Entropy-Stabilized Half-Heuslers (TiHf)_1/2(Fe_1–<i>x</i>CoNi_1+<i>x</i>)_1/3Sb with Highly Reduced Lattice Thermal Conductivity

High-entropy alloys (HEAs) have gained significant attention recently due to their exceptional physical properties. Among HEAs, entropy-stabilized alloys, where the high configurational entropy drives the structural stability, are of considerable interest in new materials discovery. Here, we combine theoretical and experimental approaches to design very low lattice thermal conductivity (κl) high-entropy materials (TiHf)1/2(Fe1–xCoNi1+x)1/3Sb belonging to the half-Heusler family. We demonstrate that (TiHf)1/2(FeCoNi)1/3Sb is entropy-stabilized, with κl at 300 K suppressed by over 80% with respect to the parent compound TiCoSb that has an unfavorably high thermal conductivity of 18 W·m–1·K–1. Further reduction of κl is achieved by tuning the Fe/Ni ratio. The lowest κl is observed in the material (TiHf)1/2(Fe0.5CoNi1.5)1/3Sb, where it approaches the theoretical minimum value of κmin ≈ 1 W·–1·K–1 at 973 K. Tuning the Fe/Ni ratio simultaneously optimizes the carrier concentration, resulting in significantly enhancing electronic properties. The electrical conductivity increases almost 5-fold, and the power factor increases from 7 to 16 μW·cm–1·K–2 as x increases from 0 to 0.5 at 973 K, making the material (TiHf)1/2(Fe0.5CoNi1.5)1/3Sb achieve a zT of 0.51 at 973 K without further optimization.