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Anomaly of temperature dependence of dielectric permittivity for perovskite-type oxides, La0.53Na0.41−xLixTiO3
Solid State Ionics 154 – 155 (2002) 795 – 799 www.elsevier.com/locate/ssi
Anomaly of temperature dependence of dielectric permittivity for perovskite-type oxides, La0.53Na0.41xLixTiO3 Tetsuhiro Katsumata *, Yoshiyuki Inaguma Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo, 171-8588, Japan Accepted 10 March 2002
Abstract We investigated the dielectric properties for La0.53Na0.41 xLixTiO3 (x < 0.26). The temperature dependence of the dielectric constant for x = 0.00 shows the quantum paraelectric behavior. For other compounds, however, the anomalies caused by the jumps of Li ions between the off-center potions in the A-site and between A-sites were observed on the temperature dependence of the dielectric properties. D 2002 Elsevier Science B.V. All rights reserved. PACS: 77.22.d Keywords: Perovskite-type oxide; A-site deficient; Li ion; Dielectric property; Ion conduction
1. Introduction A-site deficient perovskite-type lithium lanthanam titanate has been known as an intense Li ion conductor [1 –7], in which the Li ion, the La ion and the vacancy are distributed in the A-sites. Li ion, which is smaller than La ion, can easily move between these sites and the migration of Li ions through the conduction path formed by the vacancy and the Li ion is observed as the Li ion conductivity. Ion conductivity, however, is influenced by various factors, especially site percolation. According to the percolation theory, conductivity quadratically decreases with the number of objects * Corresponding author. Tel.: +81-3-3986-0221; fax: +81-35992-1029. E-mail address: [email protected] (T. Katsumata).
formed in the conduction path and diminishes when its number is less than the percolation limit. In the case of (La, Li) TiO3, the object formed in the conduction path is Li ion and the vacancy, thus ion conductivity is predicted to depend on the sum of the concentration of Li ion and the vacancy. This prediction was supported by experimental results. Inaguma and Itoh [8] investigated the variation of the Li ion conductivity with x for xLa0.55Li0.35TiO3 –(1 x)La0.5Na0.5TiO3. In their report, Li ion conductivity decreased with the sum of the concentration of the Li ion and the vacancy, 0.45x, and was too small to be measured when 0.45x is below the percolation limit of the primitive cell, which was 0.3117. Similar behavior was observed for (1 x)La055 Li 0 . 3 5 TiO 3 – xKMO 3 (M = Nb and Ta), (1 x) La0.55Li0.35TiO3 – xSrTiO3 [4] and (1 x)LiTaO3 – xSrTiO3 [5]. Therefore, Li ion conduction of the perovskite-type Li ion conductor follows a prediction of site percolation and dc conductivity decrease for a
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 4 9 4 - 0
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perovskite-type Li ion conductor in which the sum of the concentration of the Li ion and the vacancy is less than the percolation limit. In contrast, a perovskite-type lithium lanthanum titanate in which the sum of the concentration of the Li ion and the vacancy is less than percolation limit can be considered as a dielectric material. Furthermore, a dielectric property of such compound is very interesting because there are many ions in the A-site which can move more freely than that of the usual dielectric perovskite-type oxide. In this study, we synthesized such a compound, i.e. La 0.53 Na 0.41 x Li x TiO 3 (x < 0.26), and investigated those dielectric properties.
2. Experimental Samples were synthesized by conventional solid state reaction technique. Starting materials were La2O3 (4 N), Li2CO3 (3 N), Na2CO3 (3 N) and TiO2 (3 N). The La content of the La2O3 was determined by titration analysis using EDTA. An excess amount of 20% Na2CO3 was used to account for loss due to volatilization. The mixture was calcined at 1073 K for 8 h and 1323 K for 24 h. The calcined powder was ground and pressed into pellets with a diameter of 7 mm and thickness of 1.5 mm. These pellets were wrapped by a Pt foil and sintered at 1723 K for 3 h (x = 0.00 and x = 0.05) or 1523 K for 3 h (x = 0.10, x = 0.15 and x = 0.23). Phase identification was carried out by powder X-ray diffraction method (Rigaku RINT 2000). The metal content of the synthesized samples was analyzed by inductively couple plasma (ICP) spectroscopy. Dielectric constant and dielectric loss were measured by Agilent Technology 4284A precision LCR meter in a temperature range 8 K – room temperature at the frequency, f , of 1, 10, 100, 200 and 500 kHz.
