[105] Structural-Stability Study of Antiperovskite Na₃OCl for Na-Rich Solid Electrolyte

The structural phase transition of the high-symmetry cubic phase of antiperovskite Na₃OCl is investigated by computing the phonon band structures of 14 different polymorphs with distinct types of ONa₆ octahedral tilting. The resulting P-T phase diagram shows that, at high temperature and low pressure, the high-symmetry cubic structure with Pm¯3m symmetry is the most stable phase. At low temperature and high pressure, on the other hand, the monoclinic structure with P2₁/m symmetry becomes the most stable phase. In between those two, there is a region in the phase diagram where the orthorhombic structure with Bmmb symmetry is the most stable phase. To improve upon the quasiharmonic results, we do additional calculations in the framework of the self-consistent phonon (SCP) theory, including lattice anharmonicity by using cubic and quartic interatomic force constants (IFCs). This is particularly important for the high-symmetry cubic phase. We find that by decreasing the temperature, the frequency of the soft phonon at the M and R symmetry points gradually shifts to lower values. From these results, we can infer that a phase transition occurs around 166–195 K upon soft-mode condensation. Due to the proximity of the soft-mode frequencies at both symmetry points R and M, we expect a cubic-to-orthorhombic phase transition to be realized via simultaneous condensation of the two octahedral tilting modes.