Neutron diffraction and symmetry analysis of the martensitic transformation in Co-doped Ni2MnGa

Martensitic transformations are strain driven displacive transitions governing the mechanical and physical properties in intermetallic materials. This is the case in Ni2MnGa, where the martensite transition is at the heart of the striking magnetic shape memory and magnetocaloric properties. Interestingly, the martensitic transformation is preceded by a premartensite phase, and the role of this precursor and its influence on the martensitic transition and properties is still a matter of debate. In this work we report on the influence of Co doping (Ni50-xCoxMn25Ga25 with x=3 and 5) on the martensitic transformation path in stoichiometric Ni2MnGa by neutron diffraction. The use of the superspace formalism to describe the crystal structure of the modulated martensitic phases, joined with a group theoretical analysis, allows unfolding the different distortions featuring the structural transitions. Finally, a general Landau thermodynamic potential of the martensitic transformation, based on the symmetry analysis, is outlined. The combined use of phenomenological and crystallographic studies highlights the close relationship between the lattice distortions at the core of the Ni2MnGa physical properties and, more in general, on the properties of the martensitic transformations in the Ni-Mn based Heusler systems.