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.
@article{arxiv.1602.03762,
title = {Anharmonicity in the high-temperature Cmcm phase of SnSe: soft modes and three-phonon interactions},
author = {Jonathan M. Skelton and Lee A. Burton and Stephen C. Parker and Aron Walsh and Chang-Eun Kim and Aloysius Soon and John Buckeridge and Alexey A. Sokol and C. Richard A. Catlow and Atsushi Togo and Isao Tanaka},
journal= {arXiv preprint arXiv:1602.03762},
year = {2016}
}