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

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.