Titanium Doping to Enhance Thermoelectric Performance of 19-Electron VCoSb Half-Heusler Compounds with Vanadium Vacancies

The 19-electron VCoSb compounds are actually composites of an off-stoichiometric half-Heusler phase and impurities. Here the compositional adjustment is systematically studied in V_1−xCoSb to obtain single-phase V_0.955CoSb. Hall measurements suggest that such a V vacancy, as well as Ti doping, can optimize the carrier concentration, which decreases from ≈11.3 × 10²¹ cm⁻³ for VCoSb to ≈6.3 × 10²¹ cm⁻³ for V_0.755Ti_0.2CoSb. Low sound velocity contributes to the intrinsically low lattice thermal conductivity for VCoSb-based materials. The high Ti-dopant content results in enhanced point-defect scattering, which further decreases the lattice thermal conductivity. Finally, the optimized n-type V_0.855Ti_0.1CoSb is found to reach a peak ZT of ≈0.7 at 973 K. The work demonstrates that the VCoSb-based half-Heuslers are promising thermoelectric materials.