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Half-Heusler compounds XYZ, also called semi-Heusler compounds, crystallize in the C1 b MgAgAs structure, in the space group. We report a systematic examination of band gaps and the nature (covalent or ionic) of bonding in semiconducting 8-and 18-electron half-Heusler compounds through first-principles density functional calculations. We find that the most appropriate description of these compounds from the viewpoint of electronic structures is one of a YZ zinc blende lattice stuffed by the X ion. Simple valence rules are obeyed for bonding in the 8-electron compound. For example, LiMgN can be written Li++(MgN)− and (MgN)−, which is isoelectronic with (SiSi), forms a zinc blende lattice. The 18-electron compounds can similarly be considered as obeying valence rules. A semiconductor such as TiCoSb can be written Ti 4++(CoSb) 4−; the latter unit is isoelectronic and isostructural with zinc-blende GaSb …