Anharmonicity in the high-temperature Cmcm phase of SnSe: soft modes and three-phonon interactions

TL;DR

This study uses first-principles lattice dynamics to reveal that the high-temperature Cmcm phase of SnSe exhibits significant anharmonicity due to soft phonon modes and three-phonon interactions, affecting its thermal transport properties.

Contribution

The paper introduces a renormalisation scheme to quantify soft mode effects and demonstrates that anharmonicity is intrinsic to the high-temperature Cmcm phase of SnSe.

Findings

Cmcm phase is a dynamic average over lower-symmetry minima.

Enhanced three-phonon scattering causes anharmonic damping.

Soft modes significantly influence thermal transport.

Abstract

The layered semiconductor SnSe is one of the highest-performing thermoelectric materials known. We demonstrate, through a first-principles lattice-dynamics study, that the high-temperature Cmcm phase is a dynamic average over lower-symmetry minima separated by very small energetic barriers. Compared to the low-temperature Pnma phase, the Cmcm phase displays a phonon softening and enhanced three-phonon scattering, leading to an anharmonic damping of the low-frequency modes and hence the thermal transport. We develop a renormalisation scheme to quantify the effect of the soft modes on the calculated properties, and confirm that the anharmonicity is an inherent feature of the Cmcm phase. These results suggest a design concept for thermal insulators and thermoelectric materials, based on displacive instabilities, and highlight the power of lattice-dynamics calculations for materials characterization.