Ab initio molecular-dynamics study of diffusion and defects in solid Li3N

We investigate defects and diffusion in solid Li3N, a superionic conductor, using the projector-augmented-wave implementation of Car-Parrinello molecular dynamics. Static calculations are used to discuss the structure and formation of Li vacancies, where we also consider hydrogen interstitials. The barrier for lithium jumps to vacant adjacent sites in the Li2N plane (perpendicular to c) was found to be extremely small, namely, 0.004 eV, whereas jumps perpendicular to the Li2N plane (parallel to c) have a barrier of 0.58 eV. Therefore diffusion in the plane (perpendicular to c) is limited by the formation of vacancies, whereas the barrier dominates perpendicular (parallel to c) to the plane. A molecular-dynamics run at 800 K confirms the anisotropy of diffusion and leads to diffusion coefficients consistent with experiment. From the trajectories we deduce a microscopic diffusion mechanism and find that mainly isolated jumps take place.