  The dodecagonal quasicrystal classified into the five-dimensional space group
P126/mmc, recently discovered in a Mn-Cr-Ni-Si alloy, has been analyzed using
atomic-resolution spherical aberration-corrected electron microscopy, i.e.
high-angle annular dark-field scanning transmission electron microscopy
(HAADF-STEM) and conventional transmission electron microscopy. By observing
along the 12-fold axis, non-periodic tiling consisting of an equilateral
triangle and a square has been revealed, of which common edge length is a =
4.560 A. These tiles tend to form a network of dodecagons of which size is
(2+Sqrt(3))a ~ 17 A in diameter. The tiling was interpreted as an aggregate of
100 A-scale oriented domains of high- and low-quality quasicrystals with small
crystallites appearing at their boundaries. The quasicrystal domains exhibited
a densely-filled circular acceptance region in the phason space. This is the
first observation of the acceptance region in an actual dodecagonal
quasicrystal.
  Atomic structure model consistent with the electron microscopy images is a
standard Frank-Kasper decoration of the triangle and square tiles, that can be
inferred from the crystal structures of Zr4Al3 and Cr3Si. Four kinds of layers
located at z = 0, +-1/4 and 1/2 are stacked periodically along the 12-fold
axis, and the atoms at z = 0 and 1/2 form hexagonal anti-prisms consistently
with the 126-screw axis. The validity of this structure model was examined by
means of powder X-ray diffraction.
