Anharmonicity in the High-Temperature Cmcm Phase of SnSe: Soft Modes and Three-Phonon Interactions

• Published in: Physical review letters Vol. 117; no. 7; p. 075502
• Main Authors: Skelton, Jonathan M, Burton, Lee A, Parker, Stephen C, Walsh, Aron, Kim, Chang-Eun, Soon, Aloysius, Buckeridge, John, Sokol, Alexey A, Catlow, C Richard A, Togo, Atsushi, Tanaka, Isao
• Format: Journal Article
• Language: English
• Published: United States 12.08.2016
• ISSN: 1079-7114
• DOI: 10.1103/PhysRevLett.117.075502

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 renormalization 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.