Ion mobility pathways of the Li+ conductor Li0.125La0.625TiO3 by ab initio simulations

Within the LixLa2/3-x/3 square 1/3-2x/3TiO3 (LLTO) solid electrolytes with perovskite superstructures, the x = 0.125 term was selected as representative of the Li-poor compositions. Two ordered structural models, with Z= 8 formula units per 2a(p) x 2a(p) x 2a(p) unit-cell, were considered for ab initio periodic quantum-mechanical calculations (DFT-B3LYP Hamiltonian, Gaussian-atomic-orbital basis set). Least-energy optirmizations showed that model 1, with P2mm symmetry, is more stable and agrees with the Cmmm disordered structures of the x = 0.16 and 0.18 terms from experiment. Then the energy of nonequilibrium states with displaced Li positions ("frozen ion" approach) and lowered symmetry was computed, determining mobility paths of the Li+ ion within the (001) layers of both models. A one-dimensional [100] pathway with activation barrier of 0.42 eV was found in the 2La+1Li+1 square layer of model 2, and a two-dimensional one (0.47 eV barrier) in the 1La+1Li+2 square layer of model 1. Both Li+ ion transport mechanisms should operate in the real disordered material. The predicted barriers are discussed and compared with the experimental activation energy of Li-richer terms.