QUASICRYSTAL

also called quasi-periodic crystal, matter formed atomically in a manner somewhere between the amorphous solids of glasses (special forms of metals and other minerals, as well as common glass) and the precise pattern of crystals. Like crystals, quasicrystals contain an ordered structure, but the patterns are subtle and do not recur at precisely regular intervals. Rather, quasicrystals appear to be formed from two different structures assembled in a nonrepeating array, the three-dimensional equivalent of a tile floor made from two shapes of tile and having an orientational order but no repetition. Although when first discovered such structures surprised the scientific community, it now appears that quasicrystals rank among the most common structures in alloys of aluminum with such metals as iron, cobalt, or nickel. While no major commercial applications yet exploit properties of the quasicrystalline state directly, quasicrystals form in compounds noted for their high strength and light weight, suggesting potential applications in aerospace and other industries. Michael Widom also called Quasi-periodic Crystal, matter formed atomically in a manner somewhere between the amorphous solids of glasses (special forms of metals and other minerals, as well as common glass) and the precise pattern of crystals. Like crystals, quasicrystals contain an ordered structure, but the patterns are subtle and do not recur at precisely regular intervals. Rather, quasicrystals appear to be formed from two different structures assembled in a nonrepeating array, the three-dimensional equivalent of a tile floor made from two shapes of tile and having an orientational order but no repetition. Though their existence was debated for some time, the first quasicrystals were definitely discovered in 1984 by researchers working independently at the National Bureau of Standards (now the National Institute of Standards and Technology) in Gaithersburg, Md., U.S., and at the University of Pennsylvania. Afterwards, investigators at many laboratories pursued studies of this curious new material, finding many new ways to make it (including one, solid-state interdiffusion, that does not even require melting the source materials) and producing for the first time quasicrystals that are large enough to see with the naked eye and have a geometry never before seen in crystallography or mineralogy (a triaconthedron, a polyhedron with 30 identical diamond-shaped faces). Theorists speculate that some quasicrystals may have value as superconductors. Their lattice structures, which seem to be more rigid than those of crystals, make it probable that quasicrystals will prove to be harder than steel, making them potentially useful for making ultrahard tools. Additional reading Works on solids in general include Lawrence H. Van Vlack, Elements of Materials Science and Engineering, 6th ed. (1989), an elementary textbook; Charles A. Wert and Robb M. Thomson, Physics of Solids, 2nd ed. (1970), an intermediate-level text; Charles Kittel, Introduction to Solid State Physics, 6th ed. (1986), the standard college textbook; Neil W. Ashcroft and N. David Mermin, Solid State Physics (1976), an advanced textbook; George E. Bacon, The Architecture of Solids (1981), an introduction to bonding and structure; and Linus Pauling, The Nature of the Chemical Bond and the Structure of Molecules and Crystals, 3rd ed. (1960, reissued 1989), the classic reference work on chemical bonding. Gerald D. Mahan Introductions to quasicrystals in particular are available in David R. Nelson, Quasicrystals, Scientific American, 255(2):4351 (August 1986); Peter W. Stephens and Alan I. Goldman, The Structure of Quasicrystals, Scientific American, 264(4):4447, 5053 (April 1991); and P.J. Steinhardt, Icosahedral Solids: A New Phase of Matter?, Science, 238(4831):124247 (Nov. 27, 1987). Martin Gardner, Mathematical Games, Scientific American, 236(1):110112, 115121 (January 1977), discusses Penrose tilings and their remarkable properties. More technically detailed works are D.P. DiVencenzo and P.J. Steinhardt (eds.), Quasicrystals: The State of the Art (1991); and the series Aperiodicity and Order, ed. by Marko V. Jaric (1988 ). Michael Widom