  The effect of Cu substitution on the stability of the CuMnAs tetragonal phase
was studied both experimentally and by ab initio calculations. Polycrystalline
samples with various compositions Cu$_{1+x}$Mn$_{1-x}$As $(x = 0 - 0.5)$ were
synthetized. The tetragonal phase of CuMnAs is found to be stabilized by
substituting Mn by Cu in the amount of $x$ ~ 0.1 or higher. This observation is
supported by ab initio calculations of the total energy of the tetragonal and
orthorhombic phases; with increasing Cu content the tetragonal phase is
favoured. Small variations of composition thus allow to grow selectively one of
these two phases with distinct and unique features for antiferromagnetic
spintronics.
  Measurements of magnetic susceptibility and differential scanning calorimetry
have shown that the tetragonal Cu$_{1+x}$Mn$_{1-x}$As has an antiferromagnetic
behaviour with the maximum N\'eel temperature $T_N$ = 507 K for the highest Mn
content samples, decreasing with the decreasing Mn content.
