Sunday, October 30, 2011

Once-Ridiculed Discovery by Israeli’s Scientist Redefined the Term Crystal

Daniel Shechtman has the last laugh. In 1982, Shechtman, scientist  of  the Technion-Israel Institute of Technology in Haifa, discovered an alloy of aluminum and manganese that appeared to have fivefold symmetry: that is, the atoms in it formed a pattern that appeared essentially the same when rotated by a fifth of a turn, or 72˚. Other researchers scoffed, as such an arrangement was thought to be mathematically impossible. Yet scientists eventually realized that atoms in a solid can achieve such symmetry by arranging themselves in a pattern that almost but never quite repeats—a “quasicrystal.” Shechtman’s discovery has now gone full circle, from ridicule to ultimate accolade: It has netted this year’s Nobel Prize in chemistry.
          “He does deserve a Nobel Prize for ushering in this new kind of phase in chemistry: crystals that are not crystals,” says mathematician Roger Penrose of the University of Oxford in the United Kingdom, who played an indirect role in explaining the materials.
          Prior to Shechtman’s discovery, a crystal was defined as a material in which atoms are arranged in a regular pattern that repeats itself. That definition puts limits on the symmetry a crystal can have, as a simple child’s game shows. Suppose you want to cover a tabletop with identical tiles. A pattern of triangles does the trick, so it’s possible to make crystals with threefold symmetry. Squares or hexagons also work, so crystals with fourfold and sixfold symmetry can also be made. But pentagons won’t work; there will always be gaps between them. Thus, fivefold symmetry is impossible.
          Nevertheless, Shechtman saw what he saw. On 8 April 1982, while working at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland, he quickly cooled a sample of aluminum and manganese alloy to keep it from crystallizing and then fired a beam of electrons into it. If there was an orderly arrangement of atoms in the material, the electrons would “diffract” off the various planes of atoms in it and emerge at specific angles to produce a recognizable pattern in a detector. Shechtman saw a diffraction pattern unlike any he’d seen before: concentric c ircles of 10 bright dots. The tallies pointed to an impossible symmetry. “10 fold???” he recorded in his notebook.
            Shechtman says he was convinced on the first day, but he checked and rechecked his experiment and tried others over the next week to investigate the material further. When he finally told colleagues about his discovery, he was met with dismissal and ridicule. His claims caused such embarrassment that his boss asked him to leave the research group.  “That was the atmosphere at [NIST],” Shechtman says. But he persevered with the help of a few colleagues, and when he finally published in Physical Review Letters in November 1984, “then all hell broke loose.” It was a simple experiment that other labs could repeat in a matter of days, and his phone started ringing off the hook.
         The diffraction patterns may have been real, but how were the atoms arranged? The answer came from mathematicians who had been thinking about patterns of tiles. Some had been puzzling over curious mosaics that have a limited number of different-shaped tiles and that  fitted together in patterns that never repeated themselves. Such mosaics were used by Arabic artists as early as the 13th century to decorate buildings such as the Alhambra Palace in Granada, Spain. Mathematicians in the 1960s and ’70s strove to find the smallest number of tiles that could produce such a pattern. In the mid-1970s,Penrose came up with a set of just two rhombuses that did the job. Penrose’s pattern had plenty of pentagons and decagons.
           That set a number of chemists thinking about whether atoms could adopt a similar pattern. Crystallographer Alan Mackay built a model with circles representing atoms at the corners of Penrose’s tiles and calculated that it would produce a diffraction pattern with 10-fold symmetry. Paul Steinhardt, then at the University of Pennsylvania, and his student Dov Levine had also been devising theoretical structures based on Penrose tiling. When a colleague showed Steinhardt a preprint of Shechtman’s first paper in autumn 1984, “Ileapt up in the air. The two matched by eye beautifully,” says Steinhardt, who is now at Princeton University. Steinhardt and Levine published a paper shortly after Shechtman’s linking his observations to Penrose-like structures and coined the term “quasicrystal.” Not everyone was convinced. X-ray crystallographers did not accept it for 3 years, until a quasicrystal could be grown big enough to perform x-ray diffraction. “That was the turning point,” Shechtman says, that led the International Union of Crystallography in 1992 to change its defi nition of a crystal from a regular repeating array of atoms to “any solid having an essentially discrete diffraction pattern.”
           But double Nobelist Linus Pauling, a dominant figure among U.S. chemists who died in 1994, never accepted quasicrystals, despite Shechtman traveling to his lab in Palo Alto and giving him a personal hourlong lecture. Much remains mysterious about quasicrystals, including how such complex long range structures can form from single atoms. “They can’t be produced simply with local rules; there has to be some subtle kind of production,” Penrose says. Predicting their properties is also hard, Steinhardt says: “The mathematical techniques we use on crystals don’t work on quasicrystals.”
          Quasicrystals have been found in nature, in a mineral reported to have come from the Koryak Mountains in eastern Russia (Science, 5 June 2009, p. 1306). They have also been found in one of the world’s most durable steels, made by a company in Sweden for razor blades and surgical needles. They are beginning to find other industrial applications, such as nonstick coatings in pans, heat insulation in engines, and thermoelectric materials to salvage waste heat. “If there is one particular lesson we are taking from [Shechtman’s] research, it is not to underestimate the imagination of nature herself,” Andre Goodwin of the University of Oxford said in a statement.



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