Crystal structure determination of a new monoclinic phase in NiMn system in a self-assembled chessboard-like microstructure

Vacuum induction melted equi-atomic NiMn alloy has been studied through x-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD pattern of the as-solidified alloy may be matched with the tI2 eta ' -NiMn phase and the cF4 Ni3Mn phase simultaneously. In the TEM bright field image of the same alloy, alternating bright and dark contrast with chessboard-like appearance is observed. Electron diffraction patterns from the bright phase match with the tI2 NiMn phase. The electron diffraction patterns from the dark phase are related to the tetragonal phase with additional superlattice reflection along [110] and [112]. The crystal structure of the new phase has been found to be polymorphically related mP8 phase with space group P 2/m (a = 5.02 angstrom, b = 5.16 angstrom, c = 3.61 angstrom). The phase has not been reported so far in the Ni-Mn system. The chessboard-like morphology between a tetragonal and monoclinic phase is being reported for the first time.