Lithium self-diffusion in a model lithium garnet oxide Li5La3Ta2O12: A combined quasi-elastic neutron scattering and molecular dynamics study

In this work lithium self-diffusion in the model lithium garnet oxide Li5La3Ta2O12 was investigated by combining quasi-elastic neutron scattering experiments and molecular dynamics simulation. The Q-dependence of the quasi-elastic broadening measured experimentally and of the mean relaxation rate of lithium calculated by molecular dynamics simulations were both well described by the Singwi-Sjölander diffusion model. This model describes lithium nuclei that undergo diffusion via a jump-type mechanism consisting of a distribution of jumps with a mean square jump distance and residence time. The extracted mean jump length is consistent with a jump between tetrahedral (24d) to octahedral (48g and 96h) sites within the Ia3¯d symmetry structure and the residence time of Li at these sites obeys an Arrhenius relation from ~ ps timescales at 1100 K to the ~ ns range at 400 K. This result supports a lithium diffusion mechanism in which jumps are more frequent (smaller residence time) and shorter at higher temperature. The self-diffusivities of Li from both experiment and calculation were in good agreement, but deviations from those previously measured using nuclear magnetic resonance and muon spin relaxation were observed and discussed. Analysis of stretching parameter describing the relaxation of lithium calculated by molecular dynamics indicated that Li motion is more cooperative at shorter length scales, below ~ 4.4 Å, which corresponds to the distance between octahedral sites across the tetrahedral site.