Enhancement of thermoelectric efficiency in type-VIII clathrate Ba ₈Ga₁₆Sn₃₀ by Al substitution for Ga

Single-crystalline samples of type-VIII clathrate Ba₈Ga _16-xAl_xSn₃₀ (0≤x≤12) were grown from Sn flux to characterize the structural and thermoelectric properties from 300 to 600 K. The lattice parameter increases by 0.5% as x is increased to 10.5 whose value is the solubility limit of Al. The Seebeck coefficients of all samples are largely negative and the absolute values increase to approximately 300 μV/K on heating to 600 K. This large thermopower coexists with the metallic behavior in the electrical resistivity. The values of resistivity for 1≤x≤6 at 300 K are in the range 3.3-3.8 mωcm which is 70% of that for x=0. As a result, the power factor for x=4 and 6 has a rather large maximum of 1.83× 10^-3 W/m K² at 480 K. The thermal conductivity stays at a low level of 0.72 W/mK up to 480 K, and the sample with x=6 reaches a ZT value of 1.2 at 500 K.