Synthesis and thermoelectric properties of semiconducting germanium-based type-II clathrate (K,Sr)24(Ga,Ge)136

Abstract

Using the group-II element Sr as guest atoms enabled us to compensate the carrier concentrations of Ge-based type-II clathrates, resulting in semiconducting Seebeck coefficients of ∼−100 μV/K. The carrier compensation may be associated with a large contribution from the orbitals of Sr atoms to the conduction band near the Fermi energy. The type-II clathrate samples had a relatively high room-temperature carrier mobility of ∼ 50 cm2 V−1 s−1 because of a small inertial mass. However, the minimum lattice thermal conductivities were 13 mW cm−1 K−1, which was almost a double of the theoretical minimum value of Ge. The crystal structure analysis indicates that the rattling effect was not so strong in the clathrates. As a result, the maximum dimensionless figure of merit was 0.43 at 800 K.