Editing Crystal (section)

== Quasicrystals ==
[[File:Ho-Mg-ZnQuasicrystal.jpg|thumb|The material [[Holmium–magnesium–zinc quasicrystal|holmium–magnesium–zinc]] (Ho–Mg–Zn) forms [[quasicrystal]]s, which can take on the macroscopic shape of a [[dodecahedron|pentagonal dodecahedron]]. Only quasicrystals can take this 5-fold symmetry.  The edges are 2&nbsp;mm long.]]
{{Main article|Quasicrystal}}
A [[quasicrystal]] consists of arrays of atoms that are ordered but not strictly periodic. They have many attributes in common with ordinary crystals, such as displaying a discrete pattern in [[x-ray diffraction]], and the ability to form shapes with smooth, flat faces.

Quasicrystals are most famous for their ability to show five-fold symmetry, which is impossible for an ordinary periodic crystal (see [[crystallographic restriction theorem]]).

The [[International Union of Crystallography]] has redefined the term "crystal" to include both ordinary periodic crystals and quasicrystals ("any solid having an essentially discrete [[diffraction]] diagram"<ref>{{cite journal |author=International Union of Crystallography |year=1992 |title=Report of the Executive Committee for 1991 |journal=Acta Crystallogr. A |volume=48 |issue= 6|pages=922–946 |doi=10.1107/S0108767392008328|pmc=1826680 |bibcode=1992AcCrA..48..922. }}</ref>).

Quasicrystals, first discovered in 1982, are quite rare in practice. Only about 100 solids are known to form quasicrystals, compared to about 400,000 periodic crystals known in 2004.<ref>{{cite journal|author=Steurer W.|journal= Z. Kristallogr. |volume=219 |year=2004|pages= 391–446|doi=10.1524/zkri.219.7.391.35643|title=Twenty years of structure research on quasicrystals. Part I. Pentagonal, octagonal, decagonal and dodecagonal quasicrystals|issue=7–2004|bibcode = 2004ZK....219..391S |doi-access=free}}</ref> The 2011 [[Nobel Prize in Chemistry]] was awarded to [[Dan Shechtman]] for the discovery of quasicrystals.<ref>{{cite web|url=https://www.nobelprize.org/nobel_prizes/chemistry/laureates/2011/ |title=The Nobel Prize in Chemistry 2011 |publisher=Nobelprize.org |access-date=2011-12-29}}</ref>