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October 5th, 2011

With quasicrystal discovery, Daniel Shechtman earns Nobel Prize in Chemistry

Published on October 5th, 2011 | By: pwray@ceramics.org

Daniel Shechtman’s diffraction pattern was tenfold: turning the picture a tenth of a full circle (36 degrees) results in the same pattern. Credit: Shechtman; Royal Swedish Academy of Sciences.

Daniel Shechtman, while working at NIST alongside other luminaries, such as John Cahn, set the physics and materials science world atwitter (even before Twitter!) in 1984 when Physical Review Letters (doi:10.1103/PhysRevLett.53.1951) published a paper by him, Cahn, Denis Gratias and Ilan Blech reporting the discovery of a material that had a unique diffraction pattern (above) suggestive of a crystalline structure but apparently lacked a regularly ordered and repeating three-dimensional pattern.

The discovery reported was duly attributed to Shechtman. In 1982, he stumbled upon the phenomenon while studying an aluminum-mangnese alloy. The unexpected appearance of ten major dots in each concentric circle in the material’s diffraction pattern initially baffled Shechtman and others because it was unknown in crystallographic guides and seemed to violate the basic rules of crystallography. He followed up the initial data with other experiments that indicated the material had a five-fold symmetry, a characteristic that was thought to be impossible.

Cahn et al.’s contribution to Shechtman’s work was primarily to confirm his findings and conclusions about the existence of what came to be known as quasicrystals.

What Shechtman had discovered, in essence, in the Al-Mn alloy is that the five-fold symmetry creates an aperiodic regular “patternor “quasiperiodic” structure. Perhaps the easiest way to wrap one’s thinking around an regular aperiodicity is to look at the work, coincidentally done just several years before Shechtman’s discovery, by mathematicians, such as Roger Penrose, who created special mosaics with a limited number of tiles, a limitation that provides the appearance of some pattern similarities, while creating patterns that never actually repeat (see example, below). A Fibonacci sequence is another familiar example of regular aperiodicity.

Pentagonal aperiodic tiling by Roger Penrose using only two sizes of tiles. Identifying the vertices as atomic positions generates a quasiperiodic structure. Credit: R. Penrose; Royal Swedish Academy of Sciences.

The work of Penrose and others eventually provided Shechtman (and others who joined the investigation of quasicrystals) an explanation of how the material might actually be structured.

Shechtman’s assertions made him an outcast for a few years, but his dogged pursuit of an explanation of his findings eventually put him ahead of other researchers who, as it turns out, had observed similar patterns and data but had too-hastily dismissed the diffractions as being the result of twinned or intermingled crystals.

Background material provided by the Royal Swedish Academy of Sciences reports that hundreds of different types of quasicrystals have now been synthesized and that at least one natural mineral has been found to have that structure. Here is what the Academy says about the uses of quasicrystals:

When trying out different blends of metal, a Swedish company managed to create [a] steel with many surprisingly good characteristics. Analyses of its atomic structure showed that it consists of two different phases: hard steel quasicrystals embedded in a softer kind of steel. The quasicrystals function as a kind of armor. This steel is now used in products such as razor blades and thin needles made specifically for eye surgery.

Despite being very hard, quasicrystals can fracture easily, like glass. Due to their unique atomic structure, they are also bad conductors of heat and electricity, and have non-stick surfaces. Their poor thermal transport properties may make them useful as so-called thermoelectric materials … Today, scientists also experiment with quasicrystals in surface coatings for frying pans, in components for energy-saving light-emitting diodes, and for heat insulation in engines, among other things.

Here’s a great 2010 interview with Shechtman, who now works at Technion:

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