3. Results and discussion Fig. 1 shows the X-ray diffraction patterns for La0.53Na0.41 xLixTiO3. As shown in this figure, for all compounds the single phase was obtained. The reflections could be indexed on the basis of a pseudocubic cell. The weak reflection at about 2h = 38j was attributed to a superlattice with a double period of a
Fig. 1. X-ray diffraction patterns for La0.53Na0.41 xLixTiO3.
primitive cell, perovskite unit cell. The superstructure is considered to be due to the tilting of the BO6 octahedra. Table 1 shows the nominal composition, analytical composition, analytical Li content, x, sum of the concentration of Li ion and the vacancy, and relative density for La0.53Na0.41 xLixTiO3. For all compounds, the analytical composition is different from the nominal one. The amount of Li and Na in the analytical composition is smaller and larger than the nominal amount, respectively. The reason is that the excess amount was added only for Na2CO3, when starting materials were mixed. The sum of the concentration of Li ion and the vacancy concentration, however, does not exceed the percolation limit of the primitive cell, 0.3117, for all compounds. Therefore, these compounds can be considered as dielectric mate-
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Table 1 List of nominal composition, analytical composition, analytical Li content, x, sum of concentration of Li and vacancy, and relative density for La0.53Na0.41 xLixTiO3 Nominal composition
Analytical composition
x
Sum of concentration of Li and vacancy
Relative density (%)
La0.53Na0.41TiO3 La0.53Na0.36Li0.05TiO3 La0.53Na0.31Li0.10TiO3 La0.53Na0.26Li0.15TiO3 La0.53Na0.18Li0.23TiO3
La0.51Na0.42TiO2.98 La0.51Na0.41Li0.04TiO2.99 La0.52Na0.36Li0.08TiO3.00 La0.53Na0.30Li0.12TiO3.01 La0.53Na0.25Li0.17TiO3.01
0.00 0.04 0.08 0.12 0.17
0.06 0.08 0.12 0.17 0.22
83.5 88.0 92.4 94.6 95.0
rials. In fact, the dc conductivity for x = 0.17 at room temperature measured with the complex impedance method was below the measurement limit in this study, 10 8 S/cm. Fig. 2 shows the temperature dependence of the dielectric constant, er, at frequency f = 10 kHz for La0.53Na0.41 xLixTiO3. For x = 0.00 (La0.53Na0.41 TiO3), the dielectric constant monotonously increases as temperature decreases in the temperature range 50 K < T < 300 K and is almost independent when temperature is less than 50 K. Such behavior was often observed for a quantum paraelectric compound, e.g.
Fig. 2. Temperature dependence of dielectric constant, er, at frequency f = 10 kHz for La0.53Na0.41 xLixTiO3.
SrTiO3 [9]. In the case of SrTiO3, the temperature dependence of the dielectric constant was fitted with the Barrett-type formula, er ¼
A T1 =2cothðT1 =2T Þ T0
ð1Þ
For x = 0.00, the good fit is obtained at A = 1.06 105 K, T1 = 204 K and T0 = 772 K. The quantum paraelectricity was also reported for La0.5Na0.5TiO3 and the fitting parameter, A, T1 and T0, was 1.2 105, 1.8 102 and 7.7 102 K, respectively [10]. These values are almost equal to that of La0.53Na0.41TiO3, hence the vacancies in the A-site do not significantly influence the quantum paraelectricity of this solid solution system. Anomalies appear by substitution with Li ion for other compounds. While for x = 0.04 the dielectric constant increases with decreasing temperature as in the case of x = 0.00 in the temperature range 50 K < T < 300 K, this value is not constant and decreases with temperature when it drops below 50 K. For x = 0.08, the broad and asymmetric peak is observed in the vicinity of 70 K. For x = 0.12, the peak is observed more clearly and for x = 0.17, two apparent peaks are observed in the vicinity of 50 and 290 K. The dielectric constant increases during substitution with Li ion. In particular, the dielectric constant at 290 K of x = 0.17 is about four times larger than that of x = 0.00. Anomalies are also observed in the temperature dependence of the dielectric loss. Fig. 3 shows the temperature dependence of the dielectric loss, tand, at frequency f = 10 kHz for La0.53Na0.41 xLixTiO3. For x = 0.00, a peak was observed in the vicinity of 160 K. While the mechanism of this peak has not been made clear, it is speculated to be due to the vacancy introduced into the A-site because such peak was
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Fig. 3. Temperature dependence of dielectric loss, tand, at frequency f = 10 kHz for La0.53Na0.41 xLixTiO3.
