Double Half-Heuslers

Summary

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.