Double Half-Heuslers

Since their discovery around a century ago, multi-functional half-Heusler semiconductors have been studied extensively as three-component systems (nominal formula XYZ) with valence balanced compositions. From the very same set of elements and stability rules, we explore a much larger phase space of possible quaternary double (X′X″Y₂Z₂, X₂Y′Y″Z₂, and X₂Y₂Z′Z″), triple (X₂′X″Y₃Z₃), and quadruple (X₃′X″Y₄Z₄) half-Heusler compounds. Using reliable, first-principles thermodynamics on a selection (365) of previously unexplored compositions, we predict more quaternary compounds (131) than those predicted or reported extensively for the ternary systems (84). In comparison to state-of-the-art ternary half-Heusler thermoelectrics, for which performance is limited by their intrinsically high thermal conductivity (κ), κ in double half-Heuslers is dominated by smaller group velocity phonons and limited by disorder scattering. The double half-Heusler Ti₂FeNiSb₂ was synthesized and confirmed to have a significantly lower κ than TiCoSb, thereby providing a better starting point for thermoelectric efficiency optimization. Ternary half-Heusler compounds exhibit a fertile transition metal chemistry with properties relevant for themoelectrics, spintronics, topological band structures, and transparent conducting thin-film applications. For thermoelectric applications in particular, half-Heusler compounds are regularly reported with high thermoelectric power factors. However, thermoelectric efficiency in half-Heuslers suffers from intrinsically high thermal conductivities. Here, we explore a vast class of relatively unexplored double half-Heusler compounds that are constrained by crystal chemistry to exhibit much lower lattice thermal conductivities. Hence, we demonstrate a dependable strategy to assist the search for low-thermal-conductivity half-Heuslers and point toward a huge composition space for implementing it. Our findings can be extended for systematic discovery of other large families of multi-functional intermetallic semiconductors. Despite decades of research on half-Heusler thermoelectrics, they are predominantly investigated as ternary compounds. From the same chemical phase space, we explore an enormous class of double half-Heuslers based on aliovalent substitution with ordered ground states. These compounds are constrained via crystal chemistry to exhibit much lower thermal conductivities (κ_L) relative to the well-known ternary compounds. Additional avenues for κ_L reduction via alloying with their “defective” ternary components emerge that significantly advance the search for high-performance thermoelectric half-Heuslers.