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Crystal structure of martensite and intermediate phases in Ni2MnGa studied by neutron diffraction
|Hiroaki Kushida 1, Koudai Fukuda 1, Tomoyuki Terai 1, Takashi Fukuda 1, Tomoyuki Kakeshita 1, Takuya Ohba 2, Toyotaka Osakabe 3, Kazuhisa Kakurai 3, Ken-ichi Kato 4|
1. Osaka University, Department of Materials Science and Engineering, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
Ferromagnetic shape memory alloy Ni2MnGa transforms from the L21 parent phase to a martensite (M-) phase via an intermediate (I-) phase. We have investigated crystal structures of M- and I-phases in stoichiometric Ni2MnGa using neutron diffraction measurement with a single crystal specimen and synchrotron X-ray diffraction measurement with powder specimen. The neutron diffraction profile of the M-phase at T = 100 K exhibits four satellite reflections between (0 2 0)P and (2 0 0)P fundamental reflections (index is given on the basis of the parent lattice). They appear at incommensurate positions of (h 2-h 0)P with h = 0.427, 0.861, 1.139 and 1.573, which are near 3/7, 6/7, 8/7 and 11/7, respectively. Intensities of four satellite reflections relative to the fundamental ones are approximately 1/10, 1/1000, 1/1000 and 1/10, respectively. They move toward the fundamental reflection with increasing temperature. The neutron diffraction profile of the I-phase at T = 210 K exhibits two satellite reflections between (0 2 0)P and (2 0 0)P fundamental reflections. They appear also at incommensurate positions of (h 2-h 0)P with h = 0.343 and 1.657, which are near 1/3 and 5/3, respectively. Intensities of these satellite reflections relative to the fundamental ones are approximately 1/100. Temperature dependence of the satellite positions is negligibly small, while their intensities decrease with increasing temperature and disappear at T = 250 K. To determine atomic positions of Ni, Mn and Ga in M- and I-phase, powder synchrotron X-ray diffraction has been carried out. Rietveld analysis of the result revealed that atoms in both phases displace along [1 -1 0]P from the positions in L21 structure. The displacements are represented by a sine wave whose propagation vector is parallel to [1 1 0]P.
Presentation: Oral at E-MRS Fall Meeting 2007, Symposium E, by Hiroaki Kushida
See On-line Journal of E-MRS Fall Meeting 2007
Submitted: 2007-05-14 08:16 Revised: 2009-06-07 00:44