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

Abstract

The half-Heuslers NbCoSn and NbCoSb have promising thermoelectric properties. Here, an investigation of the NbCo_1+ySn_1−zSb_z (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 NbCo_1+ySn_1−zSb_z samples (z > 0). 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, ρ_323K = 27–35 mΩ cm, remains too large for these materials to be considered useful p-type materials.