Diffraction aperiodic sound4/21/2024 ![]() “It was really quite a shock when came up with this discovery.” “It required rethinking everything we knew about what forms crystals can take,” recalls David Mermin, a physicist at Cornell University in Ithaca, New York, who would go on to spend a decade developing a classification scheme for non-periodic crystals, which, in at least one direction, do not repeat their structure as you move through the material. “In no time there was a fast-growing community of young, avant-garde scientists around the world who took my discovery and turned it into a fast-growing science.” “Hell broke loose” when the PRL paper came out, Shechtman says. With Cahn, an internationally known figure in materials science, now publicly endorsing Shechtman’s revolutionary interpretation, crystallographers immediately took notice. (The original paper would not be published for another year.) ![]() The paper was published in November 1984. Cahn called up theorist Denis Gratias of Centre National de la Recherché Scientifique in Paris, France, and the three of them, along with Blech, wrote a shorter paper that they submitted to the prominent physics journal Physical Review Letters (PRL). Meanwhile, Shechtman returned to NIST for the summer of 1984 and showed the rejected manuscript to Cahn. They then sent it to a lesser-known journal, Metallurgical Transactions A. Shechtman and Blech submitted a paper to the Journal of Applied Physics, where it was immediately rejected. With their collaborators, they came to realize that the tenfold pattern in the diffraction data was actually revealing a crystal with fivefold symmetry, something equally forbidden in crystallography. Blech developed a computer model to explain the results by postulating a crystal with icosahedral symmetry, made of groups of atoms placed in the corners of a 20-sided, soccer-shaped icosahedron joining in space according to specific “matching rules” that require the sides to fit together properly. Shechtman finished his two-year appointment at NIST, and in 1983 went back to Technion, where he showed his results to Ilan Blech, a colleague in the materials engineering department. If the tenfold diffraction pattern was a result of tenfold twins, it was not interesting,” Shechtman says. “I wanted to find the twins, record them and forget about it. In theory, twinned periodic crystals could produce a diffraction pattern mimicking that of a crystal with the forbidden symmetry.Īs his logbook showed, Shechtman spent the whole afternoon of that day trying different TEM techniques to find the twins. Based on previous experiments Shechtman performed years earlier, he knew that tenfold rotational symmetry of a diffraction pattern can result from tenfold “twins.” In the twinning phenomenon, two or more crystals of different orientations become fused together. He remembers, “I said to myself in Hebrew, ‘There ain’t no such animal,’”. But a crystal producing such an arrangement would never repeat, and contradicted the laws of crystallography that everyone, including Shechtman, had learned in school. The diffraction pattern he observed seemed to have tenfold rotational symmetry: after a tenth of a full rotation, the crystal would look the same.
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