The Heusler compounds are an exciting class of intermetallics due to their ability to adopt a wide range of tunable electrical and magnetic properties, including metallicity, half metallic ferromagnetism, superconductivity, and narrow band gap semiconducting behavior with large thermoelectric power factors. Additionally, some of the Half Heuslers are predicted to be topological insulators and Weyl Semi-metals, making this family a candidate system for creating multifunctional topological heterostructures and exploring the new physics that emerges at their interfaces.
Thermoelectric Half-Heusler Alloys
A not so obvious feature of half-Heusler alloys is that those containing either 8 or 18 valence electrons per primitive unit cell tend to be semiconductors despite being composed of metallic elements. These closed shell electron arrangements (eg. d10 + s2 + p6 = 18) enable a semiconducting gap to open between the bonding and antibonding states. The rule of 18 allows for covalent hybridization of the transition elements; however, the Z element plays a critical role in not only supporting the hybridized bonding with the transition metals but in accommodating the extra d electrons necessary to stabilize the system. The small band gap semiconducting nature of the half-Heuslers and the vast compositional variations that are possible has spawned a great deal of interest in exploiting these material properties for thermoelectrics. The rapidly rising density of states and p-orbital states near the band edges are expected to contribute to the large power factors in half-Heuslers. Additionally, as the valence electron count is slightly increased or decreased through alloying, it ideally provides a means of determining if the material will be p-type or n-type.