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Hydrothermal synthesis of SrZrO3−α (M=Al, Ga, In, x≤0.20) series oxides
Solid State Ionics 108 (1998) 37–41
Hydrothermal synthesis of SrZrO 32 a (M 5 Al, Ga, In, x # 0.20) series oxides a, b a Wenjun Zheng *, Wenqin Pang , Guangyao Meng a
Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China b Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun, Jilin 130026, China
Abstract On the basis of hydrothermal synthesis of perovskite-type SrZrO 3 , SrZr 12x Mx O 32 a (M 5 Al, Ga, In, x # 0.20) series oxides were prepared. The phase, particle morphology and dopant content of the product were characterized by the techniques such as XRD, SEM, 27 Al-MASNMR, ICP and chemical analyses. The results indicated that the phase of product corresponded to orthorhombic perovskite-type SrZrO 3 , and the product had a particle size region which was 5–15 mm. Elemental analyses showed that the products have the M /(Zr 1 M) (M 5 Al, Ga, In) ratio close to that of raw materials. 1998 Elsevier Science B.V. All rights reserved. Keywords: Hydrothermal synthesis; Perovskite-type oxides; Strontium zirconate
1. Introduction SrZrO 3 -based perovskite oxides are known to exhibit proton conductivity at elevated temperatures. Their ability to conduct protons makes them potential candidates for electrolytes in some novel electrochemical devices such as solid oxide fuel cells and hydrogen or steam sensors [1]. Traditionally, SrZrO 3 -based oxides have been prepared through solid state reaction at high temperature (1350– 16008C) [1], but such reaction often leads to compositional and structural inhomogeneities in the ceramics production. Powder of oxides with high purity, narrow particle size distribution, phase homogeneity, controlled particle morphology and a high degree of crystallinity
can be produced through hydrothermal process. Therefore, we concentrated on the hydrothermal synthesis of solid electrolytes that have perovskitetype structure. Recently, Kutty et al. [2] reported that ultrafine powder of ZrO 2 was converted to SrZrO 3 powder by the hydrothermal method. The lowest reaction temperature was 3608C, which is the upper limit temperature at which Teflon liner can be used. Komarneni et al. [3] prepared SrZrO 3 powder under conventional and microwave hydrothermal conditions. In the experiments, 10 mol dm 23 KOH solution was used as the medium. In this paper, SrZrO 3 based oxides were prepared under middle hydrothermal conditions.
2. Experimental *Corresponding author. Fax: 1086-0551-3631760; e-mail: [email protected] 0167-2738 / 98 / $19.00 1998 Elsevier Science B.V. All rights reserved. PII S0167-2738( 98 )00016-2
In the hydrothermal synthesis of SrZr 12x M x O 32 a
38
W. Zheng et al. / Solid State Ionics 108 (1998) 37 – 41
(M 5 Al, Ga, In), M(OH) 3 was used as the M 31 source. M(OH) 3 (M 5 Ga, In) were prepared by the addition of NH 3 ? H 2 O to M 31 solutions that were produced through dissolving high purity metal in hot concentrated HCl. Hydrothermal synthesis was carried out in a stainless steel autoclave with a Teflon liner (100 cm 3 , capacity) under autogenous pressure. The typical synthesis procedure is described as follows: a Sr(OH) 2 solution was made in distilled water. The solution was mixed with ZrOCl 2 ? 8H 2 O, M(OH) 3 and KOH to obtain a slurry. The slurry had the molar composition of H 2 O:SrO:(ZrO 2 1 MO 1.5 ) 5 277.5:1.1:1, MO 1.5 :(ZrO 2 1 MO 1.5 ) 5 0– 0.20. The slurry was put into an autoclave, and heated at 1508C for one day. After cooling, the product was filtered, washed with dilute acetic acid and distilled water, and dried at ambient temperature. X-ray diffraction (XRD) pattern was acquired from a Rigaku D/ MAX-IIIA powder diffractometer ˚ source. The with nickel-filtered CuKa (1.5418 A) lattice parameter was refined by least-squares analysis of the XRD data. The IR spectrum was measured with a Nicolt 5DX-FT infrared spectrophotometer. The particle size and morphology were measured with a Hitachi X-156 scanning electron microscope. The 27 Al-MASNMR spectrum was recorded on a Varian XL-200 magic-angle spinning magnetic resonance spectrometer. The spinning rate is ca. 4 kHz. The chemical shifts were relative to external standard of [Al(OH) 6 ] 31 . Elemental analyses were carried out by the wet chemical analyses and spectrophotometry. Some results of elemental analyses were verified by ICP method. The ICP measurements were carried out with a Plasma-Spec(I) ICP-AES.
