Authors: Ilan A Blech John W Cahn Denis Gratias
Publish Date: 2012/07/31
Volume: 43, Issue: 10, Pages: 3411-3422
Abstract
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 LongRange Orientational Order and No Translational Symmetry”2 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 AlMn 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”3 We the three coauthors 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 significanceThe 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 limitationAccording to the thenprevailing 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 theoriesThe discovery challenged two basic principles of crystallography In the late 1700s René Just Haüy4 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 6fold rotation axes only 14 Bravais lattices 32 point groups 51 crystal forms and 230 space groups can be consistent with periodicity5 Throughout the 19th century all measured properties and external forms had been consistent with these restrictions In 1912 the diffraction of Xrays by crystals brilliantly confirmed both that Xrays 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 Haüy’s postulate With no exceptions reported in almost 200 years periodicity became the definition of a crystal and an axiom or a law of crystallographyDiffraction 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 sharp6 Quasiperiodic objects have no lattice and their diffraction spots will not form a reciprocal lattice Because of the 5fold axis a frequent ratio of spacing of spots in Figures 2 and 61 is the golden mean τ = 2 cos π/5 = 1 + √5/2 There is no reciprocal lattice in those diffraction patternsAll of the experimental work was done at NBS/NIST Robert J Schaefer initiated the study of rapid solidification of dilute AlMn alloys because of his interest in achieving plane front solidification that would produce alloys free of microsegregation7 It was Dan Shechtman’s decision to explore higher manganese content The alloys containing the quasicrystals were prepared by Schaefer and Frank Biancaniello CR Hubbard did the Xray experiments Denis Gratias travelled to NIST from the Institute of Theoretical Physics at Santa Barbara to work with us Special thanks are due to the NIST’s internal review committee which took unusual care to review the PRL manuscript quickly and thoroughly The PRL paper was reviewed and in print 5 weeks after submissionShechtman observed the electron diffraction patterns with its icosahedral symmetry in the spring of 1982 at NIST but he only showed the 10fold diffraction data to his NIST colleagues Without seeing his darkfield images in Figure 4 in the original Metallurgical Transactions A paper the common reaction was that he was seeing multiple twinning Most of us did not realize that twinning of periodic crystals would have resulted in overlapping reciprocal lattices and distortions neither are found in the patterns Quasiperiodicity was unknown to us so was Mackay’s demonstration that Penrose’s aperiodic twodimensional tiling would diffract sharply7 Discussions at NIST ceased and no further work was done on quasicrystals Before Shechtman returned to Israel in the fall of 1983 he completed another important rapid solidification work8
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