutions at abovementioned sites can play an important role in tailoring their physical properties [8– 10]. In order to explore new dielectric ceramics with high dielectric constant and low temperature coefficient of dielectric constant, we decided to prepare a group of compounds with TB structure in the BaO – Ln2O3 – SnO2 – Nb2O5 (Ln = La, Nd) system, and as there is no report available on the titled compounds, then we have systematically studied the structural and dielectric properties of the titled compounds Ba5LnSn3Nb7O30 (Ln = La, Nd) and reported the preliminary result here.
L. Fang et al. / Materials Letters 58 (2004) 2654–2657
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2. Experimental Polycrystalline samples of Ba5LnSn3Nb7O30 (Ln = La, Nd) were prepared by a high temperature solid-state reaction technique using high purity BaCO3 (99.9%), Ln 2 O 3 (Ln = La, Nd) (99.9%), SnO 2 (99.95%) and Nb2O5 (99.9%) as starting raw powders. The stoichiometric mixtures of raw powders were thoroughly ground in ethanol with ZrO2 balls for 24 h and dried by a slow evaporation technique. The mixed powders were calcined at 1300 jC for 12 h in air. The calcined powders were reground and dried, followed by mixing with 5 wt.% organic binder polyvinyl alcohol (PVA). The powders were pressed into cylindrical pellets of 11-mm diameter and 2 f 4-mm thickness under the pressure of 200 MPa. The pellets were sintered in air at 1350 jC for 4 h to yield dense ceramics. The densities of the compacts were measured by the Archimedes method. The phase identification was performed using a Rigaku D/MAX-RB X-ray diffractiometer (XRD) with CuKa radiation (k = 0.15406 nm). The fracture morphology of the samples were investigated using a Jeol JSM-5610LV scanning electron microscope (SEM). Silver paste was applied to the circular faces, then dried at 600 jC for 20 min and cooled naturally to room temperature. The temperature-dependent dielectric measurements were made using an HP4284A LCR meter equipped with a thermostat from room temperature (20 jC) to 400 jC at 1, 10, 100 kHz and 1 MHz. The temperature coefficient of the dielectric constant (se) were calculated using the data in the temperature range of 20 to 100 jC at 1 MHz.
3. Results and discussion The XRD patterns of samples are shown in Fig. 1. Ba5LaSn3Nb7O30 and Ba5NdSn3Nb7O30 were found to
Fig. 1. XRD patterns of (a) Ba5LaSn3Nb7O30 and (b) Ba5NdSn3Nb7O30.
Fig. 2. SEM micrographs of (a) Ba5LaSn3Nb7O30 and (b) Ba5NdSn3 Nb7O30 ceramics.
exhibit a single-phase tetragonal TB structure in agreement with JCPDS file No.39-255 of Ba5LaTi3Nb7O30. All peaks are indexed, and there is no evidence for any secondary phase(s) present. The unit cell parameters of those ceramics refined by the least squares method are as the following: a = 1.26170(3) nm; c = 0.39894(2) nm for Ba5LaSn3Nb7O30; a n d a = 1 . 2 6 11 8 ( 3 ) n m ; c = 0 . 3 9 7 1 0 ( 2 ) n m f o r Ba5NdSn3Nb7O30. The replacement of La with relatively smaller cation Nd leads to the smaller unit cell parameters of Ba5NdSn3Nb7O30 compared to that of Ba5LaSn3Nb7O30. The Ba5LaSn3Nb7O30 and Ba5NdSn 3Nb7O 30 compounds sintered into dense ceramics without the use of any additive. They showed a bulk density of 5.749 gcm 3 (95.1 %) and 5.842 gcm 3 (95.9%), respectively. The SEM micrographs of the fracture surfaces of Ba5LaSn3 Nb7O30 and Ba5NdSn3Nb7O30 are shown in Fig. 2a and b. Both compounds have a homogeneous microstructure. The grain sizes of Ba5LaSn3Nb7O30 ceramic are in the range 3 –10 Am with the average value of 5.6 Am compared to slightly smaller grain sizes of Ba5NdSn3Nb7O30 ceramic in the range 2– 7 Am with the average value of 3.6 Am. Temperature dependence of the dielectric constant at 1, 10, 100 kHz and 1 MHz frequency for both the compounds is shown in Fig. 3a and b. As temperature increases from 20 to 400 jC, the relative dielectric constant e of Ba5LaSn3Nb7O30 and Ba5NdSn3Nb7O30 ceramics gradually decrease, and no dielectric peak for