Fig. 1. X-ray diffraction pattern of SrZrO 3 .
3. Results and discussion The XRD pattern of undoped SrZrO 3 processed at 1508C in KOH / Zr 5 4.5 medium for one day is shown in Fig. 1. The XRD pattern shown only the characteristic lines of orthorhombic perovskite SrZrO 3 and had a higher degree of crystallinity. SEM measurements showed that the powder of products was not agglomerated, and had particle size within 5 to 15 mm (Fig. 2a). The particles are more homogeneous and the crystal shape is similar. Thus
Fig. 2. SEM morphology of SrZr 12x M x O 32 a (M5Al, Ga, In) oxides.
the product has a higher phase purity. Elemental analyses indicated that the product had a Sr / Zr ratio around 0.99 to 1.01. In the preparation of SrZrO 3 , temperature and alkalinity are important factors. When ZrOCl 2 ? 8H 2 O and Sr(OH) 2 solution were mixed in KOH medium,
W. Zheng et al. / Solid State Ionics 108 (1998) 37 – 41
amorphous ZrO 2 ? xH 2 O was formed. The mixture was heated at the temperature range of 130–1508C for several hours, the formation of SrZrO 3 was observed by XRD data. If the ratio of KOH / Zr and temperature were lower than 2.5 and 1508C, respectively, stabilized ZrO 2 as an additional phase was detected in the product. If three factors including temperature, KOH / Zr ratio and reaction time went up the crystallinity of SrZrO 3 was increased. The increased is significant when the temperature is lower than 1508C, and the KOH / Zr ratio is less than four and the duration is within 24 h. Therefore, SrZr 12x M x O 32 a series oxides were prepared at 1508C in KOH / Zr 5 4.5 medium for one day. The XRD pattern of SrZr 12x M x O 32 a (M 5 Al, Ga, In, x # 0.20) shown only the characteristic lines of orthorhombic SrZrO 3 , which is in agreement with undoped SrZrO 3 . The particle size and morphology were not changed markedly with the kind and the content of dopant (Fig. 2b, c, d). Elemental analyses indicated that the products have the M / Zr ratio close to that of raw materials. The results are listed in Table 1. In SrZr 12x M x O 32 a oxides, the dependence of the lattice parameter on M 31 ion content is shown in Fig. 3 and Fig. 4. It can been seen that the lattice parameter increased with the increase of In 31 content in SrZr 12x In x O 32 a , and reduced with the increase of dopant content in SrZr 12x M x O 32 a (M 5 Al, Ga). The change of lattice parameter is correlatable to the radii of M 31 ions with six-coordination (Table 2). In the hydrothermal synthesis of BaZr 12x Al x O 32 a , we have shown that Al-substitution in Zr sites can lead
39
Fig. 3. Relationship between lattice parameter (a, b) and dopant content in SrZr 12x M x O 32 a (M5Al, Ga, In) oxides.
Fig. 4. Relationship between lattice paremeter (c) and dopant content in SrZr 12x M x O 32 a (M5Al, Ga, In) oxides.
