Anharmonicity and phase stability of antiperovskiteLi3OCl

A lattice-dynamics study of the cubic ${\mathrm{Li}}_{3}\mathrm{OCl}$ antiperovskite, a candidate solid electrolyte in lithium-ion batteries, reveals the presence of dynamical instabilities with respect to rotations of the ${\mathrm{Li}}_{6}\mathrm{O}$ octahedra. Calculated energy landscapes in the subspace of unstable octahedral rotational modes are very shallow with at most a 1 meV per formula unit reduction in energy upon breaking the cubic symmetry. While ${\mathrm{Li}}_{3}\mathrm{OCl}$ is not stable relative to decomposition into ${\mathrm{Li}}_{2}\mathrm{O}$ and LiCl at 0 K, estimates of the vibrational free energy suggest that ${\mathrm{Li}}_{3}\mathrm{OCl}$ antiperovskite should become entropically stabilized above approximately 480 K.