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the ferroelectric –paraelectric phase transition from tetragonal 4 mm symmetry to tetragonal 4 mm symmetry is observed, indicating the Curie point is below the room temperature. Obviously, Ba5LaSn3Nb7O30 and Ba5NdSn3 Nb7O30 belong to the paraelectric phases of the tetragonal TB structure at room temperature. The temperature coefficients of the dielectric constant (se) of two ceramics at 1 MHz are 666 ppm jC 1 for Ba5LaSn3Nb7O30 and 1071 ppm jC 1 for Ba5NdSn3Nb7O30, which is much smaller than that of TB compounds in the BaO – Ln2O3 – TiO2 – Ta2O5 (Ln = La, Nd) system such as Ba5LaTi3 Ta 7 O 30 ( 1347 ppm jC 1 ) and Ba 5 NdTi 3 Ta 7 O 30 ( 1750 ppm jC 1) [10]. Fig. 4a and b demonstrates the room temperature dielectric characteristics of Ba 5 LaSn 3 Nb 7 O 30 and Ba5NdSn3Nb7O30 ceramics. Dielectric constant and dielectric loss of both ceramics varied with the frequency significantly. The dielectric constant of Ba5LaSn3Nb7O30 ceramic decreases from 175 to 171 with increasing frequency from 1 kHz to 1 MHz due to the reduction of active polarization mechanism. The dielectric loss sharply lowered from 6 10 3 to 5.1 10 4 with increasing frequency from 1 to 100 kHz, then slowly increases to 1.2 10 3 at 1 MHz. Similarly, the dielectric constant of Ba5NdSn3Nb7O30 ceramic decreases from 190 to 182 with increase of frequency, while the dielectric loss sharply Fig. 4. Frequency dependence of dielectric constant and dielectric loss of (a) Ba5LaSn3Nb7O30 and (b) Ba5NdSn3Nb7O30 ceramics.
lowers from 7.8 10 3 to 5.5 10 4 upon increase of frequency from 1 to 100 kHz, then gradually increases to 1.8 10 3 at 1 MHz.
4. Conclusions
Fig. 3. Temperature dependence of dielectric constant of (a) Ba5LaSn3 Nb7O30 and (b) Ba5NdSn3Nb7O30.
Ba5LnSn3Nb7O30 (Ln = La, Nd) ceramics in the BaO – Ln2O3 – SnO2 –Nb2O5 quaternary system were prepared and characterized. Ba5LaSn3Nb7O30 and Ba5NdSn3Nb7O30 are paraelectric phases adopting filled tetragonal TB structure at room temperature. At 1 MHz Ba 5 LaSn 3 Nb 7 O 30 and Ba5NdSn3Nb7O30 ceramics have high dielectric constants of 171 and 182, low dielectric losses of 1.2 10-3 and 1.8 10-3, and relatively low negative temperature coefficients of the dielectric constant (se) of 666 and 1071 ppm jC 1, respectively. In comparison with dielectric ceramics with TB structure in BaO – Ln2O3 – TiO2 – Ta2O5 (Ln = La, Nd) system, the temperature coefficients of the dielectric constant (se) are significantly reduced. These materials might have potential application in temperaturecompensating capacitors. Considering the microwave application, the key issue of two ceramics was to lower the dielectric loss further and to obtain near zero temperature coefficient.
L. Fang et al. / Materials Letters 58 (2004) 2654–2657
Acknowledgements This work was financially supported by Natural Science Foundation of China under Grant No. 50002007 and the Major Program of MOE of China under Grant No. 0201.
References [1] D. Liu, Y. Li, S.Q. Huang, X. Yao, J. Am. Ceram. Soc. 76 (1993) 2129. [2] T.M. Herbert, Ceramic Dielectrics and Capacitors, Gordon and Breach, New York, 1985, p. 188.
