Phonon Collapse and Second-Order Phase Transition in Thermoelectric SnSe

TL;DR

This study reveals that the high thermoelectric efficiency of SnSe is due to significant non-perturbative anharmonic effects influencing its vibrational properties and phase transition, with implications for other thermoelectric materials.

Contribution

It demonstrates the crucial role of non-perturbative anharmonicity in SnSe's phase transition and thermal transport, providing new insights into thermoelectric material behavior.

Findings

Second-order phase transition driven by phonon collapse

Spectral function shows anomalous vibrational features

Thermal conductivity matches experiments only with anharmonic effects

Abstract

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.