Weird Form of Carbon Acts as "Reversible" Diamond—A First

Diamonds are forever, and so is their extreme hardness.

But unlike diamond, a bizarre form of carbon can change its properties, going from squishy to hard and back again, if pressure on the material increases and then decreases, researchers have discovered.

Natural diamond is an allotrope, or form, of carbon crafted deep in the Earth. Other carbon allotropes include graphite, which is relatively soft, and fullerenes such as buckyballs and carbon nanotubes, which are exceptionally stable.

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The newly analyzed carbon material is a glass-like substance that factories have made for about 30 years for use in chemistry, electronics, and other purposes.

Until now, however, no one had studied what would happen to the material when placed under high pressure.

"Graphite is always soft, and diamonds are always hard," said Ho-Kwang "David" Mao, a high-pressure scientist at the Carnegie Institution for Science in Washington, D.C., and co-author of a new study on the glassy carbon, led by Stanford University graduate student Yu Lin.

"We were looking for something reversible for the kind of work we do"—studying materials at high pressure in the lab.

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How to Reverse a Diamond

The secret to a diamond's hardness is its atomic arrangement. The carbon atoms in a diamond form "3-D" bonds with their neighbors, resulting in a sturdy, pyramidlike, repeating crystalline structure.

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By contrast, graphite is soft and slippery because its carbon atoms form "flat" bonds, resulting in sandwiched sheets of atoms that weakly attract one another.

The glassy sphere allotrope used by Mao and his colleagues is also made almost entirely of flat bonds.

But when squeezed between two small diamond "anvils" to pressures similar to those hundreds of miles below Earth's crust, the bonds morphed into 3-D, diamond-like configurations—and the material rivaled the hardness of crystalline diamond.

When the pressure was released, the flat bonds returned, and the odd carbon turned back to its pliable, glassy form.

Speeding Up the Pressure

Mao said it's too soon to know of any commercial uses that might rely on the reversible carbon, because the material was studied only in experiments in which pressure was applied slowly.

In the future, Mao said, he'd like to study how the material's hardness changes under rapid pressurization—like that created by the impact of a speeding bullet, for instance, which could lead to new types of bullet-proof vests.

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In the meantime, high-pressure scientists may find clever uses for the glassy carbon in the lab, including perhaps a new way to apply extreme pressure to a material from many sides instead of sandwiching samples between two diamond anvils.

"This may help push the boundaries of what we can study," Mao said.

The reversible-diamond study will be published in the journal Physical Review Letters.