Table 1 The results of elemental analyses of SrZr 12x M x O 32 a (M5Al, Ga, In)
Table 2 Ionic radii [4] of B-site ions on SrZr 12x M x O 32 a (M5Al, Ga, In) oxides
Dopant
x
Sr / Zr / M ratio
B-site ion
Al 31
Ga 31
In 31
Zr(IV)
Al
0.05 0.10 0.15 0.20 0.05 0.10 0.15 0.20 0.05 0.10 0.15 0.20
0.99–1.1:0.92–0.95:0.04–0.06 0.99–1.1:0.89–0.91:0.08–0.11 0.99–1.1:0.83–0.87:0.12–0.15 0.99–1.1:0.78–0.81:0.18–0.20 0.99–1.1:0.92–0.96:0.03–0.06 0.99–1.1:0.88–0.91:0.09–0.11 0.99–1.1:0.82–0.87:0.11–0.15 0.99–1.1:0.79–0.80:0.17–0.19 0.99–1.1:0.91–0.95:0.03–0.06 0.99–1.1:0.88–0.90:0.08–0.10 0.99–1.1:0.81–0.86:0.11–0.14 0.99–1.1:0.77–0.81:0.16–0.18
˚ Radius / A
0.535
0.620
0.800
0.720
Ga
In
to the partial Al 31 ions becoming four-coordinated when x $ 0.10 [5]. Therefore, 27 Al-MASNMR spectrum was measured to detect coordinate state of Al 31 ions in SrZr 12x Al x O 32 a . As shown in Fig. 5, only a single coordinate state was observed in the 27 AlMASNMR spectra. Their chemical shifts are ca. 8 ppm, which appears six-coordinate Al 31 ions. It is suggested that the difference of partial Al 31 ions
40
W. Zheng et al. / Solid State Ionics 108 (1998) 37 – 41 7608C
Orthorhombic(Pbnm) ↔ Tetragonal(Bmmb) 9008C
↔ Tetragonal(I4 / mcm)
(1)
However, it is difficult that the partial structural change is observed only by XRD data. The dependence of the IR spectrum on M 31 content is shown in Fig. 6. The IR spectrum shows
Fig. 5. 27 Al-MASNMR spectra of SrZr 12x Al X O 32 a oxides (a) x50.05, (b) x50.15, (c) x50.20.
coordinate state in SrZr 12x Al x O 32 a and BaZr 12x Al x O 32 a , is due to the difference of MZrO 3 (M 5 Ba, Sr) structure. BaZrO 3 and SrZrO 3 have ideal cubic and orthorhombic perovskite-type structures, respectively. Therefore, it is possible that SrZr 12x Al x O 32 a becomes partially to better symmetry structure with the aid of produced oxygen vacancies when substituted, because the structural change is reported from orthorhombic to cubic when x $ 0.20 in Mg-substitution CaTi 12x Mg x O 32 a [6]. According to Ahtee et al. [7,8], SrZrO 3 has the following sequence of phase transitions:
Fig. 6. IR spectra of SrZr 12x M x O 32 a (M5Al, Ga, In) oxides. (a) SrZrO 3 , (b) SrZr 0.85 Al 0.15 O 32 a , (c) SrZr 0.90 Al 0.10 O 32 a , (d) SrZr 0.85 Ga 0.15 O 32 a , (e) SrZr 0.90 Ga 0.10 O 32 a , (f) SrZr 0.85 In 0.15 O 32 a , (g) SrZr 0.90 In 0.10 O 32 a .
W. Zheng et al. / Solid State Ionics 108 (1998) 37 – 41
41
Table 3 Zr–O bond lengths [7,8] in the different structure of SrZrO 3 Structure ˚ Zr–O bond length / A
Orthorhombic Pbnm (258C)
Tetragonal Bmmb (7608C)
I4 / mcm (9008C)
2.084 2.094 2.095
2.086 2.086 2.065
2.077 2.077 2.069
Note: From top to bottom, these data represent a, b and c axial Zr–O bond length, respectively.
absorption band at 500–600 cm 21 that is attributed to ZrO 6 octahedra stretching vibration [9]. The absorption band was not shifted markedly with the kind and the content of dopant. The phenomenon is due to the facts that (1) the Zr–O bond length increased around the substitutive site, and (2) the crystal lattice tends to become partially better symmetry structure on the substituting. From Table 3, it can be seen that the Zr–O bond length decreased when the symmetry becomes better.
Acknowledgements This work was partially supported by National Natural Science Foundation of China.
References [1] T. Yajima, H. Suzuki, T. Yogo, H. Iwahara, Solid State Ionics 51 (1992) 101.
[2] T.R.N. Kutty, R. Vivekanandan, S. Philip, J. Mater. Sci. 25 (1990) 3649. [3] S. Komarneni, Q. Li, K.M. Stefansson, R. Roy, J. Mater. Res. 8 (1993) 3176. [4] R.D. Shannon, Acta Crystallogr. A23 (1976) 751. [5] W.J. Zheng, C. Liu, Y. Yue, W.Q. Pang, Mater. Lett. 30 (1997) 93. [6] H. Iwahara, Solid State Ionics 52 (1992) 99. [7] A. Ahtee, M. Ahtee, A.M. Glazer, A.W. Hewat, Acta Crystallogr. B32 (1976) 3242. [8] M. Ahtee, A.M. Glazer, A.M. Hewat, Acta Crystallogr. B34 (1978) 752. [9] R.N. Nyguist, R.Q. Kagel, in: Infrared Spectra of Inorganic Compounds, Academic Press, New York and London, No. 115, 1971.
