Entropy Engineering in the Off-Stoichiometric Ti₂NiCoSn_0.5Sb_1.5 Double Half-Heusler Alloy

Lowering the thermal conductivity by phonon scattering has been previously studied in high-entropy alloys (HEAs). This concept has been extended to half-Heusler (HH) alloys in the form of entropy engineering by substituting one of the elements with multiple elements or by combining 2 HH alloys to form a double half-Heusler alloy. Here, entropy engineering of double HH Ti₂NiCoSn_0.5Sb_1.5 by the substitution of Ti with Al, Ta, and Zr was studied. Due to their low solubility in Ti, Al and Ta formed Ni-based intermetallic phases. Compositional tuning was performed based on the optimum individual dopant levels of Al, Ta, and Zr. Compositional tuning revealed that the introduction of Ta and Al improved the power factor and lowered thermal conductivity due to the formation of the TaNiCoAl quaternary full Heusler (FH) secondary phase. Zr was completely soluble in the HH alloy, lowering the thermal conductivity at the expense of the power factor. Ti_1.6Ta_0.2Al_0.2NiCoSn_0.5Sb_1.5 with a power factor of 3.83 mW/mK² had a ZT of 0.71 at 823 K, which is higher than those of other double HH alloys. Ti_0.6Ta_0.2Al_0.2ZrNiCoSn_0.5Sb_1.5 also exhibited a low lattice thermal conductivity of 2.19 W/mK at 420 K, which is comparable to that of Hf-substituted HH alloys. On the other hand, entropy engineering by equimolar substitution of elements did not lead to improvement in properties, underlining the need for compositional tuning in the HH alloys.