Super-hard graphite cracks diamond

Geometric arrangement and spacing of carbon atoms is what makes graphite and diamond differ in appearance and strength.

IT IS hard to imagine that graphite, the soft `lead' of pencils, can be transformed into a form that competes in strength with its molecular cousin diamond.

Using a diamond anvil to produce extreme pressures and the ultra-brilliant X-ray beams at the Advanced Photon Source in Illinois, scientists with the High-Pressure Collaborative Access Team (HPCAT) have surmounted experimental obstacles to probe the changes that graphite undergoes to produce this unique, super-hard substance. The recent study is reported in the journal Science.

``Researchers have speculated for years on the extreme conditions that might change the molecular structure of graphite into a super-hard form that rivals diamond,'' said Wendy Mao, from the Carnegie Institution's Geophysical Lab in Washington.

``This aims to determine quantitatively how the bonding in graphite changes under high-pressure conditions. Conventional methods limited our observations to surface studies of the material,'' she stated.

``Now, with the super high-intensity X-rays of the Argonne facility and with our team's technology to focus the entire beam to a small spot, we've been able to look at the material in the diamond-anvil cell while under high pressure ,'' she said.

Graphite and diamond are both made of carbon. The geometric arrangement and spacing of the carbon atoms is what makes the materials differ in appearance and strength. The atoms in graphite are arranged in layers that are widely spaced. But the atoms in diamond are tightly linked producing a strongly bonded structure.

The scientists subjected graphite to pressures that are equivalent to 170,000 times the pressure at sea level ( 17 gigapascals). ``We were able to see how the structure changed at the atomic level when the graphite was squeezed into the super-hard form,'' remarked co-author Dave Mao of Carnegie's Geophysical Lab.

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