Structural studies of Ni2+xMn1-xGa by powder x-ray diffraction and total energy calculations

The crystal structure of Ni2+xMn1-xGa has been studied as a function of composition (x) by powder x-ray diffraction. For Ni2.24Mn0.75Ga, 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.