Incommensurate modulated structure of the ferromagnetic shape-memory Ni2MnGa martensite

The ferromagnetic shape memory (MSM) alloy Ni2MnGa undergoes a martensitic transformation (MT) at T = 220K on cooling. The structure of this phase is studied by powder X-ray diffraction experiment. The analysis of the experimental data combined with the huge information reported in literature allowed to conclude that the Ni2MnGa martensite shows an incommensurate modulated structure closely related to a five-fold layered superstructure. The symmetry of the basic structure is found to be orthorhombic. The structure is refined by Rietveld method with superspace group Immm(00g)s00 having a = 4.2187(1) angstrom, b = 5.5534(1) angstrom and c = 4.1899(1) angstrom and modulation vector q = 0.4248(3)c*. The results show that the modulation is mainly related to the periodic shuffling of the atomic layers perpendicular to the c-axis of the orthorhombic basic structure.