Since 2014 the layered semiconductor SnSe in the high-temperature Cmcm phase is known to be the most efficient thermoelectric material. Making use of first-principles calculations we show that its vibrational and thermal transport properties are determined by huge non-perturbative anharmonic effects. We show that the transition from the Cmcm phase to the low-symmetry Pnma is a second-order phase transition driven by the collapse of a zone border phonon, whose frequency vanishes at the transition temperature. Our calculations show that the spectral function of the in-plane vibrational modes are strongly anomalous with shoulders and double-peak structures. We calculate the lattice thermal conductivity obtaining good agreement with experiments only when non-perturbative anharmonic scattering is included. Our results suggest that the good thermoelectric efficiency of SnSe is strongly affected by the non-perturbative anharmonicity. We expect similar effects to occur in other thermoelectric materials.
@article{arxiv.1807.07726,
title = {Phonon Collapse and Second-Order Phase Transition in Thermoelectric SnSe},
author = {Unai Aseginolaza and Raffaello Bianco and Lorenzo Monacelli and Lorenzo Paulatto and Matteo Calandra and Francesco Mauri and Aitor Bergara and Ion Errea},
journal= {arXiv preprint arXiv:1807.07726},
year = {2019}
}