Presence of a Doubly-Splitting Site and Its Effect on Thermoelectric Properties of Cu₄SnS₄

Abstract

Thermoelectric properties of Cu₄SnS₄ significantly change around 230 K along with a phase transition between a high temperature phase (HT-phase) and a low temperature phase (LT-phase). Here we particularly focus on a very low thermal conductivity of Cu₄SnS₄, which is even lower in HT-phase. The crystal structures of Cu₄SnS₄ are analyzed using synchrotron radiation. HT-phase is determined to be orthorhombic structure with the space group Pnma, as previously reported; however, the results of Rietveld refinement of HT-phase suggest that there are some Cu sites around saddle points of the double well potential. Phonon dispersion calculated by first-principles calculations indeed exhibits soft phonon modes of the Cu sites, consistent with the Rietveld analysis. The detailed analysis leads to a conclusion that the lower thermal conductivity in HT-phase results from an additional phonon scattering resonated with an anharmonic Cu vibration in the double well potential.