Commensurate and incommensurate "5M" modulated crystal structures in Ni-Mn-Ga martensitic phases

It is well known that the composition of ferromagnetic shape memory Ni-Mn-Ga Heusler alloys determines both temperature of martensitic transformations and the structure type of the product phase. In the present work we focused our attention on the structural study of the so-called ''5M'' modulated structure. In particular, the structure of Ni1.95Mn1.19Ga0.86 martensitic phase is analysed by powder X-ray diffraction (PXRD) and compared with that of the stoichiometric Ni2MnGa martensite. The study of the diffraction data reveals the occurrence of commensurate (C) structural modulation in Ni1.95Mn1.19Ga0.86; this contrasts with Ni2MnGa, where an incommensurate (IC) structural modulation was evident. The two phases also differ in the symmetry of the fundamental martensitic lattice. In fact, the incommensurate modulation is related to an orthorhombic basic structure, while the commensurate variant presents a monoclinic symmetry. The commensurate modulated structure has been investigated by using the superspace approach already adopted to solve the structure of Ni2MnGa martensite. The structure has been determined by Rietveld refinement of PXRD data.