Hydrothermal synthesis of SrZrO 32 a (M 5 Al, Ga, In, x # 0.20) series oxides a, b a Wenjun Zheng *, Wenqin Pang , Guangyao Meng a
Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China b Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun, Jilin 130026, China
Abstract On the basis of hydrothermal synthesis of perovskite-type SrZrO 3 , SrZr 12x Mx O 32 a (M 5 Al, Ga, In, x # 0.20) series oxides were prepared. The phase, particle morphology and dopant content of the product were characterized by the techniques such as XRD, SEM, 27 Al-MASNMR, ICP and chemical analyses. The results indicated that the phase of product corresponded to orthorhombic perovskite-type SrZrO 3 , and the product had a particle size region which was 5–15 mm. Elemental analyses showed that the products have the M /(Zr 1 M) (M 5 Al, Ga, In) ratio close to that of raw materials. 1998 Elsevier Science B.V. All rights reserved. Keywords: Hydrothermal synthesis; Perovskite-type oxides; Strontium zirconate
1. Introduction SrZrO 3 -based perovskite oxides are known to exhibit proton conductivity at elevated temperatures. Their ability to conduct protons makes them potential candidates for electrolytes in some novel electrochemical devices such as solid oxide fuel cells and hydrogen or steam sensors [1]. Traditionally, SrZrO 3 -based oxides have been prepared through solid state reaction at high temperature (1350– 16008C) [1], but such reaction often leads to compositional and structural inhomogeneities in the ceramics production. Powder of oxides with high purity, narrow particle size distribution, phase homogeneity, controlled particle morphology and a high degree of crystallinity
can be produced through hydrothermal process. Therefore, we concentrated on the hydrothermal synthesis of solid electrolytes that have perovskitetype structure. Recently, Kutty et al. [2] reported that ultrafine powder of ZrO 2 was converted to SrZrO 3 powder by the hydrothermal method. The lowest reaction temperature was 3608C, which is the upper limit temperature at which Teflon liner can be used. Komarneni et al. [3] prepared SrZrO 3 powder under conventional and microwave hydrothermal conditions. In the experiments, 10 mol dm 23 KOH solution was used as the medium. In this paper, SrZrO 3 based oxides were prepared under middle hydrothermal conditions.
2. Experimental *Corresponding author. Fax: 1086-0551-3631760; e-mail: [email protected] 0167-2738 / 98 / $19.00 1998 Elsevier Science B.V. All rights reserved. PII S0167-2738( 98 )00016-2
In the hydrothermal synthesis of SrZr 12x M x O 32 a
38
W. Zheng et al. / Solid State Ionics 108 (1998) 37 – 41
(M 5 Al, Ga, In), M(OH) 3 was used as the M 31 source. M(OH) 3 (M 5 Ga, In) were prepared by the addition of NH 3 ? H 2 O to M 31 solutions that were produced through dissolving high purity metal in hot concentrated HCl. Hydrothermal synthesis was carried out in a stainless steel autoclave with a Teflon liner (100 cm 3 , capacity) under autogenous pressure. The typical synthesis procedure is described as follows: a Sr(OH) 2 solution was made in distilled water. The solution was mixed with ZrOCl 2 ? 8H 2 O, M(OH) 3 and KOH to obtain a slurry. The slurry had the molar composition of H 2 O:SrO:(ZrO 2 1 MO 1.5 ) 5 277.5:1.1:1, MO 1.5 :(ZrO 2 1 MO 1.5 ) 5 0– 0.20. The slurry was put into an autoclave, and heated at 1508C for one day. After cooling, the product was filtered, washed with dilute acetic acid and distilled water, and dried at ambient temperature. X-ray diffraction (XRD) pattern was acquired from a Rigaku D/ MAX-IIIA powder diffractometer ˚ source. The with nickel-filtered CuKa (1.5418 A) lattice parameter was refined by least-squares analysis of the XRD data. The IR spectrum was measured with a Nicolt 5DX-FT infrared spectrophotometer. The particle size and morphology were measured with a Hitachi X-156 scanning electron microscope. The 27 Al-MASNMR spectrum was recorded on a Varian XL-200 magic-angle spinning magnetic resonance spectrometer. The spinning rate is ca. 4 kHz. The chemical shifts were relative to external standard of [Al(OH) 6 ] 31 . Elemental analyses were carried out by the wet chemical analyses and spectrophotometry. Some results of elemental analyses were verified by ICP method. The ICP measurements were carried out with a Plasma-Spec(I) ICP-AES.
