Double Half-Heuslers

Abstract

Since their discovery around a century ago, the structure and chemistry of the multi-functional half-Heusler semiconductors have been studied extensively as three component systems. The elemental groups constituting these ternary compounds with the nominal formula XYZ are well established where most stable compositions follow a valence balanced rule (a generalization of the 18 electron rule). From the very same set of well-known elements and stability guidelines 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 a reliable, first-principles thermodynamics methodology on a selection of 365 novel compositions, we predict 131 novel quaternary compounds to be stable which is already larger in number than those reported extensively for ternary systems (82). Thermoelectric performance of the state-of-the-art ternary half-Heusler compounds are limited by their intrinsically high lattice thermal conductivity (κ_L). In comparison to ternary half-Heuslers, thermal transport in double half-Heuslers is dominated by low frequency phonon modes with smaller group velocities and limited by disorder scattering. The double half-Heusler composition Ti₂FeNiSb₂ was synthesized and confirmed to have a significantly lower lattice thermal conductivity (factor of 3 at room temperature) than TiCoSb, thereby providing a better starting point for thermoelectric efficiency optimization. We demonstrate a dependable strategy to assist the search for low thermal conductivity half-Heuslers and point towards a huge composition space for implementing it. Our findings can be extended for systematic discovery of other large families of multi-component intermetallic semiconductors.