not observed for La0.5Na0.5TiO3. For other compounds, the peak is observed in the vicinity of 50 K. Another peak is also observed in the vicinity of 150 and 225 K for x = 0.12 and x = 0.17, respectively. The relaxation time at the peak temperature can be evaluated from the measurement frequency f to be 1/ 2pf. Fig. 4 shows the Arrhenius plot of the relaxation time for peaks in the vicinity of 50 K. The activation energy, E, estimated by this plot is 0.02, 0.08, 0.12 and 0.17 eV for x = 0.04, x = 0.08, x = 0.12 and x = 0.17, respectively. These values are smaller than the activation energy, 0.36 eV, for Li ion conduction of La0.63Li0.1TiO3 [3]. In these compounds, this peak is considered to be caused by the movement of Li ions between off-center positions in the A-site. In the case of (La,Li)TiO3, the existence of off-center positions in the A-site was suggested by the simple energetic calculation [11] and NMR study [12]. According to the potential energy calculation for the Li ion, the energy minimums appeared in the vicinity of A-site when the lattice parameter of the perovskite unit cell was smaller than 0.46 nm. This predicts that off-center positions are more energetically stable than the Asites. Parı´s et al. performed NMR spectroscopy for
La0.61Li0.18TiO3 in the temperature range 200 –350 K and found that the satellite transition ( 3/2, 1/2 and 1/2, 3/2 transition) was diminished at lower temperatures. Satellite transition is associated with electric quadrupolar interaction with electric field gradients; thus, their absence suggests the existence of the off-center positions or the presence of some disorder effects in the structure. Therefore, the stable off-center positions probably exist in the vicinity of A-site for La0.53Na0.41 xLixTiO3 and the mechanism which induces these anomalies can be described by the following. The energetically stable positions for a Li ion intrinsically exist in the vicinity of an A-site and separate with E, estimated by Fig. 3. While Li ions trapped at these stable positions are not activated below the temperature of the anomaly, Li ion is thermally activated enough to jump between these positions at the temperature of the anomaly, but far enough to jump from A-site to A-site. It is speculated that the increase in E with x is caused by the increase in the structural distortion surrounding the Li ion, which is promoted by substitution with Li ion. On the other hand, anomalies at higher temperature would be attributed to the jump of Li ion between A-
Fig. 4. Arrhenius plot of the relaxation time, t, for the peaks in the vicinity of 50 K observed in Fig. 2.
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4. Summary We investigated the dielectric properties of La0.53 Na0.41 xLixTiO3 (x < 0.26). The temperature dependence of the dielectric constant for x = 0.00 shows quantum paraelectric behavior. However, this temperature dependence deviates from the quantum paraelectric behavior with increasing x and the large peak appears in the vicinity of 270 K for x = 0.17. The reason of the large peak is speculated to be due to the existence of Li ions, which can move between A-sites, in this compound and the dipole induced by the motion of these Li ions causes the increase in the dielectric constant. Anomalies on the temperature dependence of the dielectric loss were also observed in the vicinity of 50 K for all compounds except for x = 0.00. These anomalies are attributed to the jump of Li ions between off-center positions in the vicinity of the A-site. Fig. 5. Arrhenius plot of the relaxation time, t, estimated by the peak frequency in the imposed figure, which is the frequency dependence of the imaginary part of the complex dielectric constant, eW, for x = 0.17.
