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Breaking the rules in crystallography.

Rummaging through rubbish bins, crystallographers are taking a fresh look at crystal structure -- dusting off old photographs, retrieving discarded data, remeasuring key paramters. Furthermore, results from recent experiments are adding to the slew of unconventional crystal patterns into which atoms now appear to settle.

This burst of activity was sparked by reports late last year of a crystal pattern that violates some long-established crystallographic rules (SN: 1/1/85, p.37; 3/23/85, p.188). These crystals, usually aluminum alloys containing small amounts of manganese, iron or chromium, seem to have an icosahedral crystal structure, which shows a fivefold symmetry. Blocks of atoms in such an arrangement can't be piled in a regularly repeating pattern.

In the July 29 PHYSICAL REVIEW LETTERS, a team of Japanese scientists adds a nickel-chromium alloy to the list of substances that may have an unusual crystal form. In electron diffraction patterns, tiny particles of this alloy show a 12-fold symmetry. The researchers suggest that the nickel and chromium atoms are arranged in a network of incomplete 12-sided figures (dodecagons) resulting in a "crystalloid" state that falls somewhere between the orderliness of a regular crystal and the completely disordered amorphous state.

"It's certainly quite exciting to see something noncrystallographic in an alloy that isn't aluminum-based," says physicist David R. Nelson of Harvard University. But much more work must be done to confirm the Japanese results and to see how their structure fits into the rapidly evolving theory of nonperiodic crystals.

At AT&T Bell Laboratories in Murray Hill, N.J., researchers are reexamining puzzling data taken as long as 40 years ago from very complex diffraction patterns. The patterns imply that this particular crystal, an aluminum-manganese-silicon alloy, may have thousands of atoms in its "unit cells," the building blocks that make up the complete crystal. Now, the investigators suspect that each unit cell may contain atoms arranged in the form of icosahedra. But on a large scale, the blocks form a conventional crystal pattern.

An Indian scientist recently found a similar combination of structures in a magnesium-zinc-aluminum alloy. This led to the production by rapid cooling of the first icosahedral crystals made from an alloy that is not largely aluminum.

Equally intriguing is a newly created hybrid crystal structure that consists of sheets of atoms arranged so that they show a 10-fold symmetry within a layer -- yet the layers can be stacked to form a periodic crystal lattice.
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Author:Peterson, Ivars
Publication:Science News
Date:Aug 17, 1985
Words:404
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