Fig. 1: A 1s rotation diffraction pattern of a decagonal Al-Co-Ni quasicrystal recorded with monochromatic X-rays on a MarResearch imaging plate detector system, exposure time: 120 seconds, wavelength: 0.875 Å, crystal-to-detector distance: 100 mm.
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Structural investigations of crystalline materials have contributed much to the present day understanding of the solid state. The discovery of quasicrystals, crystalline materials with an altogether different kind of ordering scheme, has extended the traditional concept that crystalline matter is a periodic arrangement of identical units (atoms, cluster of atoms or molecules). The typical quasicrystal is an intermetallic compound in which the building blocks are arranged in a non-periodic but highly ordered way. In fact, the long-range order can be as good as in perfect silicon crystals. Some of these novel materials, e.g. Al-Co-Ni alloys, show even a transition from periodic to aperiodic state and vice versa upon heating. In the course of this transition a rearrangement of atoms takes places which locally can cause disorder. It is one of the fascinating aspects of quasicrystals that they can exhibit perfect long-range order in the presence of short-range disorder.
