Thermoelectric properties of Sn-based clathrates

Inorganic clathrates consisting in group-IV elements have crystal structures with large cages. Electronic structure and transport properties of Sn-based clathrate semiconductors have been studied by the ab-initio band structure calculating method. In this calculation the FLAPW method with the GGA is used and the Ga configuration of occupied sites is determined by the comparison of the total energy E_tot among various configurations. The band gap of the calculated electronic structure E_g is 0.16 eV, which agrees with the experimental result, estimated by the temperature dependence of the electric conductivity. However the band gap E_g=0.16 eV does not explain the experimental Seebeck coefficient α. When we assume a lager band gap E_g=0.45 eV and a carrier concentration n_e=8×10¹⁸/cm³, the calculated α reasonably explains the experimental data.