Stabilities and defect-mediated lithium-ion conduction in a ground state cubic Li3N structure

Abstract

A stable ground state structure with cubic symmetry of Li₃N (c-Li₃N) is found by an ab initio initially symmetric random-generated crystal structure search method. Gibbs free energy, calculated within quasi-harmonic approximation, shows that c-Li₃N is the ground state structure for a wide range of temperatures. The c-Li₃N structure has a negative thermal expansion coefficient at temperatures lower than room temperature, mainly due to two transverse acoustic phonon modes. This c-Li₃N phase is a semiconductor with an indirect band gap of 1.90 eV within hybrid density functional calculations. We also investigate the migration and energetics of native point defects in c-Li₃N, including lithium and nitrogen vacancies, interstitials, and anti-site defects. Lithium interstitials are found to have a very low migration barrier (∼0.12 eV) and the lowest formation energy among all possible defects. The ionic conduction in c-Li₃N is thus expected to occur via an interstitial mechanism, in contrast to that in the well-known α-Li₃N phase which occurs via a vacancy mechanism.