spiral staircase

Spirals add mystery to how crystals form

The discovery of a different way crystals grow may lead to new pharmaceuticals and electronic materials. The findings could also clarify the pathway for the formation of a particularly nefarious type of kidney stone.

The findings at first glance seemed to confound 50 years of theory and deepen the crystal mystery—but researchers have found appearances can be deceiving.

While the interest in L-cystine crystals is limited to the biomedical arena, understanding the details of crystal growth, especially the role of defects—or imperfections in crystals—is critical to the advancement of emerging technologies that aim to use organic crystalline materials, researchers say.

Scientists have been examining defects in crystals called screw dislocations—features on the surface of a crystal that resemble a spiraled ham. Dislocations were first posed by William Keith Burton, Nicolás Cabrera, and Sir Frederick Charles Frank in the late 1940s as essential for crystal growth.

The so-called BCF theory posited that crystals with one screw dislocation would form hillocks that resembled a spiral staircase while those with two screw dislocations would merge and form a structure similar to a Mayan pyramid—a series of stacked “island” surfaces that are closed off from each other.

For the new study published in Proceedings of the National Academy of Sciences, the Molecular Design Institute team used atomic force microscopy to examine both kinds of screw dislocations in L-cystine crystals at nanoscale resolution.

Spirals and islands

Their results showed exactly the opposite of what BCF theory predicted—crystals with one screw dislocation seemed to form stacked hexagonal “islands” while those with two proximal screw dislocations produced a six-sided spiral staircase.

A re-examination of these micrographs by Molecular Design Institute scientist Alexander Shtukenberg, in combination with computer simulations, served to refine the actual crystal growth sequence and found that, in fact, BCF theory still held.

In other words, while the crystals’ physical appearance seemed at odds with the long-standing theory, they actually did grow in a manner predicted decades ago.

“These findings are remarkable in that they didn’t, at first glance, make any sense,” says Michael Ward, professor of chemistry at New York University. “They appeared to contradict 60 years of thinking about crystal growth, but in fact revealed that crystal growth is at once elegant and complex, with hidden features that must be extracted if it is to be understood.

“More importantly, this example serves as a warning that first impressions are not always correct.”

The National Science Foundation and the NSF Materials Research Science and Engineering Center Program supported the research.

Source: New York University

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