quasicrystal

The Columbia Encyclopedia, 6th ed.

quasicrystal

quasicrystal (kwā´zīkrĬs´təl, kwäz´ē–) or quasiperiodic solid, solid body that exhibits such crystalline features as symmetry and repeating patterns of unit cells (regular arrangements of atoms, molecules, or ions) but—unlike a crystal—requires more than one type of unit cell to achieve large-scale order, i.e., the structure cannot consist of the repetition of a single cell. Quasicrystals exhibit symmetries (e.g., icosahedral and decagonal) not seen in crystals. Quasicrystals seem to forge a link between conventional crystals and materials called metallic glasses, which are solids formed when molten metals are cooled so rapidly that their constituent atoms do not have adequate time to form a crystal lattice. The first quasicrystal was discovered in a rapidly cooled sample of an aluminum-manganese alloy by a team led by Daniel S. Shechtman at the National Bureau of Standards (now the National Institute of Standards and Technology) in Gaithersburg, Md., in 1982.

Three models have been advanced to explain the structure of quasicrystals. The Penrose model, derived from the work of British mathematician Roger Penrose by Dov Levine and Paul J. Steinhardt at the Univ. of Pennsylvania, suggests that quasicrystals are composed of two or more unit cells that fit together according to specific rules. The glass model, as refined by American physicists Peter W. Stephens and Alan J. Goldman, suggests that clusters of atoms join in a somewhat random way determined by local interactions. The random-tiling model, which combines some of the best features of the other two models, suggests that the strict matching rules of the Penrose model need not be obeyed so long as local interactions leave no gaps in the structure.

Quasicrystals have been found to be common structures in alloys of aluminum with such metals as cobalt, iron, and nickel. Unlike their constituent elements, quasicrystals are poor conductors of electricity. Quasicrystals have stronger magnetic properties and exhibit greater elasticity at higher temperatures than crystals. Because they are extremely hard and resist deformation, quasicrystals form high-strength surface coatings, which has led to their commercial use as a surface treatment for aluminum skillets.

See M. V. Jaric, ed., Introduction to Quasicrystals (1988); C. Janot, Quasicrystals: A Primer (1994); M. Senechal, Quasicrystals and Geometry (1995).

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