  Tetragonal CuMnAs was the first antiferromagnet where reorientation of the
N\'eel vector was reported to occur by an inverse spin galvanic effect. A
complicating factor in the formation of phase-pure tetragonal CuMnAs is the
formation of an orthorhombic phase with nearly the same stoichiometry.
Pure-phase tetragonal CuMnAs has been reported to require an excess of Cu to
maintain a single phase in traditional solid state synthesis reactions. Here we
show that subtle differences in diffraction patterns signal pervasive
inhomogeneity and phase separation, even in Cu-rich Cu$_{1.18}$Mn$_{0.82}$As.
From calorimetry and magnetometry measurements, we identify two transitions
corresponding to the N\'eel temperature (T$_N$) and an antiferromagnet to weak
ferromagnet transition in Cu$_{1.18}$Mn$_{0.82}$As and
CuMn$_{0.964}$As$_{1.036}$. These transitions have clear crystallographic
signatures, directly observable in the lattice parameters upon in-situ heating
and cooling. The immiscibility and phase separation could arise from a
spinoidal decomposition that occurs at high temperatures, and the presence of a
ferromagnetic transition near room temperature warrants further investigation
of its effect on the electrical switching behavior.
