Stabilities and defect-mediated lithium-ion conduction in a ground state cubic Li 3 N structure
A stable ground state structure with cubic symmetry of Li 3 N (c-Li 3 N) is found by ab initio initially symmetric random-generated crystal structure search method. Gibbs free energy, calculated within quasi-harmonic approximation, shows that c-Li 3 N is the ground state structure for a wide range of temperature. The c-Li 3 N structure has a negative thermal expansion coefficient at temperatures lower than room temperature, due mainly to two transverse acoustic phonon modes. This c-Li 3 N phase is a semiconductor with an indirect band gap of 1.90 eV within hybrid density functional calculation. We also investigate the migration and energetics of native point defects in c-Li 3 N, 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. Thus, the ionic conduction in c-Li 3 N is expected to occur via an interstitial mechanism, in contrast to that in the well-known α-Li 3 N phase which occurs via a vacancy mechanism.