Ion Mobility Pathways of the Li+ Conductor Li0.125La0.625TiO3 by Ab Initio Simulations

Within the Li_xLa_2/3-x/3□ _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 × 2a_p × 2a_p unit-cell, were considered for ab initio periodic quantum-mechanical calculations (DFT-B3LYP Hamiltonian, Gaussian-atomic-orbital basis set). Least-energy optimizations 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□ layer of model 2, and a two-dimensional one (0.47 eV barrier) in the 1La+1Li+2□ 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. © 2008 American Chemical Society.