Li Mobility in the Orthorhombic Li_0.18La_0.61TiO₃ Perovskite Studied by NMR and Impedance Spectroscopies

Abstract

Electrical conductivity and NMR relaxation times (T1 and T2) have been determined in the Li0.18La0.61TiO3 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 T1 and T2 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.