Li Mobility in the Orthorhombic Li 0.18 La 0.61 TiO 3 Perovskite Studied by NMR and Impedance Spectroscopies M. A. Parı ´s and J. Sanz* Instituto de Ciencia de Materiales de Madrid (CSIC), Cantoblanco, 28049 Madrid, Spain C. Leo ´n and J. Santamarı ´a Departamento de Fı ´sica Aplicada III, Facultad de Ciencias Fı ´sicas, Universidad Complutense de Madrid, 28040 Madrid, Spain J. Ibarra and A. Va ´ rez Departamento de Materiales, Escuela Polite ´ cnica Superior, Universidad Carlos III de Madrid, 28911 Legane ´ s, Spain Received August 6, 1999. Revised Manuscript Received March 23, 2000 Electrical conductivity and NMR relaxation times (T 1 and T 2 ) have been determined in the Li 0.18 La 0.61 TiO 3 perovskite. At room temperature, the unit cell constants are a ) 3.865- (1), b ) 3.876(1), and c ) 7.788(2) Å and the space group Pmmm (orthorhombic). In this doubled perovskite, the Rietveld analysis of the X-ray powder pattern showed that La ions occupy preferentially one type of sites (z/c ) 0), and Li and vacancies accommodate with the remaining La at the second site (z/c ) 0.5). From this fact, Li motion should be favored in the plane ab; however, exchanges of Li between contiguous layers are detected above 200 K by NMR spectroscopy. From T 1 and T 2 NMR data, two main relaxation mechanisms have been detected, which have been ascribed to localized exchanges (200-273 K) and extended motions of Li (above 273 K). The dc conductivity shows a non-Arrhenius temperature dependence, and local activation energies of 0.41 and 0.26 eV were obtained in the low- and high-temperature ranges, respectively. Both NMR and electrical conductivity relaxations are described by “stretched exponential” functions, characteristic of correlated ion motions. 1. Introduction Interest in solid electrolytes for use in solid-state batteries has increased in recent years. Lithium-based systems are attractive due to high energy densities and high open circuit potentials. In particular, lithium lanthanum titanates with perovskite structure exhibit one of the highest conductivities reported at room temperature (10 -3 S/cm). 1 In this family of compounds, the poor coordination of Li and the presence of abundant vacant equivalent sites enhances Li mobility. 1,2 On the other hand, ionic conductivity depends on cations oc- cupying A sites of the perovskite; lanthanide ions with a smaller radius than La depress Li mobility, while Sr with a larger radius, improves slightly the conductiv- ity. 2,3 The progressive substitution of Li for La in Li 3x La 2/3-x TiO 3 perovskites reduces slightly the unit cell size but increases the conductivity. 4 At present, the influence of the structure on ionic conductivity has not been established, being necessary additional work to understand the causes that enhance the Li mobility in these compounds. NMR spin-lattice relaxation (SLR) and electrical conductivity relaxation (ECR) techniques have been often proposed to study cation mobility in solids. In lithium lanthanum perovskites, relaxation functions describing either SLR or ECR show significant devia- tions from the simple exponential behavior, character- istic of ideal Bloembergen-Purcell-Pound (BPP) or Debye-like relaxations. 5,6 These deviations have been described using stretched exponentials of the Kohl- rausch-Williams-Watts (KWW) form, 7 f(t) ) exp[-(t/ τ)  ], with  ≈ 0.4 taking into account of correlation effects in ion motion. Activation energies corresponding to short- and long-range motions of Li in these perovs- kites verified the expression E m ) E M , 5 deduced from the coupling and jump relaxation models. 8,9 Alterna- tively, SLR and ECR data taken in different perovskites (1) Inaguma, Y.; Chen, L.; Itoh, M.; Nakamura, T.; Uchida, T.; Ikuta, M.; Wakihara, M. Solid State Commun. 1993, 86, 689. (2) Inaguma, Y.; Chen, L.; Itoh, M.; Nakamura, T. Solid State Ionics 1994, 70/71, 196. (3) Itoh, M.; Inaguma, Y.; Jung, W.; Chen, L.; Nakamura, T. Solid State Ionics 1994, 70/71, 203. (4) Kawai, H.; Kuwano, J. J. Electrochem. 1994, 141, L78. (5) Leo ´n, C.; Lucı ´a, M. L.; Santamarı ´a, J.; Parı ´s, M. A.; Sanz, J.; Va ´rez, A. Phys. Rev. B 1996, 54, 184. (6) Emery, J.; Buzare, J. Y.; Bohnke, O.; Fourquet, J. L. Solid State Ionics 1997, 99, 41. (7) Kohlrausch, R. Ann. Phys. Lpz. 1847, 72, 393. (8) Ngai, K. L. Comments Solid State Phys. 1979, 9, 121; 1980,9, 141. Ngai, K. L. Effects of Disorder on Relaxational Processes; Springer- Verlag: Berlin, 1994. (9) Funke, K. Prog. Solid State Chem. 1993, 22, 111. 1694 Chem. Mater. 2000, 12, 1694-1701 10.1021/cm9911159 CCC: $19.00 © 2000 American Chemical Society Published on Web 05/16/2000