Semiconducting ternary Si clathrates

Abstract

This article reviews the recent developments in semiconducting ternary Si clathrates, which have been regarded as energy-conversion materials. Further, it elucidates their synthesis methods, crystal structure, calculated results associated with the band structure, and physical characteristics such as optical, transport, and thermoelectric properties. Currently, eight ternary type-I Si clathrates encapsulating alkali-metal atoms have been reported to exhibit a semiconducting behavior with band gaps ranging from 0.64 to 1.4 eV. Their relatively high electrical resistivities can be attributed to carrier conduction by the variable range hopping mechanism. Their Seebeck coefficients and thermal conductivities are comparable in magnitude to the other group 14 element clathrates. As a result, the dimensionless thermoelectric figure of merit of these clathrates are low due to their high electrical resistivity as compared with clathrates of Ge and Sn. Future issues associated with their energy conversion applications have also been discussed herein.