  For a material that is a half-metal, there should exist a range of
compositions for half-metallicity. This compositional range can be expressed in
terms of electron count and computed. We investigate electronic and magnetic
properties of doped full- and half-Heusler alloys (stoichiometry XYZ2 and XYZ,
respectively) with elements X from groups 13-16 and periods 3-6 of the Periodic
Table, Y={Mn, Fe}, and Z={Co, Ni}. Using spin density functional theory, we
predict shifts of the Fermi energy in the doped and solid-solution alloys.
These predictions can be used for band-gap engineering of multicomponent
half-metals and provide the viable range of compositions, such as for a range
of n=x+y+z in (Co$_{2-z}Ni_z)$(Mn$_{1-y}$Fe$_{y}$)(Sn$_{1-x}$Sb$_{x}$). This
methodology for doped and chemically disordered half-metallic alloys offers a
design approach to electronic-structure engineering that can accelerate
development of half-metals for novel electronic and spintronic applications.
