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Shape oscillations mark crystal growth.

Salt crystals grown by slow evaporation of water from concentrated brine generally develop a characteristic cubic shape with sharp corners and flat faces. In contrast, crystals produced by vapor deposition - in effect, one atom at a time - can actually oscillate in shape between a sharp-cornered and a roundcornered form as they grow.

This novel, cyclic phenomenon, first observed by researchers at the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y., is most apparent when the growing crystal is small - only 1 micron or so in size. As the crystal gets larger, the physical conditions responsible for the shape oscillations exert less influence, and the crystal reverts to its sharp-cornered form.

Under certain growth conditions, "small crystals differ from large ones in a fascinating and unanticipated way," Jerry D. Tersoff, A.W. Denier van der Gon, and Rudolf M. Tromp report in the Feb. 22 PHYSICAL REVIEW LETTERS. Their theoretical analysis reveals that shape oscillations are a "fundamental feature of the equilibrium shape of small crystals."

Tromp and Van der Gon first noticed these surprising oscillations while studying the growth of silver crystals by depositing silver atoms onto a silicon surface. At first, the researchers assumed that the shape oscillations they unexpectedly observed were an artifact of their technique. But further analysis by Tersoff suggested thermodynamic reasons why such cyclic shape changes could occur naturally under the given conditions.

Made up of ragged steps, a crystal's rounded corners are actually quite rough on an atomic scale. Thus, a small, rounded crystal has rough corners and smooth, flat faces.

Because atoms require less energy to settle in a rough region than to start the formation of a new layer on a smooth face, they first go exclusively to the corners. Eventually the corners fill in, and deposited atoms have no other choice but to start forming an "island" on a smooth face.

Once established, this island acts as a reservoir, absorbing extra atoms from the corners, which become rounded again. After the island grows into a complete layer on one face, additional atoms have nowhere to go but the rounded corners, and the cycle begins again.
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Author:Peterson, Ivars
Publication:Science News
Article Type:Brief Article
Date:Mar 6, 1993
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