Structural, transport, and thermal properties of the single-crystalline type-VIII clathrate Ba8Ga16Sn30 -: art. no. 075113

We report the electrical resistivity rho, Hall coefficient R-H, thermoelectric power S, specific heat C, and thermal conductivity kappa on single crystals of the type-VIII clathrate Ba8Ga16Sn30 grown from Sn-flux. Negative S and R-H over a wide temperature range indicate that electrons dominate electrical transport properties. Both rho(T) and S(T) show typical behavior of a heavily doped semiconductor. The absolute value of S increases monotonically to 243 mu V/K with increasing temperature up to 550 K. The large S may originate from the low carrier concentration n=3.7x10(19) cm(-3). Hall mobility mu(H) shows a maximum of 62 cm(2)/V s around 70 K. The analysis of temperature dependence of mu(H) suggests a crossover of a dominant scattering mechanism from ionized impurity to acoustic phonon scattering with increasing temperature. The existence of local vibration modes of Ba atoms in cages composed of Ga and Sn atoms is evidenced by analysis of experimental data of structural refinement and specific heat, which give an Einstein temperature of 50 K and a Debye temperature of 200 K. This local vibration of Ba atoms should be responsible for the low thermal conductivity (1.1 W/m K at 150 K). The potential of type-VIII clathrate compounds for thermoelectric application is discussed.