  The need for spin-injectors having the same zincblende-type crystal structure
as conventional semiconductor substrates has created significant interests in
theoretical predictions of possible metastable ``half-metallic'' zincblende
ferromagnets, which are normally more stable in other structure-types, e.g.,
NiAs. Such predictions were based in the past on differences $\Delta_{\rm
bulk}$ in the total-energies of the respective {\em bulk} crystal forms
(zincblende and NiAs). We show here that the appropriate criterion is comparing
difference $\Delta_{\rm epi}(a_s)$ in {\em epitaxial} total-energies. This
reveals that even if $\Delta_{\rm bulk}$ is small, still for MnAs, CrSb, CrAs,
CrTe, $\Delta_{\rm epi}(a_s) > 0$ for all substrate lattice constant $a_s$, so
the zincblende phase is not stabilized. For CrS we find $\Delta_{\rm epi}(a_s)
<0$, but the system is antiferromagnetic, thus not half-metallic. Finally,
zincblende CrSe is predicted to be epitaxially stable for $a_s > 6.2$ \AA\ and
half-metallic.
