Ab initiomolecular-dynamics study of diffusion and defects in solidLi3N

We investigate defects and diffusion in solid ${\mathrm{Li}}_3$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 ${\mathrm{Li}}_2$N plane ($\perp c$) was found to be extremely small, namely, 0.004 eV, whereas jumps perpendicular to the ${\mathrm{Li}}_2$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, whereas 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.