Crystal structure of three Ni-Mn-Ga alloys in powder and bulk materials

In NiMnGa alloy the crystal structure of low temperature phase is strongly related to transformation temperature. In the present study three NiMnGa alloys with different transformation temperatures were studied. Alloy A has the martensitic start temperature (M-s) near ambient temperature and M. of Alloy B is near Curie point (T-C) whereas M-s of Alloy C M-s is well above T-C. The crystal structures of three alloys were studied by X-ray diffraction in powder and bulk materials. The low temperature phase of single crystal in Alloy A was tetragonal phase with c < a, in Alloy B it was mixture of orthorhombic phase and tetragonal phase with c > a, and in Alloy C such a phase was tetragonal phase with c > a. The crystal structure of Alloy A in bulk material is in good agreement with powder diffraction, but in Alloys B and C there is a lack of conformity between bulk and powder. Furthermore, the difference of crystal structure in powder and bulk is discussed in more detail.