Brief comm 20/1/00 mx

brief communications
Alloys
Atomic structure of the
quasicrystal Al
72
Ni
20
Co
8
Steinhardt et al.
1
reported experimental evi-
dence in support of the coverage model
2–4
by presenting a structure for the high-per-
fection decagonal quasicrystal Al
72
Ni
20
Co
8
.
The coverage model describes the decagon-
al quasicrystals by a single type of tile, a
decagon, the basic 2-nm cluster of which is
allowed to overlap so as to cover the surface.
Although their Z-contrast image (also
referred to as a high-angle annular dark-
field image) supports the coverage model,
we find that the atomic structure they pro-
pose for the decagon has significant short-
comings which are inconsistent with our
Z-contrast images.
The correct atomic structure of the
decagon is critical to understanding how
and why quasicrystals form. We have inves-
tigated the same high-perfection decagonal
quasicrystal Al
72
Ni
20
Co
8
(provided by A. P.
Tsai) by Z-contrast imaging, but at a higher
spatial resolution (~0.13 nm) than Stein-
hardt et al.
1
.
The main feature in the proposed struc-
ture of the decagon
1
is the extensive broken
Figure 1 Comparison of the proposed “best-fit candidate model”
1
for the 2-nm cluster of the high-perfection decagonal quasicrystal
Al
72
Ni
20
Co
8
with our higher resolution 
Z-contrast image. a, The proposed model contains broken decagonal symmetry for the entire clus-
ter: see, for example, the four yellow columns indicated by green arrows. b,
Z-contrast image of a 2-nm cluster of the same high-perfec-
tion decagonal quasicrystal Al
72
Ni
20
Co
8 
along the ten-fold axis. The bright yellow features show the locations of highest intensity
corresponding to high transition-metal (TM) occupancy. Al columns have lower intensity and are seen as red features: see, for example,
the ten red spokes around the central ring. These spokes connect the central ring to the ring of ten TM columns (yellow) and show the
basic ten-fold symmetry of the cluster. c, The model cluster of Steinhardt 
et al. superimposed on our Z-contrast image shows that their
four proposed TM columns indicated by green arrows are not present.
Figure 2 Cluster showing broken decagonal symmetry within the
central ring, but there are still ten columns present, five of which
have higher intensities (yellow), indicating high TM occupancy,
whereas the other five show high Al occupancy (red). One of the
five TM columns is a single column; the others form closely
spaced column pairs, very similar to those in the outer 2-nm ring. 
© 2000 Macmillan Magazines Ltd