Fig. 1. X-ray diffraction pattern of SrZrO 3 .
3. Results and discussion The XRD pattern of undoped SrZrO 3 processed at 1508C in KOH / Zr 5 4.5 medium for one day is shown in Fig. 1. The XRD pattern shown only the characteristic lines of orthorhombic perovskite SrZrO 3 and had a higher degree of crystallinity. SEM measurements showed that the powder of products was not agglomerated, and had particle size within 5 to 15 mm (Fig. 2a). The particles are more homogeneous and the crystal shape is similar. Thus
Fig. 2. SEM morphology of SrZr 12x M x O 32 a (M5Al, Ga, In) oxides.
the product has a higher phase purity. Elemental analyses indicated that the product had a Sr / Zr ratio around 0.99 to 1.01. In the preparation of SrZrO 3 , temperature and alkalinity are important factors. When ZrOCl 2 ? 8H 2 O and Sr(OH) 2 solution were mixed in KOH medium,
W. Zheng et al. / Solid State Ionics 108 (1998) 37 – 41
amorphous ZrO 2 ? xH 2 O was formed. The mixture was heated at the temperature range of 130–1508C for several hours, the formation of SrZrO 3 was observed by XRD data. If the ratio of KOH / Zr and temperature were lower than 2.5 and 1508C, respectively, stabilized ZrO 2 as an additional phase was detected in the product. If three factors including temperature, KOH / Zr ratio and reaction time went up the crystallinity of SrZrO 3 was increased. The increased is significant when the temperature is lower than 1508C, and the KOH / Zr ratio is less than four and the duration is within 24 h. Therefore, SrZr 12x M x O 32 a series oxides were prepared at 1508C in KOH / Zr 5 4.5 medium for one day. The XRD pattern of SrZr 12x M x O 32 a (M 5 Al, Ga, In, x # 0.20) shown only the characteristic lines of orthorhombic SrZrO 3 , which is in agreement with undoped SrZrO 3 . The particle size and morphology were not changed markedly with the kind and the content of dopant (Fig. 2b, c, d). Elemental analyses indicated that the products have the M / Zr ratio close to that of raw materials. The results are listed in Table 1. In SrZr 12x M x O 32 a oxides, the dependence of the lattice parameter on M 31 ion content is shown in Fig. 3 and Fig. 4. It can been seen that the lattice parameter increased with the increase of In 31 content in SrZr 12x In x O 32 a , and reduced with the increase of dopant content in SrZr 12x M x O 32 a (M 5 Al, Ga). The change of lattice parameter is correlatable to the radii of M 31 ions with six-coordination (Table 2). In the hydrothermal synthesis of BaZr 12x Al x O 32 a , we have shown that Al-substitution in Zr sites can lead
39
Fig. 3. Relationship between lattice parameter (a, b) and dopant content in SrZr 12x M x O 32 a (M5Al, Ga, In) oxides.
Fig. 4. Relationship between lattice paremeter (c) and dopant content in SrZr 12x M x O 32 a (M5Al, Ga, In) oxides.
Table 1 The results of elemental analyses of SrZr 12x M x O 32 a (M5Al, Ga, In)
Table 2 Ionic radii [4] of B-site ions on SrZr 12x M x O 32 a (M5Al, Ga, In) oxides
Dopant
x
Sr / Zr / M ratio
B-site ion
Al 31
Ga 31
In 31
Zr(IV)
Al
0.05 0.10 0.15 0.20 0.05 0.10 0.15 0.20 0.05 0.10 0.15 0.20
0.99–1.1:0.92–0.95:0.04–0.06 0.99–1.1:0.89–0.91:0.08–0.11 0.99–1.1:0.83–0.87:0.12–0.15 0.99–1.1:0.78–0.81:0.18–0.20 0.99–1.1:0.92–0.96:0.03–0.06 0.99–1.1:0.88–0.91:0.09–0.11 0.99–1.1:0.82–0.87:0.11–0.15 0.99–1.1:0.79–0.80:0.17–0.19 0.99–1.1:0.91–0.95:0.03–0.06 0.99–1.1:0.88–0.90:0.08–0.10 0.99–1.1:0.81–0.86:0.11–0.14 0.99–1.1:0.77–0.81:0.16–0.18
˚ Radius / A
0.535
0.620
0.800
0.720
Ga
In
to the partial Al 31 ions becoming four-coordinated when x $ 0.10 [5]. Therefore, 27 Al-MASNMR spectrum was measured to detect coordinate state of Al 31 ions in SrZr 12x Al x O 32 a . As shown in Fig. 5, only a single coordinate state was observed in the 27 AlMASNMR spectra. Their chemical shifts are ca. 8 ppm, which appears six-coordinate Al 31 ions. It is suggested that the difference of partial Al 31 ions
40
W. Zheng et al. / Solid State Ionics 108 (1998) 37 – 41 7608C
Orthorhombic(Pbnm) ↔ Tetragonal(Bmmb) 9008C
↔ Tetragonal(I4 / mcm)
(1)
However, it is difficult that the partial structural change is observed only by XRD data. The dependence of the IR spectrum on M 31 content is shown in Fig. 6. The IR spectrum shows
Fig. 5. 27 Al-MASNMR spectra of SrZr 12x Al X O 32 a oxides (a) x50.05, (b) x50.15, (c) x50.20.
