Structural Modifications Induced by High-Temperature Quenching Treatments in the Fast Ion Conductor Li 0.18 La 0.61 TiO 3 : A Neutron Diffraction Study

The crystal structure of the fast ionic conductor Li 0.18 La 0.61 TiO 3 perovskite-type was determined at 5 K by neutron diffraction (ND) experiments in samples cooled slowly and quenched from 1600 K. In both cases, the perovskite displayed superstructures of the elementary perovskite. In the slowly cooled sample, a 2a p,2a p,2a p superstructure with orthorhombic symmetry was detected ( Cmmm space group). The main characteristics of this structure are the out-of-phase tilting of TiO 6 octahedra in the [010] direction, and the ordering of La vacancies in alternate planes along the c -axis. In samples quenched in liquid nitrogen from high temperature, La and vacancies become disordered and the structure adopts a rhombohedral √2a p,√2a p, 2√3a p symmetry ( R 3̄ c space group). In this structure the octahedral tilting is produced along the three crystallographic axes as a consequence of the disordered arrangement of the La and the vacancies. A detailed analysis of Fourier difference maps in both samples revealed the preference of Li for the 4-fold coordination at the center of square windows that connect contiguous A-sites of the perovskite.