  We search for new fully compensated half metals, in which only one electronic
spin channel is conducting and there exists no net magnetic moment. We focus on
half Heusler alloys and we examine the physical consequence of different
crystal structures found in the literature for these compounds, XMnZ, with a
transition metal element, such as Cr, Mn, and Fe for X and a nonmetallic
element, such as P, Sb and Si for Z. The structures differ in the placement of
voids in the L2$_1$ structure of the full Heulser alloy. One structure has the
void at (1/4, 3/4, 1/4)a and the other places the void at (0.0, 0.0, 1/2)a. The
first structure is expected to have greater d-p hybridization between Mn and
the Z atom. The other exhibits strong d-d hybridization between the nearest
neighboring transition metal elements. Five XMnZ compounds are considered along
with the previously studied CrMnSb in the second structure, which serves as a
reference. Besides the CrMnSb, only one other alloy, MnMnSi, shows fully
compensated half metallic properties in both structures. Both these alloys obey
the Slater-Pauling electron counting rule for half Hesuler alloys. The
differences between CrMnSb and MnMnSi in the two structures are discussed based
on their atomic properties. In the search for fully compensated half metals in
transition metal-based half Heusler alloys, we suggest using the counting rule
as a guide.
