Synthesis and thermoelectric properties of high-entropy half-Heusler MFe_1-xCo_xSb (M = equimolar Ti, Zr, Hf, V, Nb, Ta)

The application of the high-entropy concept has generated many interesting results for both alloys and ceramics. However, there are very few reports on high entropy thermoelectric materials. In this work, a single phase high-entropy half-Heusler compound MFe_1-xCo_xSb with 6 equimolar elements (Ti, Zr, Hf, V, Nb and Ta) on the M site was successfully synthesized by a simple method of mechanical alloying, and the single phase was maintained after densification by spark plasma sintering. The multi-elements are homogenously distributed in the samples. The samples are stable and there is no phase separation after annealing at 1073 K in argon for 72 h, which could be attributed to their high configurational entropy. Due to the phonon scattering introduced by multi-elements, the lattice thermal conductivity is largely suppressed with a lowest value of ~ 1.8–1.5 Wm⁻¹K⁻¹ (300–923 K) for MCoSb. By adjusting the Fe/Co ratio, the samples can show both n-type and p-type semiconductor behavior. Maximum zT values of 0.3 and 0.25 are achieved for n-type MCoSb and p-type MFe_0.6Co_0.4Sb, respectively. The results suggest that the high-entropy concept is a promising strategy to extend the composition range and tune the thermoelectric properties for half-Heusler materials, which could potentially be applied in other thermoelectric materials.