Extremely space and time restricted thermal transport in the high temperature Cmcm phase of thermoelectric SnSe

Abstract

Phonon dispersions and linewidths of layered thermoelectric SnSe in the high temperature Cmcm phase have been mapped by inelastic neutron scattering measurements. Downturns in the phonon dispersion reveal soft phonon energies at larger wave vectors for phonon modes propagating in the crystallographic basal a-c plane. The downturns can be described by a q2-dependency, indicative of the appearance of strong electron-phonon scattering in the Cmcm phase. A q3-dependency is also needed for a satisfactory description of the dispersions, revealing the existence of a huge lattice anharmonicity. Group velocities of the transverse acoustic phonons are reduced by as much as 37% through the structural phase transition at 798 K. The scattering of phonons is so strong that they propagation only over a few unit cells in length, with the lifetime as short as ~0.3 ps. The very short phonon lifetimes and very limited phonon propagation length, together with negative phonon group velocity at large wavevectors restricts heat transport at high temperatures. Our results reveal the origin of the extremely low thermal conductivity of SnSe in the Cmcm phase.