Lattice Dynamics and Ionic Transport in Antiperovskites M₃XCh (M = Li, Na; X = H, F; Ch = S, Se, Te): A First-Principles Study Incorporating Anharmonicity and Vacancy Effects

Phonon properties from lattice dynamic serve as descriptors for ionic conduction and are widely used to screen solid electrolytes. However, most phonon calculations assume perfect crystals, ignoring anharmonicity and defects. To better understand phonon-ionic conduction relationship, we perform first-principles harmonic and anharmonic lattice dynamic calculations for perfect and defective antiperovskites, M3XCh (M = Li, Na; X = H, F; Ch = S, Se, Te), recognized as lithium/sodium conductors. Anharmonic calculations reveal a significant increase in a specific phonon frequency, associated with octahedral rotation and critical for ionic conduction, while harmonic calculations with vacancies show minimal effects. Importantly, a lower specific frequency leads to a lower energy barrier for Li+/Na+ migration; this relationship persists even after accounting for anharmonicity and vacancy effects. Correlations between the average phonon frequency of migrating ions and the energy barriers are rather weak, indicating the importance of the specific phonon mode for promoting ionic conduction. Structural analysis shows that the phonon-ionic conduction interactions may be weakened when anharmonicity increases the specific frequency. These results suggest that selecting materials with low specific frequencies, even at operating temperatures, could facilitate the discovery of fast ionic conductors. Moreover, the anharmonic stabilization mechanism of the cubic phases observed in these compounds may broaden a chemical space to explore solid electrolytes.