Effect of Au substitution on thermoelectric properties of silicon clathrate compounds

The effect of Au substitution on the electronic and thermoelectric properties has been investigated on polycrystalline silicon clathrate compounds with the formula Ba₈Au_xGa_ySi_46-x-y (x=1, 2, 3, 4, 5; y=16-3x). The conductivity type changes from n-type for x=1, 2, 3 to p-type for x=4, 5 with increasing Au composition x. The Seebeck coefficient for n-type samples is enhanced as compare to n-type Ba₈Ga₁₆Si₃₀ and the Seebeck coefficient for p-type samples is larger than that for n-type samples at similar carrier concentrations. The effective mass values for n-type and p-type samples are estimated to be about 2.4 m₀ and 6.0 m₀, respectively. The temperature dependence of the thermoelectric properties suggests that the band gap decreases with increasing Au composition. These electronic properties qualitatively agree with the characteristics of band structure reported for Ba₈Au₆Si₄₀. An appropriate amount of Au substitution (x=1, 2) is effective in increasing the power factor due to the enhanced Seebeck coefficient at high temperatures for n-type samples.