Enhanced thermoelectric figure-of-merit in ‘defective’ half-Heusler Nb0.8CoSb

The half Heusler alloys are potential mid-to-high temperature range thermoelectrics due to their high power factor, solid structural stability and high mechanical strength. However, the high lattice thermal conductivity inherent to these materials reduces the zT, rendering them less useful for practical applications. The ‘defective’ half-Heuslers provide an attractive alternative to mitigate these issues. In these materials, the intrinsic lattice defects present in the form of vacancies lower the lattice thermal conductivity substantially. Here, we study the effect of excess Nb and Sn doping in the defective half Heuslers Nb0.8+δCoSb1−xSnx. We show that Sn doping allows for: (i) optimizing the carrier concentration with a much finer control than if only δ is increased, and (ii) incorporation of a higher concentration of Nb in the structure. A combination of optimized carrier concentration and synergistic changes in the band structure, including the appearance of a new flat band near 50 meV above the Fermi energy due to excess Nb, suppresses the bipolarity and enhances the thermopower further at high temperatures as the average band effective mass of the carriers increases. We, therefore, obtain a high zT exceeding 1 at 1100 K for Nb0.85CoSb0.95Sn0.05. This value is ≈ 15% higher than the highest zT previously reported.