High solubility of Al and enhanced thermoelectric performance due to resonant states in Fe₂VAl_x

Recently, n-type Fe2VAl-based full-Heusler systems, exhibiting high thermoelectric power factors, have sparked new interest in this material class for thermoelectric applications. In this paper, the aluminum-rich region of the L2(1) phasespace in Fe-V-Al is explored by a step-by-step increase in the Al content in Fe2VAlx. We reveal a promising route to improve the thermoelectric properties of p-type Heusler alloys. First, we find an ultrahigh solubility of Al in Fe2VAlx and confirm the presence of a single Heusler phase up to x = 2 using x-ray diffraction analysis and scanning electron microscopy. Second, thermoelectric transport properties, measured in a wide temperature range from 4 to 800 K, show a substantial increase in the thermopower by over 100% and a significant decrease in the thermal conductivity by up to 80% for the Al-rich samples. Detailed analysis of the carrier-concentration-dependent thermopower as well as Hall measurements indicate the formation of a resonant state at the valence band edge as a likely origin for this enhancement. This is further corroborated by density-functional-theory calculations of the electronic density of states. Our work sets the stage for p-type full-Heusler materials with enhanced thermoelectric performance, applying the principle of resonant states to this material class. (c) 2022 Author(s).