Short-range order and Li+ ion diffusion mechanisms in Li5La9□2(TiO3)16 (LLTO)

The Li-rich phases of superionic LLTO lithium conductor were modelled by a 2ap × 2ap × 4ap perovskite supercell hosting one Li5La9⎕2(TiO3)16 formula unit. The disordered La/Li/⎕ distribution in the A-type perovskite cages was resolved into several ordered configurations, characterized by a short-range order parameter σ related to the number of nearest-neighbour La-vacant A sites. Quantum-mechanical ab initio calculations of the total energy were performed by the periodic code CRYSTAL09, on the basis of a B3LYP Hamiltonian and Gaussian-type localized basis sets. The structural configurations were least-energy optimized within the Pm monoclinic symmetry, finding a progressive stabilization for larger σ values, i.e. with clustering of La-vacancies and then with increasing connectivity of LaO12 coordination polyhedra. By the frozen ion technique, one- and two-dimensional mechanisms of lithium diffusion within (001) layers of thickness ap were investigated. The lowest energy barrier of 0.31 eV, in good agreement with the experimental activation energy of 0.35 eV, was determined for the 1D pathways. The result holds both for one and for two mobile Li atoms per 2ap × 2ap layer unit-cell, according to a peculiar correlation between the motions of neighbouring ions.

Research Highlights
► The energy of structural models of Li0.3125La0.5625TiO3 is computed ab initio.
► Clustering of Li vacancies is shown to be energetically favoured.
► 1D and 2D pathways of Li mobility are determined inside the structure.
► The computed activation barrier fits well with experimental data.