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[3] R. Ubic, I.M. Reaney, W.E. Lee, Int. Mater. Rev. 43 (1998) 205. [4] M.T. Sebastian, J. Mater. Sci., Mater. Electron. 10 (1999) 475. [5] D.F.K. Hennings, B.S. Schreinemacher, J. Eur. Ceram. Soc. 14 (1994) 450. [6] R.P. Rao, S.K. Ghosh, P. Koshy, J. Mater. Sci., Mater. Electron. 12 (2001) 729. [7] P. Koshy, L.P. Kumari, M.T. Sebastian, J. Mater. Sci., Mater. Electron. 9 (1998) 43. [8] X.M. Chen, J.S. Yang, J. Eur. Ceram. Soc. 19 (1999) 139. [9] L. Fang, H. Zhang, J.F. Yang, F.H. Meng, R.Z. Yuan, Mater. Lett. 58 (2004) 1777. [10] X.M. Chen, J.S. Yang, J. Eur. Ceram. Soc. 23 (2003) 1571. [11] H. Zhang, L. Fang, B.L. Wu, Acta Phys. Chim. Sin. 17 (2001) 749. [12] L. Fang, H. Zhang, J.F. Yang, X.K. Hong, F.C. Meng, J. Mater. Sci., Mater. Electron. 15 (2004) 355.
Structural and dielectric properties of Ba5LnSn3Nb7O30 (Ln=La, Nd) ceramics Liang Fang *, L. Chen, H. Zhang, C.L. Diao, R.Z. Yuan State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, People’s Republic of China Received 10 November 2003; received in revised form 18 March 2004; accepted 19 March 2004 Available online 28 May 2004
Abstract Two novel Ba5LnSn3Nb7O30 (Ln = La, Nd) ceramics were prepared and characterized. Both compounds are paraelectric phases adopting the filled tetragonal tungsten bronze (TB) structure at room temperature. Ba5LaSn3Nb7O30 and Ba5NdSn3Nb7O30 ceramics show high dielectric constant of 171 and 182 together with low dielectric loss 1.2 10-3 and 1.8 10-3 at 1 MHz, respectively. In comparison with dielectric ceramics with TB structure in BaO – Ln2O3 – TiO2 – Ta2O5 (Ln = La, Nd) system, the temperature coefficients of the dielectric constant (se) are significantly reduced. D 2004 Elsevier B.V. All rights reserved. Keywords: X-ray diffraction; Dielectrics; Tungsten bronze; Niobates; Microstructure
1. Introduction Electronic materials with high dielectric constants, high Q-values and good stability of temperature coefficient of resonant frequency have been extensively studied because of their applications in discrete and multilayer capacitors (MLC), microwave telecommunication applications and low loss substrates for microwave integrated circuits [1 – 4]. Dielectric materials for temperature stable and temperature compensating capacitor applications should have reasonable dielectric constants with good thermal stability and low dielectric loss [5]. So far, many dielectric materials have been studied such as BaO – TiO2, BaO – Nd2O3 – TiO2 [4]. However, these materials (e < 100) are insufficient for miniaturization of microelectronic devices. Hence, the development of new materials with dielectric constants in excess of 100 has become very important. Rao et al. [6] and Koshy et al. [7] proposed some promising compounds with tungsten bronze (TB) structure under the formula Ba 3 (RE) 3 Ti5Nb5O30 (RE = La, Y, Sm, Nd) for their high dielectric
* Corresponding author. Tel.: +86-27-87884448; fax: +86-2787879468. E-mail address: [email protected] (L. Fang). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.03.042
constants of above 130. Chen and Yang [8,10] and Fang et al. [9,12] and Zhang et al. [11] also reported the presence of dielectric materials with filled TB structure in the BaO – Ln 2 O 3 – TiO 2 – Ta 2 O 5 and SrO – Ln 2 O 3 – TiO 2 – Ta 2 O 5 (Ln = La, Sm, Nd) system with high dielectric constant above 103 and low dielectric loss in the order of 10-3 –10-4. The problem for ceramics with TB structure in the above systems is the relatively large negative temperature coefficient of dielectric constant [8 –10]. The TB structure consists of a complex array of distorted BO6 octahedra sharing corners in such a way that three different types of interstices (A, B and C) are available for a wide variety of cations occupying in the general formula (A1)2(A2)4(C)4(B1)2(B2)8O30. It has been found that different ionic substitutions at abovementioned sites can play an important role in tailoring their physical properties [8– 10]. In order to explore new dielectric ceramics with high dielectric constant and low temperature coefficient of dielectric constant, we decided to prepare a group of compounds with TB structure in the BaO – Ln2O3 – SnO2 – Nb2O5 (Ln = La, Nd) system, and as there is no report available on the titled compounds, then we have systematically studied the structural and dielectric properties of the titled compounds Ba5LnSn3Nb7O30 (Ln = La, Nd) and reported the preliminary result here.
