Octahedral tilting and ordering of vacancies in the fast ion conductor Li0.12La0.63TiO3 perovskite: a neutron diffraction study

Abstract

We have examined by neutron powder diffraction (NPD) a Li-poor member of the Li_3xLa_{⅔ − x}TiO₃ series, with x = 0.04, which exhibits one of the highest reported values of ionic conductivity. The unit cell deduced, a 2a_o × 2a_o × 2a_o superstructure of the elementary perovskite unit (a_o ≈ 3.8 Å), results from two superimposed effects: the out-of-phase tilting of the TiO₆ octahedra along the b crystallographic axis, and the ordering of La vacancies, preferentially occupying the z/c = 0.5 over the z/c = 0 layers. In this superstructure, defined in the Cm2m space group, a bi-dimensional conductivity is favored by the alternating disposition of lanthanum and cation vacancies in consecutive planes along the c-axis. A difference Fourier synthesis suggests that Li cations are located close to the square windows defined by four TiO₆ octahedra, as previously shown in the Li-rich member of the series, La_0.5Li_0.5TiO₆.