A paper, ‘‘The Microstructure of Rapidly Solidified Al6Mn,’’ [1] was submitted for publication in October 1984 by D. Shechtman and I. Blech to the Metallurgical Transactions A (now Metallurgical and Materials Transactions) after having been rejected by The Journal of Applied Physics (JAP) in the summer of 1984. A second paper, ‘‘Metallic Phase with Long-Range Orientational Order and No Translational Symmetry,’’ was submitted within a week by Dan Shechtman and coworkers to the Physical Review Letters (PRL). Both papers announced the creation by rapid solidification, at the National Bureau of Standards (NBS)—now the National Institute of Standards and Technology (NIST)— of a sharply diffracting metallic Al-Mn solid phase that, because of its icosahedral symmetry, could not be periodic. In 2011, Dan Shechtman was awarded the Nobel Prize in Chemistry for this discovery. The Award cites him for ‘‘changing the way chemists looked at the solid state.’’ We, the three co-authors of these papers, are pleased to have been invited by the editor of Metallurgical and Materials Transactions to recount our participation in this work and to summarize its significance. The two papers differ in several ways. The Physical Review Letters paper was confined to the compelling case made by the NBS experiments alone that challenged several prevailing paradigms of crystallography. The Metallurgical Transactions A paper had, in addition, a model created by Ilan Blech, then at the Technion, demonstrating that an icosahedral electron diffraction pattern could result from a special sort of an icosahedral glass in which the translational symmetry is broken while retaining icosahedral orientational symmetry. This model was referred to, but left out of the Physical Review Letters, for three main reasons: (1) The experimental case by itself was strong and sufficient to force a change in thinking, (2) the model was open to criticism and might distract attention from the experiments, and (3) Physical Review Letters has a page limitation. According to the then-prevailing crystallographic theories, crystals with icosahedral symmetry could not exist. Within a short time of publication, the existence of many others with forbidden symmetries were reported. Their undeniable existence and properties formed a classic example of the truism that experiments are unsurpassed at disproving theories. Nothing else was needed to force a change in the prevailing theories. The discovery challenged two basic principles of crystallography. In the late 1700s, Rene Just Hauy postulated that all crystals were made up of clusters of atoms repeated periodically in three dimensions. The severe restrictions that periodicity places on crystals became a cornerstone of crystallography. In the 19th century, it was established that only 1-, 2-, 3-, 4-, and 6-fold rotation axes, only 14 Bravais lattices, 32 point groups, 51 crystal forms, and 230 space groups can be consistent with periodicity. Throughout the 19th century, all measured properties and external forms had been consistent with these restrictions. In 1912, the diffraction of X-rays by crystals brilliantly confirmed both that X-rays were short wavelength light and that crystals were periodic. The point group symmetries of the diffraction patterns, which are the same as those of the objects, also conformed to what was allowed by Hauy’s postulate. With no exceptions reported in almost 200 years, periodicity became the definition of a crystal and an axiom or a law of crystallography. Diffraction from periodic objects results in sharp spots arrayed on a reciprocal lattice. The converse, that sharp diffraction spots could only come from a periodic object, was a widely accepted fallacy. By the definition of quasiperiodicity, the diffraction from quasiperiodic objects is sharp. Quasiperiodic objects have no lattice, and their diffraction spots will not form a reciprocal lattice. Because of the 5-fold axis, a frequent ratio of spacing of spots in Figures 2 and 6* is the golden mean, ILAN A. BLECH, 25551 Burke Lane, Los Altos Hills, CA 94022. JOHN W. CAHN, Affiliate Professor, UW Physics, Senior Fellow Emeritus, is with the National Institute of Standards & Technology, Gaithersburg, MD 20899. Contact e-mail: jwcahn@gmail.com DENIS GRATIAS, Research Director, is with the Laboratoire d’Etudes des Microstructures CNRS ONERA, 92322 Châtillon, France. Article published online July 31, 2012 *Figure numbers refer to figures in the Metallurgical Transactions A paper.
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Jun 25, 2020
Fateh Boudardara + 1
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