When is a crystal not a crystal? Until 1982, scientists thought they knew the answer well.
Along came Israeli physicist Daniel Shechtman, whose discovery of "quasicrystals" in metals – crystals with oddball structures scientists held to be impossible for crystals – initially cost him a research job, drew scorn from several colleagues, but now has earned him the 2011 Nobel Prize in Chemistry.
Since Dr. Shechtman and colleagues published the first controversial paper on quasicrystals in 1984, researchers have been looking for ways to incorporate their unique properties into technologies as high tech as light-emitting diodes and as mundane as non-stick frying pans.
And where quasicrystals have been produced in the lab for decades, researchers in 2009 found naturally occurring quasicrystals in a mineral found in eastern Russia.
Shechtman's discovery injected fresh energy into a field that at the time was becoming a scientific backwater, notes Michael Widom, a physicist at Carnegie Mellon University in Pittsburgh who studies the structure and properties of novel materials, including quasicrystals.
"People had well established techniques for determining the structure of crystals" he says, and the mathematical framework for understanding crystals was well-established and reliable.
"Along came quasicrystals, which threw an incredible monkey wrench into people's understanding. That led to a widespread revolution of interest and progress in metallurgy and crystallography," he says.
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What are quasicrystals?
Crystals are a form of matter in which the atoms that make up the crystal are arranged in orderly patterns that repeat themselves.
A diamond is one of the most common examples of a crystal – in this case, of carbon.
To qualify for the crystal club, the arrangement of atoms had to be symmetrical when the sample was viewed from different angles.
For instance, each atom might sit in the middle of a triangle formed by its neighbors. Turn a sample, and the series of overlapping triangles form a pattern that reappears with every 120 degrees of rotation. Atoms centered in a pattern of rectangles, squares, hexagons also exhibit this kind of symmetry, although at different angles.
But atoms in patterns made up of geometric shapes with five sides, or more than seven, don't show symmetry at any angle and hence materials with those arrangements were not deemed crystals. Or so many scientists thought.
Shechtman was mixing molten aluminum and manganese, then cooling it quickly, to study the properties of the resulting alloy.
When he viewed the sample through a electron microscope, however, he was stunned. The microscope's image displayed atoms arranged in pentagons. But the pentagons themselves were arranged in such a way that they also formed concentric circles of 10 dots apiece. These circles displayed the kinds of symmetry associated with the structures of widely accepted crystals. In the case of Shechtman's alloy, the geometric shape formed by the 10 dots formed a repeated pattern for each 36 degrees the sample was rotated.
Initially, even Shechtman didn't believe his results, according to a biographical sketch provided by the Nobel Committee. It took two years to produce a paper scientific journals were willing to publish, in essence because three highly respected coauthors, who found his results convincing, agreed to join him in writing up the results.
Uses for quasicrystals
Quasicrystals have fascinating properties, but those properties also are challenging for researchers hunting for ways to use the materials, Widom says.
They are metal. But they are brittle. And unlike most metals, they are very poor conductors of electricity. Scratch electronics and construction from the list of potential applications.
Still, they have their strong points. A quasicrystal's unusual structure makes it tough for defects such as fractures to migrate through a sample. And it has properties that could be useful in fields such as optical computing, where light replaces electrons in carrying and processing data.
Even as researchers probe the practical possibilities of quasicrystals, others are still trying to figure out why they exist at all.
Despite the time that has elapsed since Shechter's discovery, the structures of quasicrystals in metal alloys have not been precisely described, Widom says. And no one yet knows why they form.
If history is any indication when it comes to Nobel Prizes for discoveries in materials science, the scientist who can explain why a Nobel-winning discovery behaves as it does could well be in line for a trip to Stockholm as well.