A quasicrystal is a strange type of solid whose atomic structure is very regular but never
quite repeats. Quasicrystalline structures don't have a simple unit cell
that can be repeated periodically in all directions to fill space, although
they do have local patterns that repeat almost periodically. They also have
local rotational symmetries, such as those of a pentagon, that can't exist
in ordinary crystals.
|This pattern is the first evidence of non-spherical
particles coalescing into a "quasicrystal". Researchers reporting
in Nature used computer simulations to show how four-sided pyramids
organise themselves into different motifs (as shown by the dice) and
then into the dense 3-D pattern.
Prior to the discovery of quasicrystals, it was thought that five-fold crystal
symmetry was impossible, because there are no space-filling periodic tilings
of this kind. The best known examples of quasicrystals resemble Penrose
tilings, which use repeated copies of two different rhombi to cover
an infinite plane in intricate, interlocking patterns. In fact, some quasicrystals
can be sliced in such a way that the atoms on the surface follow the exact
pattern of the Penrose tiling.
- Senechal, M. Quasicrystals and Geometry. New York: Cambridge
University Press, 1995.