Improving thermoelectric performance of half-Heusler Ti(0.2)Hf(0.8)CoSb(0.8)Sn(0.2) compounds via the introduction of excessive Ga and Co-deficiencies

(Ti/Zr/Hf)CoSb0.8Sn0.2 compounds are promising p-type thermoelectric materials. In this work, excessive Ga and Co deficiencies were introduced into Ti0.2Hf0.8CoSb0.8Sn0.2 compounds. The microstructure, electrical, and thermal transport properties were investigated in the temperature range of 300 K < T < 1000 K. The excessive Ga formed nanoinclusions with Ti and Sn at the grain boundaries. The Co deficiencies lowered the thermal conductivity and simultaneously improved the electrical conductivity. As a result, the power factor was enhanced to 3.2 mW/m K2, and the maximum Z.T. of ∼0.93 at 988 K was obtained in the Ti0.2Hf0.8Co0.99Sb0.8Sn0.2–0.01Ga sample, which is an increase of ∼48% compared to that of the pristine Ti0.2Hf0.8CoSb0.8Sn0.2 sample.