Determination of hyperfine fields and atomic ordering in NiMnFeGe exhibiting martensitic transformation

The hyperfine fields and atomic ordering in Ni_1−xFe_xMnGe (x = 0.1, 0.2, 0.3) alloys were investigated using X-ray diffraction and Mössbauer spectroscopy at room temperature. The X-ray diffraction measurements show that the samples with x = 0.2, 0.3 crystallized in the hexagonal Ni₂In-type of structure, whereas in the sample with x = 0.1, the coexistence of two phases, Ni₂In- and orthorhombic TiNiSi-type of structures, were found. The Mössbauer spectra measured with x = 0.2, 0.3 show three doublets with different values of isomer shift (IS) and quadrupole splitting (QS) related to three different local surroundings of Fe atoms in the hexagonal Ni₂In-type structure. It was shown that Fe atoms in the hexagonal Ni₂In-type structure of as-cast Ni_1−xFe_xMnGe alloys are preferentially located in Ni sites and small amount of Fe is located in Mn and probably in Ge sites. The spectrum for x = 0.1 shows the doublets in the central part of spectrum and a broad sextet. The doublets originate from the Fe atoms in the paramagnetic state of hexagonal Ni₂In-type structure, whereas the sextet results from the Fe atoms in orthorhombic TiNiSi-type structure.