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Single crystalline samples of type-VIII clathrate Ba 8 Ga 16-x Cu x Sn 30 (0 x 0.033) were prepared by the Sn-flux method. Upon substituting Cu for Ga, the carrier mobility at 300 K increases twice while the carrier density stays in the range 3:1 À 4:2 Â 10 19 =cm 3. Consequently, the electrical resistivity is decreased from 5.3 mXcm for x ¼ 0 to 3.2 mXcm for x ¼ 0.033. Irrespective of x, the Seebeck coefficient is largely negative and linearly changes with temperature in the range 300 < T < 600 K. The thermal conductivity is in the range 0:68 À 0:74 W/Km at 300 K for all samples. The dimensionless figure of merit ZT for x ¼ 0.033 reaches the maximum of 1.35 at 540 K.

We have grown single-crystalline samples of Zn-substituted type-VIII clathrate Ba 8 Ga 16 Sn 30 with n-type carriers by Sn-flux method. The actual compositions of the single crystals were found to be described as Ba 8 Ga 15.8À2y Zn y Sn 30.2þy (y ¼ 0 $ 0.54), where the charge balance is well maintained. As y goes from 0 to 0.42, the resistivity at 300 K decreases from 5.3 to 3.0 mX cm gradually, but the effective mass is essentially constant at 1.2 $ 1.5m 0 , indicating intact band structure near the conduction band minima upon Zn substitution for Ga. At elevated temperatures, the ambipolar effect on the thermal conductivity becomes less pronounced upon Zn doping, and the dimensionless figure of merit ZT for y ¼ 0.07 and 0.42 remains at rather high values compared with the nondoped sample. V

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