Charge-Compensated Compound Defects in Ga-containing Thermoelectric Skutterudites

Heavy doping changes an intrinsic semiconductor into a metallic conductor by the introduction of impurity states. However, Ga impurities in thermoelectric skutterudite CoSb_3 with lattice voids provides an example to the contrary. Because of dual-site occupancy of the single Ga impurity charge-compensated compound defects are formed. By combining first-principle calculations and experiments, we show that Ga atoms occupy both the void and Sb sites in CoSb_3 and couple with each other. The donated electrons from the void-filling Ga (GaVF) saturate the dangling bonds from the Sb-substitutional Ga (Ga_(Sb)). The stabilization of Ga impurity as a compound defect extends the region of skutterudite phase stability toward Ga_(0.15)Co_4Sb_(11.95) whereas the solid–solution region in other directions of the ternary phase diagram is much smaller. A proposed ternary phase diagram for Ga-Co-Sb is given. This compensated defect complex leads to a nearly intrinsic semiconductor with heavy Ga doping in CoSb_3 and a much reduced lattice thermal conductivity (κ_L) which can also be attributed to the effective scattering of both the low- and high-frequency lattice phonons by the dual-site occupant Ga impurities. Such a system maintains a low carrier concentration and therefore high thermopower, and the thermoelectric figure of merit quickly increases to 0.7 at a Ga doping content as low as 0.1 per Co_4Sb_(12) and low carrier concentrations on the order of 10^(19) cm^(−3).