sites. Fig. 5 shows the frequency dependence of the imaginary part of the complex dielectric constant, eW, for x = 0.17 and the Arrhenius plot of the relaxation time estimated by the peak frequency of eW. The activation energy estimated by the Arrhenius plot is 0.37 eV, which is close to the activation energy of Li ion conductivity for La0.63Li0.1TiO3 [3]. Li ion conduction, however, was not observed for x = 0.17— thus, this anomaly must be induced by a different mechanism. With an increase in the Li content, the area in which Li ions and vacancies are connected is formed in the crystal like an island. Li ions in such a location can hop between A-sites and the dipole induced by such motion is observed as the increase in the dielectric constant. The fact that the apparent peaks were not observed except for x = 0.17 is due to the small number of areas mentioned above accompanied by the low concentration of Li and vacancy in A-site. When Li content is increased, this area enlarges and increases and the number of Li ions that can easily move from one A-site to another also increases, and then the apparent peak appears.
Acknowledgements This work was partially supported by Asahi Glass Foundation and Saneyoshi Scholarship Foundation.
References [1] A.G. Belous, G.N. Novitiskaya, S.V. Polyanetskaya, I. Gornikov, Izv. Akad. Nauk. SSSR, Neorg. Mater. 23 (1987) 470. [2] Y. Inaguma, L. Chen, M. Itoh, T. Nakamura, T. Uchida, M. Ikuta, M. Wakihara, Solid State Commun. 86 (1993) 689. [3] Y. Inaguma, L. Chen, M. Itoh, T. Nakamura, Solid State Ionics 70 – 71 (1994) 196. [4] Y. Inaguma, Y. Matsui, Y.J. Shan, M. Itoh, T. Nakamura, Solid State Ionics 79 (1995) 91. [5] T. Katsumata, Y. Matsui, Y. Inaguma, M. Itoh, Solid State Ion. 86 – 88 (1996) 165. [6] H.T. Chung, J.G. Kim, H.G. Kim, Solid State Ion. 107 (1998) 153. [7] Y. Harada, H. Watanabe, J. Kuwano, J. Power Sources 81 – 82 (1999) 777. [8] Y. Inaguma, M. Itoh, Solid State Ionics 86 – 88 (1996) 257. [9] K.A. Mu¨ller, H. Burkard, Phys. Rev. B 19 (1979) 3593. [10] Y. Inaguma, J.H. Sohn, S. Kim, M. Itoh, T. Nakamura, J. Phys. Soc. Jpn. 61 (1992) 3831. [11] Y. Inaguma, Y. Matsui, J.D. Yu, Y.J. Shan, T. Nakamura, M. Itoh, J. Phys. Chem. Solids 58 (1997) 843. [12] M.A. Parı´s, J. Sanz, C. Le´on, J. Santamarı´a, J. Ibarra, A. Va´rez, Chem. Mater. 12 (2000) 1964.
Anomaly of temperature dependence of dielectric permittivity for perovskite-type oxides, La0.53Na0.41xLixTiO3 Tetsuhiro Katsumata *, Yoshiyuki Inaguma Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo, 171-8588, Japan Accepted 10 March 2002
Abstract We investigated the dielectric properties for La0.53Na0.41 xLixTiO3 (x < 0.26). The temperature dependence of the dielectric constant for x = 0.00 shows the quantum paraelectric behavior. For other compounds, however, the anomalies caused by the jumps of Li ions between the off-center potions in the A-site and between A-sites were observed on the temperature dependence of the dielectric properties. D 2002 Elsevier Science B.V. All rights reserved. PACS: 77.22.d Keywords: Perovskite-type oxide; A-site deficient; Li ion; Dielectric property; Ion conduction
1. Introduction A-site deficient perovskite-type lithium lanthanam titanate has been known as an intense Li ion conductor [1 –7], in which the Li ion, the La ion and the vacancy are distributed in the A-sites. Li ion, which is smaller than La ion, can easily move between these sites and the migration of Li ions through the conduction path formed by the vacancy and the Li ion is observed as the Li ion conductivity. Ion conductivity, however, is influenced by various factors, especially site percolation. According to the percolation theory, conductivity quadratically decreases with the number of objects * Corresponding author. Tel.: +81-3-3986-0221; fax: +81-35992-1029. E-mail address: [email protected] (T. Katsumata).
