Ab Initio Thermoelectric Transport Calculations of Alloyed Half-Heuslers

We examine the effect of alloying in the electronic transport of half-Heusler thermoelectric materials from fully ab initio transport considerations. For this, we develop a novel method that computes appropriate supercells of alloys and couples them with Boltzmann transport considering the full energy/momentum dependence of all relevant scattering processes, described using first-principles extracted parameters. In alloys, such as ${\mathrm{Ti}}_{1-x}{\mathrm{Zr}}_x\mathrm{NiSn}$ , ${\mathrm{Zr}}_{1-x}{\mathrm{Hf}}_x\mathrm{NiSn}$ , and ${\mathrm{Hf}}_{1-x}{\mathrm{Ti}}_x\mathrm{NiSn}$ , only minor electronic structure changes are observed with x. This translates into minor TE power factor changes upon alloying. For TaFeSb-type materials, isovalent (Nb) and aliovalent (Ti/Zr/Hf) alloying/doping change the band structure significantly, making it more susceptible to band splitting, which leads to significant PF reductions by up to 50%. Thus, although zone-end high valley degeneracy is favorable for TEs, alloying reduces part of this PF advantage. On the other hand, the additional benefit of thermal conductivity reduction upon alloying still improves the figure of merit $zT$ , showing that in some cases the ultimate performance of these materials can reach a $zT$ = 2 at high temperatures.