Ab-Initio Molecular Dynamics Study of Diffusion and Defects in Solid Li(sub3)N

We investigate defects and diffusion in solid Li$_3$N, 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 Li$_2$N plane ($\perp c$) was found to be extremely small, namely 0.004~eV, where\-as jumps perpendicular to the Li$_2$N plane ($\parallel c$) have a barrier of 0.58~eV. Therefore diffusion in the plane ($\perp c$) is limited by the formation of vacancies, where\-as the barrier dominates perpendicular ($\parallel c$) 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.