Defects and diffusion in this solid superionic conductor were investigated using the projector augmented-wave implementation of Car-Parrinello dynamics. Static calculations were used to treat the structure and formation of Li vacancies, and hydrogen interstitials were also considered. The barrier to lithium jumps to vacant adjacent sites in the Li2N plane (⊥c) was found to be extremely small (0.004eV) whereas jumps perpendicular to the Li2N plane (∥c) encountered a barrier of 0.58eV. Diffusion in the plane (⊥c) was therefore limited by the formation of vacancies, whereas the barrier predominated perpendicular (∥c) to the plane. A molecular dynamics run at 800K confirmed the diffusion anisotropy and led to diffusion coefficients which were consistent with experiment. From the trajectories, a microscopic diffusion mechanism was deduced and it was found that mainly isolated jumps took place.

Ab initio Molecular-Dynamics Study of Diffusion and Defects in Solid Li3N. Sarnthein, J., Schwarz, K., Blöchl, P.E.: Physical Review B, 1996, 53[14], 9084-91