Structure and thermoelectric properties of the n-type clathrate Ba8Cu5.1Ge40.2Sn0.7

We study type I clathrate Ba8Cu5.1Ge40.2Sn0.7 single crystals (space group Pm (3) over barn, no. 223, a = 10.7151(3)) grown using a Sn flux method. Microprobe analysis and single-crystal X-ray diffraction reveal a small amount of Sn embedded in the Cu/Ge framework, which increases disorder at the guest Ba sites. Ba8Cu5.1Ge40.2Sn0.7 is diamagnetic with a susceptibility of similar to 2.8 x 10(-7) emu g(-1) and shows metal-like behavior (d rho/dT > 0) with a low charge carrier concentration of 0.5 e(-) per unit cell at 300 K. The single crystals show a relatively high carrier mobility (mu (300 K) = 11.9 cm(2) V-1) and very low lattice thermal conductivity (similar to 0.6 W m(-1) K-1). The thermoelectric figure of merit ZT of Ba8Cu5.1Ge40.2Sn0.7 single crystals reaches a maximum value of 0.6 at 773 K, which can be further improved by adjusting the chemical composition.