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25 August 2000 7:00 pm
Like a politician on the campaign trail, a quasicrystal has a structure that regularly repeats itself, but never says exactly the same thing twice. Now a series of rapid-fire photographs has exposed another political talent of those odd lattices: effortlessly changing its position. These molecular flip-flops, called phasons, rearrange quasicrystals in a way unlike anything else in nature.
For more than 150 years, scientists believed that everything becomes crystalline if it gets cold enough. But in 1985, the discovery of quasicrystals proved them wrong. Instead of freezing like water into a regular, repeating, wire-frame pattern of identical clusters of atoms, quasicrystals have a not-quite-regular structure that never exactly repeats. Some quasicrystals, for example, mix two distinct three-dimensional structures, one hexagonal, the other pentagonal. The assortment of shapes suggested to theorists that quasicrystals could wiggle in unusual ways. If you bang on a regular crystal, a vibration called a phonon hums through it. Theorists believed that, in addition to phonons, quasicrystals support an extra kind of oscillation called a phason. Phasons, theoretically, rearrange the quasicrystal structures by making individual atoms jump as much as a few angstroms as the oscillation passes. But no one had ever seen the wiggles caused by a passing phason.
To catch phasons in the act, physicist Keiichi Edagawa and his collaborators at the University of Tokyo used a high-resolution electron-tunneling microscope to record the metamorphosis of a quasicrystal. They first heated an aluminum-copper-cobalt mixture to 1173°C, then cooled it to room temperature to form a quasicrystal of interlocking hexagonal and pentagonal rhombi. A series of photographs revealed a column of atoms jumping approximately one nanometer, the team reports in the 21 August Physical Review Letters. The jump changes a hexagonal rhombus to a pentagonal one and makes an adjacent pentagonal rhombus become hexagonal. After a few seconds to minutes, the column jumps back and flips the rhombi back to the original configuration.
"This is a breakthrough because we can now see the dynamical effects of phasons," says physicist Paul Steinhardt of Princeton University in New Jersey. The new imaging technique may also help scientists figure out whether quasicrystals ever solidify into a predictable state--or if they lock unpredictably into one of a nearly infinite number of equally likely configurations, says physicist Michael Widom of Carnegie Mellon University in Pittsburgh.