quasicrystal

thumb|Potential energy surface for silver depositing on an aluminium–[[palladium–manganese (Al–Pd–Mn) quasicrystal surface. Similar to Fig. 6 in Ref.]]

A quasiperiodic crystal, or quasicrystal, is a structure that is ordered but not periodic. A quasicrystalline pattern can continuously fill all available space, but it lacks translational symmetry. While crystals, according to the classical crystallographic restriction theorem, can possess only two-, three-, four-, and six-fold rotational symmetries, the Bragg diffraction pattern of quasicrystals shows sharp peaks with other symmetry orders—for instance, five-fold.