Alloy composition of half-Heusler compounds for high thermoelectric performance

We performed first-principles-based calculations of density of states, electrical conductivity, Seebeck coefficient, and thermal conductivity of a p-type half-Heusler compounds (ZrCoSb) to identify the alloy composition that gives rise to a high thermoelectric figure-of-merit. The strategy here is to substitute constituent atoms of ZrCoSb with lighter or heavier homologous element to reduce thermal conductivity without appreciably altering electrical properties. The density-of-states calculations reveal that substitution of Sb does not singnificantly alter the electronic states near Fermi level. The small influence on electrical properties was also confirmed by performing calculations for ZrCoBi, where the power factor is slightly larger than that of ZrCoSb. The effect of alloying on thermal conductivity reduction was quantified by equilibrium molecular dynamics simulations of ZrCoSb1-xBix with force fields obtained from first principles. With increasing alloy fraction x, thermal conductivity rapidly reduces to less than 15% of that of ZrCoSb and nearly saturates above x～20%, suggesting ZrCoSb0.8Bi0.2 to be a reasonable alloy composition for high thermoelectric performance.