The layered semiconductor SnSe is one of the highest-performing thermoelectric materials known. We demonstrate, through a first-principles lattice-dynamics study, that the high-temperature Cmcm phase is a dynamic average over lower-symmetry minima separated by very small energetic barriers. Compared to the low-temperature Pnma phase, the Cmcm phase displays a phonon softening and enhanced three-phonon scattering, leading to an anharmonic damping of the low-frequency modes and hence the thermal transport. We develop a renormalization scheme to quantify the effect of the soft modes on the calculated properties, and confirm that the anharmonicity is an inherent feature of the Cmcm phase. These results suggest a design concept for thermal insulators and thermoelectric materials, based on displacive instabilities, and highlight the power of lattice-dynamics calculations for materials characterization.
Bibliographical noteFunding Information:
We gratefully acknowledge helpful discussions with Dr J.M. Frost. J.M.S. is supported by an EPSRC programme grant (Grant No.EP/K004956/1). L.A.B. is currently supported by the JSPS (Grant No.26.04792). J.B. acknowledges support from the EPSRC (Grant No.EP/K016288/1). Calculations were carried out on the ARCHER supercomputer, accessed through membership of the UK HPC Materials Chemistry Consortium, which is funded by the EPSRC (Grant No.EP/L000202). We also made use of the Bath University HPC cluster, which is maintained by the Bath University Computing Services, and the SiSu supercomputer at the IT Center for Science (CSC), Finland, via the Partnership for Advanced Computing in Europe (PRACE) Project No.13DECI0317/IsoSwitch.
All Science Journal Classification (ASJC) codes
- Physics and Astronomy(all)