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

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 similar to-100 mu V/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 similar to 50 cm(2) 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. (C) 2016 Elsevier B.V. All rights reserved.