Impact of Nb vacancies and p-type doping of the NbCoSn-NbCoSb half-Heusler thermoelectricsElectronic supplementary information (ESI) available. See DOI: 10.1039/c7cp07521a

• Main Authors: Ferluccio, Daniella A, Smith, Ronald I, Buckman, Jim, Bos, Jan-Willem G
• Format: Article
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c7cp07521a

The half-Heuslers NbCoSn and NbCoSb have promising thermoelectric properties. Here, an investigation of the NbCo 1+ y Sn 1− z Sb 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+ y Sn 1− z Sb 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, S 2 / 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. Nb vacancies maintain a semiconducting electron count and cause strong mass fluctuation phonon scattering enabling good thermoelectric performance.