Thermoelectric properties and band structures of vacancy-containing Sn-based clathrates K8Sn44−xGex

Abstract

The type-I clathrate K8Sn44 showed a change in the arrangement of Sn vacancies around 320 ​K. Below the temperature, it exhibited a 2 ​× ​2 ​× ​2 superstructure of the conventional type-I clathrate with ordered Sn vacancies; above it, the ordering disappeared. K8Sn42Ge2 and K8Sn40Ge4 showed no structural transition and no ordering of Sn vacancies. The thermoelectric and transport properties of K8Sn44 changed at the transition temperature. The vacancy-disordered K8Sn44 in the high-temperature phase had large carrier concentrations by one order of magnitude, low carrier mobility, and large effective masses, compared to that in the low-temperature phase. The band structure of K8Sn44 in the high-temperature phase contains localized states, probably due to disordered Sn vacancies, whereas that in the low-temperature phase has no localized states. The high-temperature phase, containing localized states, is expected to provide higher thermoelectric performance because of its larger density-of-states effective mass. Also, the room-temperature lattice thermal conductivity of K8Sn44 was lower than those of the related clathrates Rb8Sn44 and K8Ga8Sn38. It may be related to the framework's Sn-weighted thermal vibration modes of the guest atom K, in addition to the complexity of its crystal structure with disordered vacancies.