formed in the conduction path and diminishes when its number is less than the percolation limit. In the case of (La, Li) TiO3, the object formed in the conduction path is Li ion and the vacancy, thus ion conductivity is predicted to depend on the sum of the concentration of Li ion and the vacancy. This prediction was supported by experimental results. Inaguma and Itoh [8] investigated the variation of the Li ion conductivity with x for xLa0.55Li0.35TiO3 –(1 x)La0.5Na0.5TiO3. In their report, Li ion conductivity decreased with the sum of the concentration of the Li ion and the vacancy, 0.45x, and was too small to be measured when 0.45x is below the percolation limit of the primitive cell, which was 0.3117. Similar behavior was observed for (1 x)La055 Li 0 . 3 5 TiO 3 – xKMO 3 (M = Nb and Ta), (1 x) La0.55Li0.35TiO3 – xSrTiO3 [4] and (1 x)LiTaO3 – xSrTiO3 [5]. Therefore, Li ion conduction of the perovskite-type Li ion conductor follows a prediction of site percolation and dc conductivity decrease for a
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 4 9 4 - 0
796
T. Katsumata, Y. Inaguma / Solid State Ionics 154 – 155 (2002) 795–799
perovskite-type Li ion conductor in which the sum of the concentration of the Li ion and the vacancy is less than the percolation limit. In contrast, a perovskite-type lithium lanthanum titanate in which the sum of the concentration of the Li ion and the vacancy is less than percolation limit can be considered as a dielectric material. Furthermore, a dielectric property of such compound is very interesting because there are many ions in the A-site which can move more freely than that of the usual dielectric perovskite-type oxide. In this study, we synthesized such a compound, i.e. La 0.53 Na 0.41 x Li x TiO 3 (x < 0.26), and investigated those dielectric properties.
2. Experimental Samples were synthesized by conventional solid state reaction technique. Starting materials were La2O3 (4 N), Li2CO3 (3 N), Na2CO3 (3 N) and TiO2 (3 N). The La content of the La2O3 was determined by titration analysis using EDTA. An excess amount of 20% Na2CO3 was used to account for loss due to volatilization. The mixture was calcined at 1073 K for 8 h and 1323 K for 24 h. The calcined powder was ground and pressed into pellets with a diameter of 7 mm and thickness of 1.5 mm. These pellets were wrapped by a Pt foil and sintered at 1723 K for 3 h (x = 0.00 and x = 0.05) or 1523 K for 3 h (x = 0.10, x = 0.15 and x = 0.23). Phase identification was carried out by powder X-ray diffraction method (Rigaku RINT 2000). The metal content of the synthesized samples was analyzed by inductively couple plasma (ICP) spectroscopy. Dielectric constant and dielectric loss were measured by Agilent Technology 4284A precision LCR meter in a temperature range 8 K – room temperature at the frequency, f , of 1, 10, 100, 200 and 500 kHz.
3. Results and discussion Fig. 1 shows the X-ray diffraction patterns for La0.53Na0.41 xLixTiO3. As shown in this figure, for all compounds the single phase was obtained. The reflections could be indexed on the basis of a pseudocubic cell. The weak reflection at about 2h = 38j was attributed to a superlattice with a double period of a
Fig. 1. X-ray diffraction patterns for La0.53Na0.41 xLixTiO3.
