Low thermal conductivity and promising thermoelectric performance in A_{: X}CoSb (A = V, Nb or Ta) half-Heuslers with inherent vacancies

Half-Heuslers with vacancies that are stabilised by a semiconducting electron count offer new opportunities for discovering good thermoelectric performance. Here, we present a comparative study of A_xCoSb half-Heuslers (A = V, Nb or Ta) with intrinsic vacancies. Structural analysis reveals an increasing vacancy concentration from V (13%) to Nb (15%) to Ta (19%) with evidence for ∼3% V/Co inversion. This decrease in ability to n-type dope these materials is caused by an increase in conduction band dispersion, evident from a decreasing density of states mass from Hall data, leading to a higher cost of populating these antibonding states. V_0.87CoSb has an ultralow lattice thermal conductivity, κ_lat ∼ 2.2 W m^-1 K^-1, which cannot be explained within the Callaway framework. Coupled to a promising power factor, S²/ρ = 2.25 mW m^-1 K^-2, this results in ZT = 0.6 at 950 K. Nb_0.85CoSb has a power factor of S²/ρ = 2.75 mW m^-1 K^-2 with κ ∼ 4.75 W m^-1 K^-1, yielding a similar ZT = 0.5 at 950 K. Ta_0.81CoSb has a microstructure consisting of smaller grains than the other samples, impacting both the carrier and thermal transport, yielding a power factor S²/ρ = 0.75 mW m^-1 K^-2 and ZT = 0.3 at 950 K. The ultralow κ_lat for V_0.87CoSb may be linked to porosity effects that do not strongly impact on the charge transport, thus affording a new route towards improved performance.