Half‐Heusler materials as model systems for phase‐separated thermoelectrics

Semiconducting half‐Heusler compounds based on NiSn and CoSb have attracted attention because of their good performance as thermoelectric materials. Nanostructuring of the materials was experimentally established through phase separation in (T1−x′Tx″)T(M1−yMy′)alloys when mixing different transition metals (T, T′, T″) or main group elements (M, M′). The electric transport properties of such alloys depend not only on their micro‐ or nanostructure but also on the atomic‐scale electronic structure. In the present work, the influence of the band structure and density of states on the electronic transport and thermoelectric properties is investigated in detail for the constituents of phase‐separated half‐Heusler alloys. The electronic structure is calculated using different theoretical schemes for ordered and disordered materials. It is found that chemical disorder scattering influences the electronic transport properties in all substituted materials. Substitution in NiSn‐based compounds leads to high performance n‐type materials but only moderate p‐type thermoelectric properties. The latter is caused by the influence of the valence band on the conductivity. For CoSb‐based compounds, it is found that Sb substitution with Sn keeps the bands close to the Fermi energy intact. The resulting substituted alloys are excellent p‐type materials because of the characteristic valence band structure in the Λdirection. The figure shows the fcccrystal structure (C1b) of the half‐Heusler compounds (prototype: MgAgAs, cF12, F:4‾3m, 216).