primitive cell, perovskite unit cell. The superstructure is considered to be due to the tilting of the BO6 octahedra. Table 1 shows the nominal composition, analytical composition, analytical Li content, x, sum of the concentration of Li ion and the vacancy, and relative density for La0.53Na0.41 xLixTiO3. For all compounds, the analytical composition is different from the nominal one. The amount of Li and Na in the analytical composition is smaller and larger than the nominal amount, respectively. The reason is that the excess amount was added only for Na2CO3, when starting materials were mixed. The sum of the concentration of Li ion and the vacancy concentration, however, does not exceed the percolation limit of the primitive cell, 0.3117, for all compounds. Therefore, these compounds can be considered as dielectric mate-
T. Katsumata, Y. Inaguma / Solid State Ionics 154 – 155 (2002) 795–799
797
Table 1 List of nominal composition, analytical composition, analytical Li content, x, sum of concentration of Li and vacancy, and relative density for La0.53Na0.41 xLixTiO3 Nominal composition
Analytical composition
x
Sum of concentration of Li and vacancy
Relative density (%)
La0.53Na0.41TiO3 La0.53Na0.36Li0.05TiO3 La0.53Na0.31Li0.10TiO3 La0.53Na0.26Li0.15TiO3 La0.53Na0.18Li0.23TiO3
La0.51Na0.42TiO2.98 La0.51Na0.41Li0.04TiO2.99 La0.52Na0.36Li0.08TiO3.00 La0.53Na0.30Li0.12TiO3.01 La0.53Na0.25Li0.17TiO3.01
0.00 0.04 0.08 0.12 0.17
0.06 0.08 0.12 0.17 0.22
83.5 88.0 92.4 94.6 95.0
rials. In fact, the dc conductivity for x = 0.17 at room temperature measured with the complex impedance method was below the measurement limit in this study, 10 8 S/cm. Fig. 2 shows the temperature dependence of the dielectric constant, er, at frequency f = 10 kHz for La0.53Na0.41 xLixTiO3. For x = 0.00 (La0.53Na0.41 TiO3), the dielectric constant monotonously increases as temperature decreases in the temperature range 50 K < T < 300 K and is almost independent when temperature is less than 50 K. Such behavior was often observed for a quantum paraelectric compound, e.g.
Fig. 2. Temperature dependence of dielectric constant, er, at frequency f = 10 kHz for La0.53Na0.41 xLixTiO3.
SrTiO3 [9]. In the case of SrTiO3, the temperature dependence of the dielectric constant was fitted with the Barrett-type formula, er ¼
A T1 =2cothðT1 =2T Þ T0
ð1Þ
For x = 0.00, the good fit is obtained at A = 1.06 105 K, T1 = 204 K and T0 = 772 K. The quantum paraelectricity was also reported for La0.5Na0.5TiO3 and the fitting parameter, A, T1 and T0, was 1.2 105, 1.8 102 and 7.7 102 K, respectively [10]. These values are almost equal to that of La0.53Na0.41TiO3, hence the vacancies in the A-site do not significantly influence the quantum paraelectricity of this solid solution system. Anomalies appear by substitution with Li ion for other compounds. While for x = 0.04 the dielectric constant increases with decreasing temperature as in the case of x = 0.00 in the temperature range 50 K < T < 300 K, this value is not constant and decreases with temperature when it drops below 50 K. For x = 0.08, the broad and asymmetric peak is observed in the vicinity of 70 K. For x = 0.12, the peak is observed more clearly and for x = 0.17, two apparent peaks are observed in the vicinity of 50 and 290 K. The dielectric constant increases during substitution with Li ion. In particular, the dielectric constant at 290 K of x = 0.17 is about four times larger than that of x = 0.00. Anomalies are also observed in the temperature dependence of the dielectric loss. Fig. 3 shows the temperature dependence of the dielectric loss, tand, at frequency f = 10 kHz for La0.53Na0.41 xLixTiO3. For x = 0.00, a peak was observed in the vicinity of 160 K. While the mechanism of this peak has not been made clear, it is speculated to be due to the vacancy introduced into the A-site because such peak was
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Fig. 3. Temperature dependence of dielectric loss, tand, at frequency f = 10 kHz for La0.53Na0.41 xLixTiO3.
