Preparation and thermoelectric properties of sintered type-I clathrates K₈Ga_xSn_46−x

Polycrystalline clathrate samples of nominal K₈Ga_xSn_46−x were prepared by the spark plasma sintering method to investigate their crystal structures, mobilities and thermoelectric properties. The samples almost had a single-phase type-I clathrate structure, and their relative densities reached as high as 98%. The room-temperature mobility of the K₈Ga₈Sn₃₈ sample was 25 cm² V⁻¹ s⁻¹, which substantially exceeded the reported mobilities of Rb- and Cs-containing Sn clathrates. Moreover, the mobility was larger than those of the type-I Ba₈Ga₁₆Sn₃₀, which had more Ga substituting atoms in unit cell. A higher mobility might accordingly be achieved in thermoelectric clathrates with a smaller number of substituting atoms. Their electrical conductivities and Seebeck coefficients in the temperature range 300–450 K were typical of n-type doped semiconductors in the extrinsic region while their room-temperature lattice thermal conductivities were as low as 11 mW cm⁻¹ K⁻¹. The maximum dimensionless figure of merit ZT was estimated to be 0.27 at 490 K from the Seebeck coefficient of −262 μV K⁻¹ and the electrical conductivity of 96 S cm⁻¹ for the K₈Ga₈Sn₃₈ sample with a carrier concentration of 2.9 × 10¹⁹ cm⁻³