Abstract
Half-Heusler compounds XYZ. also called serni-Heusler compounds,
crystallize in the C1(b) MgAgAs structure, in the space group F (4) over
bar 3m. 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 Ti4+ + (CoSb)(4-); the latter unit is isoelectronic and
isostructural with zinc-blende GaSb. For both the 8- and the 18-electron
compounds, when X is fixed as some electropositive cation, the computed
band gap varies approximately as the difference in Pauling
electronegativities of Y and Z. What is particularly exciting is that
this simple idea of a covalently bonded YZ lattice can also be extended
to the very important magnetic half-Heusler phases; we describe these as
valence compounds, i.e. possessing a band gap at the Fermi energy albeit
only in one spin direction. The local moment in these magnetic compounds
resides on the X site.
