Structure of Fast Ion Conductors Li₃xLa2/3₋xTiO₃ Deduced from Powder Neutron Diffraction Experiments

High-temperature neutron diffraction (ND) experiments have been carried out in two representative members of the Li 3 x La 2/3 - x TiO 3 series ( x = 0.067 and 0.167), to analyze the influence of temperature on the perovskite structure. In the orthorhombic Li-poor ordered member Li 0.2 La 0.6 TiO 3 ( Cmmm space group), prepared by slowly cooling from 1673 K, the heating of the sample produced the elimination of the octahedral tilting along the b -axis ( a 0 b - c 0 scheme) at 873 K, requiring higher temperatures to induce the vacancies disordering. In the rhombohedral Li-rich disordered member Li 0.5 La 0.5 TiO 3 ( R 3̄ c space group), prepared by quenching from 1673 K, the sample heating produced the elimination of the octahedral tilting along the [111] direction ( a - a - a - scheme) at 1073 K. The elimination of octahedral tilting drived the orthorhombic−tetragonal transformation in the first perovskite and the rhombohedral−cubic transformation in the second perovskite. A detailed analysis of four identified phases showed that distortions of LaO 12 cubooctahedra depends on the octahedral tilting and on the vacancy distribution adopted by the perovskite. Finally, the influence of these two parameters, octahedral tilting and vacancy ordering, on the Li mobility of the perovskite series has been discussed.