The crystal structure of ${\mathrm{Ni}}_{2+x}{\mathrm{Mn}}_{1-x}\mathrm{Ga}$ has been studied as a function of composition $\left(x\right)$ by powder x-ray diffraction. For ${\mathrm{Ni}}_{2.24}{\mathrm{Mn}}_{0.75}\mathrm{Ga}$, where one-fourth of the Mn atoms are replaced by Ni, the experimentally determined lattice constants are in good agreement with theoretical equilibrium lattice constants calculated by minimization of total energy using full potential linearized augmented plane-wave method. For $0.15⩽x⩽0.35$, a nonmodulated tetragonal martensitic phase is obtained at room temperature, whose lattice constant $c$ increases and $a$ decreases linearly with increasing $x$ following Vegard’s law. A $7M$ modulated monoclinic phase is obtained for $x=0.2$ due to annealing. The small width of the hysteresis of the martensitic transition shows its thermoelastic nature that is the characteristic of a shape memory alloy. Phase coexistence is observed for $0.1⩽x⩽0.15$ at room temperature, confirming the first-order nature of the martensitic transition.

• Received 31 July 2006

DOI:https://doi.org/10.1103/PhysRevB.75.104107