L. Fang et al. / Materials Letters 58 (2004) 2654–2657
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2. Experimental Polycrystalline samples of Ba5LnSn3Nb7O30 (Ln = La, Nd) were prepared by a high temperature solid-state reaction technique using high purity BaCO3 (99.9%), Ln 2 O 3 (Ln = La, Nd) (99.9%), SnO 2 (99.95%) and Nb2O5 (99.9%) as starting raw powders. The stoichiometric mixtures of raw powders were thoroughly ground in ethanol with ZrO2 balls for 24 h and dried by a slow evaporation technique. The mixed powders were calcined at 1300 jC for 12 h in air. The calcined powders were reground and dried, followed by mixing with 5 wt.% organic binder polyvinyl alcohol (PVA). The powders were pressed into cylindrical pellets of 11-mm diameter and 2 f 4-mm thickness under the pressure of 200 MPa. The pellets were sintered in air at 1350 jC for 4 h to yield dense ceramics. The densities of the compacts were measured by the Archimedes method. The phase identification was performed using a Rigaku D/MAX-RB X-ray diffractiometer (XRD) with CuKa radiation (k = 0.15406 nm). The fracture morphology of the samples were investigated using a Jeol JSM-5610LV scanning electron microscope (SEM). Silver paste was applied to the circular faces, then dried at 600 jC for 20 min and cooled naturally to room temperature. The temperature-dependent dielectric measurements were made using an HP4284A LCR meter equipped with a thermostat from room temperature (20 jC) to 400 jC at 1, 10, 100 kHz and 1 MHz. The temperature coefficient of the dielectric constant (se) were calculated using the data in the temperature range of 20 to 100 jC at 1 MHz.
3. Results and discussion The XRD patterns of samples are shown in Fig. 1. Ba5LaSn3Nb7O30 and Ba5NdSn3Nb7O30 were found to
Fig. 1. XRD patterns of (a) Ba5LaSn3Nb7O30 and (b) Ba5NdSn3Nb7O30.
Fig. 2. SEM micrographs of (a) Ba5LaSn3Nb7O30 and (b) Ba5NdSn3 Nb7O30 ceramics.
exhibit a single-phase tetragonal TB structure in agreement with JCPDS file No.39-255 of Ba5LaTi3Nb7O30. All peaks are indexed, and there is no evidence for any secondary phase(s) present. The unit cell parameters of those ceramics refined by the least squares method are as the following: a = 1.26170(3) nm; c = 0.39894(2) nm for Ba5LaSn3Nb7O30; a n d a = 1 . 2 6 11 8 ( 3 ) n m ; c = 0 . 3 9 7 1 0 ( 2 ) n m f o r Ba5NdSn3Nb7O30. The replacement of La with relatively smaller cation Nd leads to the smaller unit cell parameters of Ba5NdSn3Nb7O30 compared to that of Ba5LaSn3Nb7O30. The Ba5LaSn3Nb7O30 and Ba5NdSn 3Nb7O 30 compounds sintered into dense ceramics without the use of any additive. They showed a bulk density of 5.749 gcm 3 (95.1 %) and 5.842 gcm 3 (95.9%), respectively. The SEM micrographs of the fracture surfaces of Ba5LaSn3 Nb7O30 and Ba5NdSn3Nb7O30 are shown in Fig. 2a and b. Both compounds have a homogeneous microstructure. The grain sizes of Ba5LaSn3Nb7O30 ceramic are in the range 3 –10 Am with the average value of 5.6 Am compared to slightly smaller grain sizes of Ba5NdSn3Nb7O30 ceramic in the range 2– 7 Am with the average value of 3.6 Am. Temperature dependence of the dielectric constant at 1, 10, 100 kHz and 1 MHz frequency for both the compounds is shown in Fig. 3a and b. As temperature increases from 20 to 400 jC, the relative dielectric constant e of Ba5LaSn3Nb7O30 and Ba5NdSn3Nb7O30 ceramics gradually decrease, and no dielectric peak for
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the ferroelectric –paraelectric phase transition from tetragonal 4 mm symmetry to tetragonal 4 mm symmetry is observed, indicating the Curie point is below the room temperature. Obviously, Ba5LaSn3Nb7O30 and Ba5NdSn3 Nb7O30 belong to the paraelectric phases of the tetragonal TB structure at room temperature. The temperature coefficients of the dielectric constant (se) of two ceramics at 1 MHz are 666 ppm jC 1 for Ba5LaSn3Nb7O30 and 1071 ppm jC 1 for Ba5NdSn3Nb7O30, which is much smaller than that of TB compounds in the BaO – Ln2O3 – TiO2 – Ta2O5 (Ln = La, Nd) system such as Ba5LaTi3 Ta 7 O 30 ( 1347 ppm jC 1 ) and Ba 5 NdTi 3 Ta 7 O 30 ( 1750 ppm jC 1) [10]. Fig. 4a and b demonstrates the room temperature dielectric characteristics of Ba 5 LaSn 3 Nb 7 O 30 and Ba5NdSn3Nb7O30 ceramics. Dielectric constant and dielectric loss of both ceramics varied with the frequency significantly. The dielectric constant of Ba5LaSn3Nb7O30 ceramic decreases from 175 to 171 with increasing frequency from 1 kHz to 1 MHz due to the reduction of active polarization mechanism. The dielectric loss sharply lowered from 6 10 3 to 5.1 10 4 with increasing frequency from 1 to 100 kHz, then slowly increases to 1.2 10 3 at 1 MHz. Similarly, the dielectric constant of Ba5NdSn3Nb7O30 ceramic decreases from 190 to 182 with increase of frequency, while the dielectric loss sharply Fig. 4. Frequency dependence of dielectric constant and dielectric loss of (a) Ba5LaSn3Nb7O30 and (b) Ba5NdSn3Nb7O30 ceramics.
