[81] Computational predictions of stable phase for antiperovskite Na₃OCl via tilting of Na₆O octahedra

We study the structural stability of crystalline Na₃OCl in terms of cohesive energies and phonon spectra through the tilting of Na₆O octahedra. We prove that the crystal Na₃OCl can be stabilized through octahedral tilts by comparing 14 tilted structures that are consistent with the Howard and Stokes’ group-theoretical analysis of the octahedral tilting in perovskites. We found that all the 14 tilted structures of Na₃OCl have lower energies than the cubic Pm3¯ 3¯m structure by about 11 to 16 meV per five-atom unit cell. The tilt angles along the pseudocubic [100], [010], and [001] directions vary in the range of 2.6°–9.3°. While the Pnma and P2₃/m structures of Na₃OCl are found as the two most stable ones, only the P2₁/m phase has stable phonon vibrations with a direct band gap of 3.38 eV at the Γ point.