  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.