lowers from 7.8 10 3 to 5.5 10 4 upon increase of frequency from 1 to 100 kHz, then gradually increases to 1.8 10 3 at 1 MHz.
4. Conclusions
Fig. 3. Temperature dependence of dielectric constant of (a) Ba5LaSn3 Nb7O30 and (b) Ba5NdSn3Nb7O30.
Ba5LnSn3Nb7O30 (Ln = La, Nd) ceramics in the BaO – Ln2O3 – SnO2 –Nb2O5 quaternary system were prepared and characterized. Ba5LaSn3Nb7O30 and Ba5NdSn3Nb7O30 are paraelectric phases adopting filled tetragonal TB structure at room temperature. At 1 MHz Ba 5 LaSn 3 Nb 7 O 30 and Ba5NdSn3Nb7O30 ceramics have high dielectric constants of 171 and 182, low dielectric losses of 1.2 10-3 and 1.8 10-3, and relatively low negative temperature coefficients of the dielectric constant (se) of 666 and 1071 ppm jC 1, respectively. In comparison with dielectric ceramics with TB structure in BaO – Ln2O3 – TiO2 – Ta2O5 (Ln = La, Nd) system, the temperature coefficients of the dielectric constant (se) are significantly reduced. These materials might have potential application in temperaturecompensating capacitors. Considering the microwave application, the key issue of two ceramics was to lower the dielectric loss further and to obtain near zero temperature coefficient.
L. Fang et al. / Materials Letters 58 (2004) 2654–2657
Acknowledgements This work was financially supported by Natural Science Foundation of China under Grant No. 50002007 and the Major Program of MOE of China under Grant No. 0201.
References [1] D. Liu, Y. Li, S.Q. Huang, X. Yao, J. Am. Ceram. Soc. 76 (1993) 2129. [2] T.M. Herbert, Ceramic Dielectrics and Capacitors, Gordon and Breach, New York, 1985, p. 188.
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[3] R. Ubic, I.M. Reaney, W.E. Lee, Int. Mater. Rev. 43 (1998) 205. [4] M.T. Sebastian, J. Mater. Sci., Mater. Electron. 10 (1999) 475. [5] D.F.K. Hennings, B.S. Schreinemacher, J. Eur. Ceram. Soc. 14 (1994) 450. [6] R.P. Rao, S.K. Ghosh, P. Koshy, J. Mater. Sci., Mater. Electron. 12 (2001) 729. [7] P. Koshy, L.P. Kumari, M.T. Sebastian, J. Mater. Sci., Mater. Electron. 9 (1998) 43. [8] X.M. Chen, J.S. Yang, J. Eur. Ceram. Soc. 19 (1999) 139. [9] L. Fang, H. Zhang, J.F. Yang, F.H. Meng, R.Z. Yuan, Mater. Lett. 58 (2004) 1777. [10] X.M. Chen, J.S. Yang, J. Eur. Ceram. Soc. 23 (2003) 1571. [11] H. Zhang, L. Fang, B.L. Wu, Acta Phys. Chim. Sin. 17 (2001) 749. [12] L. Fang, H. Zhang, J.F. Yang, X.K. Hong, F.C. Meng, J. Mater. Sci., Mater. Electron. 15 (2004) 355.