Local structure of the Li1/8La5/8TiO3(LLTO) ionic conductor by theoretical simulations

Some locally ordered arrangements of the lithium-lanthanum-vacancy distribution were devised, within the disordered perovskite superstructure of the LixLa2/3−x/3 1/3−2x/3TiO3 ion conductors. By consideration of the Li1/8La5/8 1/4TiO3 composition, two ordered superstructures are built up on the basis of 8 formula units/primitive cell (with monoclinic Pm space group symmetry). Periodic quantum-mechanical calculations were carried out at the B3LYP level by the CRYSTAL06 code, using ECP for lanthanum and all-electron basis sets for all other atoms. For each ordered model the complete structure was fully relaxed by energy minimization, finding that the 4La/1La+1Li+2 arrangement is more stable than 3La+1 /2La+1Li+1. Hence the average disordered Cmmm structures of Li0.16La0.613TiO3 and Li0.18La0.607TiO3 (neutron diffraction data) can be rationalized. The optimized Li positions were found to lie in peripheral sites within the A-type cavities, close to separating windows and in flattened square-pyramidal coordination. Some ambiguous aspects of the corresponding results of diffraction experiments can then be clarified.