Defects and diffusion in the solid superionic conductor were investigated by using the projector augmented-wave version of Car-Parrinello molecular dynamics. Static calculations were used to analyze the structure and formation of Li vacancies and H interstitials. It was found that the barrier to Li jumps into vacant adjacent sites in the Li2N plane (c) was extremely small (0.004eV). On the other hand, jumps perpendicular to the Li2N plane (||c) had to overcome a barrier of 0.58eV. Therefore, diffusion in the plane (c) was limited by the formation of vacancies, whereas the diffusion barrier governed migration perpendicular to the plane. A molecular dynamics simulation at 800K confirmed the diffusive anisotropy, and predicted diffusion coefficient values which were consistent with experimental data. On the basis of the trajectories, the operation of a microscopic diffusion mechanism was deduced; in which mainly isolated jumps occurred.

J.Sarnthein, K.Schwarz, P.E.Blöchl: Physical Review B, 1996, 53[14], 9084-91