World's most complex crystal simulated

WASHINGTON: US researchers have simulated the most complex crystal structure ever - an icosahedral quasicrystal.

The "icosahedral quasicrystal" simulated by researchers at the University of Michigan looks ordered to the eye, but has no repeating pattern.

At the same time, it's symmetric when rotated, like a soccer ball with five-fold and six-fold patches.

This property, called icosahedral symmetry, is frequently found on small scales around a single point. It's in virus shells or buckyballs - molecules of 60 carbon atoms. But it is forbidden in a conventional crystal.

Like trying to tile a bathroom floor with pentagons, icosahedra do not nicely fill space, said Michael Engel, a research investigator in the Department of Chemical Engineering and first author of a paper on the findings published in Nature Materials.

"An icosahedral quasicrystal is nature's way of achieving icosahedral symmetry in the bulk. This is only possible by giving up periodicity, which means order by repetition. The result is a highly complicated structure," Engel said.

Engineers are still searching for efficient ways to make icosahedral quasicrystals, commonly found in metal alloys, with other materials.

Due to their high symmetry under rotation, they can have a property called a " photonic band gap."

A photonic band gap occurs when the spacing between the particles is similar to that of light. Particles arranged in this way could trap and route light coming from all directions.

"If icosahedral quasicrystals could be made from nano- and micro-metre sized particles, they could be useful in a variety of applications including communication and display technologies, and even camouflage," said Sharon Glotzer, the Stuart W Churchill Collegiate Professor of Chemical Engineering at U-M.

The researchers said the most exciting aspect of the findings is the insight they provide into how icosahedral quasicrystals form.

"When researchers study quasicrystals in the lab, they typically lack direct information about where the atoms are. They look at how the materials scatter light to figure that out," Glotzer said.

"No one has ever gotten one with icosahedral symmetry to self-assemble thermodynamically in a computer model that's not built by hand, and researchers have been trying for decades," Glotzer said.

The simulation will allow researchers for the first time to observe how icosahedral symmetry develops.

The U-M simulation was done using only one type of particle, which is unique. Typically, two or even three atomic elements are required to achieve a quasicrystal structure.