Searching for hexagonal analogues of the half-metallic half-Heusler XYZ compounds

The XYZ half-Heusler crystal structure can conveniently be described as a tetrahedral zinc blende YZ structure which is stuffed by a slightly ionic X species. This description is well suited to understanding the electronic structure of semiconducting 8-electron compounds such as LiAlSi (formulated Li⁺[AlSi]⁻) or semiconducting 18-electron compounds such as TiCoSb (formulated Ti⁴⁺[CoSb]⁴⁻). The basis for this is that [AlSi]⁻ (with the same electron count as Si₂) and [CoSb]⁴⁻ (the same electron count as GaSb) are both, structurally and electronically, zinc blende semiconductors. The electronic structure of half-metallic ferromagnets in this structure type can then be described as semiconductors with stuffing magnetic ions which have a local moment: for example, 22-electron MnNiSb can be written Mn³⁺[NiSb]³⁻. The tendency in the 18-electron compound for a semiconducting gap—believed to arise from strong covalency—is carried over in MnNiSb to a tendency for a gap in one-spin direction. Here we similarly propose the systematic examination of 18-electron hexagonal compounds for semiconducting gaps; these would be the ‘stuffed wurtzite’ analogues of the ‘stuffed zinc blende’ half-Heusler compounds. These semiconductors could then serve as the basis for possibly new families of half-metallic compounds, attained through appropriate replacement of non-magnetic ions by magnetic ones. These semiconductors and semimetals with tunable charge carrier concentrations could also be interesting in the context of magnetoresistive and thermoelectric materials.