Competing tetragonal and monoclinic phases in $Ni2.2Mn0.80Ga$

Temperature dependent synchrotron x-ray powder diffraction, differential scanning calorimetry, and magnetic measurements were performed on $Ni2+xMn1−xGa$ (⁠$x=0.20$ and 0.35) magnetic shape memory alloys. For $x=0.20$⁠, though the monoclinic phase is thermodynamically stable, a trace of residual stress can stabilize a tetragonal phase. The residual-stress-induced tetragonal phase transforms to the cubic austenite phase over an unusually large temperature range $(348K≤T≤693K)$⁠, suggesting extremely slow kinetics of transformation. In contrast to $x=0.20$⁠, the thermodynamically stable phase of $x=0.35$ is tetragonal and this composition exhibits the usual features of a reversible martensitic transformation. The results suggest that for $x=0.20$ the monoclinic and tetragonal phases are nearly degenerate.