Ferromagnetic Ni-Mn-Ga Heusler alloys have recently attracted a great deal of interest due to the giant magnetic field-induced strain effect. The composition of Ni-Mn-Ga alloys determines both the temperature of martensitic transformations and the structure type of the product phase. Martensitic structural transformation consists in an uniform lattice distortion (the so-called Bain deformation) of the parent phase showing a cubic L2₁-ordered structure. In addition, a periodic shuffling of the close-packed atomic planes, frequently called as a structural modulation can be observed. Depending on the number of satellites, s, between the main reflections of diffraction patterns, martensitic phases in Ni-Mn-Ga alloys have been associated to superstructures, involving 5, 7, 10 or even 12 lattice modulation periodicity and indicated as 5M, 7M, 10M and 12M, where nM is determined by the relation n=s+1.

In the present work, the so-called “5M” and “7M” martensitic modulated structures occurred in Ni-Mn-Ga alloy system are thoroughly studied by a powder X-ray diffraction. The results made possible a detailed description of these structures in terms of symmetry, atomic coordinates, occupancy factors and thermal parameters. Our findings indicate the presence of incommensurate and commensurate modulation properly described within superspace approach. Moreover, the presented results clarify a series of critical points and discrepancies emerging from the literature dealing with the structural studies of Ni-Mn-Ga martensitic phases. A broadly used classification of martensitic modulated structures based on the number of observed satellites must be reconsidered. In fact, it will be demonstrated that the number of satellites does not always determine the type of possible martensitic superstructures .

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