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.

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