not observed for La0.5Na0.5TiO3. For other compounds, the peak is observed in the vicinity of 50 K. Another peak is also observed in the vicinity of 150 and 225 K for x = 0.12 and x = 0.17, respectively. The relaxation time at the peak temperature can be evaluated from the measurement frequency f to be 1/ 2pf. Fig. 4 shows the Arrhenius plot of the relaxation time for peaks in the vicinity of 50 K. The activation energy, E, estimated by this plot is 0.02, 0.08, 0.12 and 0.17 eV for x = 0.04, x = 0.08, x = 0.12 and x = 0.17, respectively. These values are smaller than the activation energy, 0.36 eV, for Li ion conduction of La0.63Li0.1TiO3 [3]. In these compounds, this peak is considered to be caused by the movement of Li ions between off-center positions in the A-site. In the case of (La,Li)TiO3, the existence of off-center positions in the A-site was suggested by the simple energetic calculation [11] and NMR study [12]. According to the potential energy calculation for the Li ion, the energy minimums appeared in the vicinity of A-site when the lattice parameter of the perovskite unit cell was smaller than 0.46 nm. This predicts that off-center positions are more energetically stable than the Asites. Parı´s et al. performed NMR spectroscopy for
La0.61Li0.18TiO3 in the temperature range 200 –350 K and found that the satellite transition ( 3/2, 1/2 and 1/2, 3/2 transition) was diminished at lower temperatures. Satellite transition is associated with electric quadrupolar interaction with electric field gradients; thus, their absence suggests the existence of the off-center positions or the presence of some disorder effects in the structure. Therefore, the stable off-center positions probably exist in the vicinity of A-site for La0.53Na0.41 xLixTiO3 and the mechanism which induces these anomalies can be described by the following. The energetically stable positions for a Li ion intrinsically exist in the vicinity of an A-site and separate with E, estimated by Fig. 3. While Li ions trapped at these stable positions are not activated below the temperature of the anomaly, Li ion is thermally activated enough to jump between these positions at the temperature of the anomaly, but far enough to jump from A-site to A-site. It is speculated that the increase in E with x is caused by the increase in the structural distortion surrounding the Li ion, which is promoted by substitution with Li ion. On the other hand, anomalies at higher temperature would be attributed to the jump of Li ion between A-
Fig. 4. Arrhenius plot of the relaxation time, t, for the peaks in the vicinity of 50 K observed in Fig. 2.
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799
4. Summary We investigated the dielectric properties of La0.53 Na0.41 xLixTiO3 (x < 0.26). The temperature dependence of the dielectric constant for x = 0.00 shows quantum paraelectric behavior. However, this temperature dependence deviates from the quantum paraelectric behavior with increasing x and the large peak appears in the vicinity of 270 K for x = 0.17. The reason of the large peak is speculated to be due to the existence of Li ions, which can move between A-sites, in this compound and the dipole induced by the motion of these Li ions causes the increase in the dielectric constant. Anomalies on the temperature dependence of the dielectric loss were also observed in the vicinity of 50 K for all compounds except for x = 0.00. These anomalies are attributed to the jump of Li ions between off-center positions in the vicinity of the A-site. Fig. 5. Arrhenius plot of the relaxation time, t, estimated by the peak frequency in the imposed figure, which is the frequency dependence of the imaginary part of the complex dielectric constant, eW, for x = 0.17.
sites. Fig. 5 shows the frequency dependence of the imaginary part of the complex dielectric constant, eW, for x = 0.17 and the Arrhenius plot of the relaxation time estimated by the peak frequency of eW. The activation energy estimated by the Arrhenius plot is 0.37 eV, which is close to the activation energy of Li ion conductivity for La0.63Li0.1TiO3 [3]. Li ion conduction, however, was not observed for x = 0.17— thus, this anomaly must be induced by a different mechanism. With an increase in the Li content, the area in which Li ions and vacancies are connected is formed in the crystal like an island. Li ions in such a location can hop between A-sites and the dipole induced by such motion is observed as the increase in the dielectric constant. The fact that the apparent peaks were not observed except for x = 0.17 is due to the small number of areas mentioned above accompanied by the low concentration of Li and vacancy in A-site. When Li content is increased, this area enlarges and increases and the number of Li ions that can easily move from one A-site to another also increases, and then the apparent peak appears.
Acknowledgements This work was partially supported by Asahi Glass Foundation and Saneyoshi Scholarship Foundation.
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