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A team of
international researchers has discovered a new type of structural anomaly, or defect,
which can appear in quasicrystals, a unique material with some crystal-like
properties but a more complex structure.
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.
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.
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.
“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.
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.
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.
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.
“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.”
