Optimization of thermoelectric properties of type-VIII clathrate Ba ₈Ga₁₆Sn₃₀ by carrier tuning

Single crystals of type-VIII clathrate Ba₈Ga₁₆Sn ₃₀ (BGS) with p- and n-type carriers were grown from Ga flux and Sn flux, respectively. With the increase of the initial flux amount, both the electrical resistivity ρ and the absolute value of the Seebeck coefficient α are decreased, indicative of effective carrier doping. In the optimally doped samples, the dimensionless thermoelectric figure of merit ZT has the maximum values of 1.0 and 0.9 at 450 K for p-and n-type samples, respectively. In aiming at further increase of the ZT value, Sb was substituted for Sn in BGS. It is found that the Ga content in the crystals unexpectedly increases with the increase of Sb content and thus the crystal composition is described as Ba ₈Ga_16+xSn_30-x-ySb_y (x < 0.9, y < 0.9). The ZT value for the p-type sample with x = y = 0.7 has the maximum of 1.0 at around 480 K.