Feb 17, 2012 |
Researchers find strange new nanoregion can form in quasicrystals
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(Nanowerk News) A team of international researchers has discovered a new type of structural anomaly, or defect, that can appear in quasicrystals, a unique material with some crystal-like properties but a more complex structure (see paper in Physical Review Letters: "Nanodomains due to Phason Defects at a Quasicrystal Surface"). Pat Thiel, senior chemist at the U.S. Department of Energy's Ames Laboratory, led the international team, which includes scientists from the Institut Jean Lamour at Nancy-Université in France.
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In crystals, a "defect" refers to any departure from perfect structural symmetry. While the term suggests an undesirable quality, not all defects are bad; many control or influence key material properties, such as chemical purity, mechanical strength, conductivity, color, corrosivity or surface properties. Rubies, for instance, are red due to a defect that turns an otherwise non-descript crystal into a valuable gem.
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Quasicrystals were already known to have a type of defect called a phason flip, which can form at the surface. The new defect type was discovered after researchers observed mysterious nano-sized areas on quasicrystal surfaces. Unlike the phason flip, however, the new defect type extends beyond the surface region and into the bulk of the quasicrystal.
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"Quasicrystals are such fascinating materials -- they seem to always exhibit features that are unexpected, starting with their very existence," said Thiel, who is also Iowa State University's John D. Corbett Distinguished Professor of Chemistry.
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It wasn't until 1982, in fact, when Dan Shechtman observed the seemingly impossible -- a well-defined but non-repeating arrangement of atoms under his electron microscope -- that quasicrystals were found to exist. It took even longer for the scientific community to accept their existence. Shechtman, a materials scientist with Ames Lab, Iowa State University and Technion-Israel Institute of Technology, won the 2011 Nobel Prize in Chemistry for his discovery.
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The recent discovery of the new defect type shows quasicrystals are still yielding surprises. While the nanodomain defect isn't always present -- it only occurs under certain circumstances to help balance competing energetic issues -- its formation at those times enables higher-energy transition-metal-rich surfaces to be exposed rather than the expected lower-energy aluminum-rich surfaces.
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Because nanostructures show promise for use in a range of applications, from medical to electronics, understanding the relationship between surface and bulk defects in materials may yield greater insights into why nanostructures are often unusually strong.
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"It's already known that in nanowires, their strength is related to the fact that the surface can 'erase' the bulk defects," Thiel said. "But then eventually under extreme conditions even a nanowire can fail, and the surface seems to play a role in that event as well. So the relationship between surface and bulk defects really is very important."
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