Impact of Nb vacancies and p-type doping of the NbCoSn-NbCoSb half-Heusler thermoelectrics

The half-Heuslers NbCoSn and NbCoSb have promising thermoelectric properties. Here, an investigation of the NbCo1+ySn1-zSbz (y = 0, 0.05; 0 ≤ z ≤ 1) solid-solution is presented. In addition, the p-type doping of NbCoSn using Ti and Zr substitution is investigated. Rietveld analysis reveals the gradual creation of Nb vacancies to compensate for the n-type doping caused by the substitution of Sb in NbCoSn. This leads to a similar valence electron count (~18.25) for the NbCo1+ySn1-zSbz samples (z > 0). The mass fluctuation disorder due to the Nb vacancies strongly decreases the lattice thermal conductivity from 10 W m-1 K-1 (z = 0) to 4.5 W m-1 K-1 (z = 0.5, 1). This is accompanied by a transition to degenerate semiconducting behaviour leading to large power factors, S2/ρ = 2.5-3 mW m-1 K-2 and figures of merit, ZT = 0.25-0.33 at 773 K. Ti and Zr can be used to achieve positive Seebeck values, e.g. S = +150 μV K-1 for 20% Zr at 773 K. However, the electrical resistivity, ρ323K = 27-35 m cm, remains too large for these materials to be considered useful p-type materials.