coordinate state in SrZr 12x Al x O 32 a and BaZr 12x Al x O 32 a , is due to the difference of MZrO 3 (M 5 Ba, Sr) structure. BaZrO 3 and SrZrO 3 have ideal cubic and orthorhombic perovskite-type structures, respectively. Therefore, it is possible that SrZr 12x Al x O 32 a becomes partially to better symmetry structure with the aid of produced oxygen vacancies when substituted, because the structural change is reported from orthorhombic to cubic when x $ 0.20 in Mg-substitution CaTi 12x Mg x O 32 a [6]. According to Ahtee et al. [7,8], SrZrO 3 has the following sequence of phase transitions:
Fig. 6. IR spectra of SrZr 12x M x O 32 a (M5Al, Ga, In) oxides. (a) SrZrO 3 , (b) SrZr 0.85 Al 0.15 O 32 a , (c) SrZr 0.90 Al 0.10 O 32 a , (d) SrZr 0.85 Ga 0.15 O 32 a , (e) SrZr 0.90 Ga 0.10 O 32 a , (f) SrZr 0.85 In 0.15 O 32 a , (g) SrZr 0.90 In 0.10 O 32 a .
W. Zheng et al. / Solid State Ionics 108 (1998) 37 – 41
41
Table 3 Zr–O bond lengths [7,8] in the different structure of SrZrO 3 Structure ˚ Zr–O bond length / A
Orthorhombic Pbnm (258C)
Tetragonal Bmmb (7608C)
I4 / mcm (9008C)
2.084 2.094 2.095
2.086 2.086 2.065
2.077 2.077 2.069
Note: From top to bottom, these data represent a, b and c axial Zr–O bond length, respectively.
absorption band at 500–600 cm 21 that is attributed to ZrO 6 octahedra stretching vibration [9]. The absorption band was not shifted markedly with the kind and the content of dopant. The phenomenon is due to the facts that (1) the Zr–O bond length increased around the substitutive site, and (2) the crystal lattice tends to become partially better symmetry structure on the substituting. From Table 3, it can be seen that the Zr–O bond length decreased when the symmetry becomes better.
Acknowledgements This work was partially supported by National Natural Science Foundation of China.
References [1] T. Yajima, H. Suzuki, T. Yogo, H. Iwahara, Solid State Ionics 51 (1992) 101.
[2] T.R.N. Kutty, R. Vivekanandan, S. Philip, J. Mater. Sci. 25 (1990) 3649. [3] S. Komarneni, Q. Li, K.M. Stefansson, R. Roy, J. Mater. Res. 8 (1993) 3176. [4] R.D. Shannon, Acta Crystallogr. A23 (1976) 751. [5] W.J. Zheng, C. Liu, Y. Yue, W.Q. Pang, Mater. Lett. 30 (1997) 93. [6] H. Iwahara, Solid State Ionics 52 (1992) 99. [7] A. Ahtee, M. Ahtee, A.M. Glazer, A.W. Hewat, Acta Crystallogr. B32 (1976) 3242. [8] M. Ahtee, A.M. Glazer, A.M. Hewat, Acta Crystallogr. B34 (1978) 752. [9] R.N. Nyguist, R.Q. Kagel, in: Infrared Spectra of Inorganic Compounds, Academic Press, New York and London, No. 115, 1971.