Phonon Collapse and Second-Order Phase Transition in Thermoelectric SnSe
Since 2014 the layered semiconductor SnSe in the high-temperature Cmcm phase is known to be the most efficient intrinsic thermoelectric material. Making use of first-principles calculations we show that its vibrational and thermal transport properties are determined by huge nonperturbative 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 nonperturbative anharmonic scattering is included. Our results suggest that the good thermoelectric efficiency of SnSe is strongly affected by the nonperturbative anharmonicity.
© 2019 American Physical Society. Received 19 July 2018; revised manuscript received 29 November 2018; published 22 February 2019. The authors acknowledge fruitful discussions with O. Delaire. Financial support was provided by the Spanish Ministry of Economy and Competitiveness (FIS2016-76617-P), the Department of Education, Universities and Research of the Basque Government and the University of the Basque Country (IT756-13). U. A. is also thankful to the Material Physics Center for support. Computer facilities were provided by the Donostia International Physics Center (DIPC), the Spanish Supercomputing Network (FI-2017-2-0007), and PRACE (2017174186).
Submitted - 1807.07726.pdf
Published - PhysRevLett.122.075901.pdf
Supplemental Material - supplementary_material.pdf