  Understanding the crystal structure of magnetic shape memory alloys is
crucial for insights into their unique properties, such as the high mobility of
twin boundaries and magnetic field functionality. The complex neutron
diffraction patterns betweeen 10-300 K indicate an incommensurate and
simultaneously anharmonic modulation function (AMF) in Ni50.0Mn27.7Ga22.3 10M
martensite. Our identification of the dominant Fourier components in the AMF
allowed for a comparison between calculated diffraction patterns and
experiments. The AMF explains the appearance of peculiar small-intensity
diffraction peaks when nearly commensurate AMF at 300 K turns to incommensurate
AMF upon cooling. Further analysis reveals that divergent periodicity between
the incommensurate modulation and the lattice leads to the formation of
additional nanodomains. Interpreting the modulation displacements within the
nanodomains in the terms of the (2-3)2 stacking sequence of basal (110) planes
enables to understand that these nanodomains are emerging a/b nanotwins. The
nanotwinning interlinked with incommensurate modulation also explains the
transition from the tetragonal to the orthorhombic symmetry upon cooling. We
identify specific low-temperature orthorhombic structures, like 34O, 24O, 14O,
which are significant as their unit cell simultaneously represents an
a/b-nanotwin. The ab initio calculations confirm the stability and low and
comparable energy of all found nanotwinned structures. Based on the presumed
stability of the nanotwinned state and literature comparisons, we propose that
the low temperature state of the martensite nominally marked as five-layered
modulated, 5M or 10M, is one of the specific a/b-nanotwinned configurations
such as 34O, 24O, 14O. The exact choice depends on the composition, but 14O
resulting from q = 3/7 is the ultimate limit with a corresponding smallest twin
domain size of 3.5 nm.
