Enhanced thermoelectric performance of nominal 19-electron half-Heusler compound NbCoSb with intrinsic Nb and Sb vacancies

Since 2015, when we first reported that the half-Heusler alloy NbCoSb with a nominal 19-valence-electrons composition exhibits the moderate thermoelectric properties of a heavily doped n-type semiconductor, we have been studying the preparation of pure-phase materials. Here we report, for the first time, the effects of Nb and Sb deficiencies on the phase composition and thermoelectric properties of NbCoSb. The experimental results show that phase-pure material can be achieved if the composition is slightly Nb and Sb deficient and that intrinsic vacancy defects have a remarkable effect on the thermoelectric performance. The ZT value is doubled and a maximum ZT of 0.8 is achieved in Nb0·97CoSb0.99 at 973 K, benefiting from the elimination of impurity phases and the optimization of carrier concentration. This work proves that introducing vacancy defects is an effective strategy to improve the thermoelectric properties of some half-Heusler alloys, especially of nominal 19-electron half-Heusler compounds.