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Mineralogy
Super-hard graphite

Graphite, the gray-black, soft, greasy “lead” used in pencils, is chemically identical to diamond, the hardest known mineral in the world. But the way their carbon atoms bond to each other makes them unique minerals, with decidedly different properties.

Mineralogists have long hypothesized about what happens to graphite under extreme pressure while at ambient temperatures. Some suspected that changes to the carbon bonds would transform it into hexagonal diamond. But the technology required to answer the long-standing question did not yet exist.

“With this technique, you can actually probe what is going on at high pressure in compressed graphite.”

Wendy Mao, University of Chicago and the
Carnegie Institution of Washington’s Geophysical Laboratory

Now, by combining the use of a diamond anvil to squeeze the graphite and high-intensity X-rays to observe the bonding changes, scientists have been able to peer inside the graphite while it is under pressure. “With this technique, you can actually probe what is going on at high pressure in compressed graphite,” says Wendy Mao, a graduate student at the University of Chicago and the Carnegie Institution of Washington’s Geophysical Laboratory.

As reported in the Oct. 17 Science, Mao and her colleagues found that the compressed graphite does not become diamond, but instead becomes a “super-hard” form of graphite. “The bonding does change in graphite, but it doesn’t become one of the known diamond forms — hexagonal or cubic,” Mao says.

In graphite, the carbon atoms are layered in sheets, and although the bonds within a sheet are strong, the bonds between the layers are not. The strong, covalent bonds within the sheet are called sigma bonds. The longer, weaker bonds between the layers are called pi bonds. Pi bonds are what allow the sheets to slip past each other easily, making graphite a popular industrial lubricant. In diamond, however, the carbon atoms are all strongly connected with sigma bonds.

In order for the graphite to become diamond, all the bonds would have had to change to sigma bonds. But when the researchers compressed the graphite to 17 gigapascals (170,000 times the air pressure at sea level), only half of the bonds changed.

“If you look down on sheets of graphite, only three out of the six carbons in each hexagon are directly above or below another carbon in an adjacent sheet,” Mao says. “So that’s why only those three carbons can easily form a sigma bond. The other three just remain pi bonds because there’s no adjacent carbon.”

The study, funded by the Departments of Defense and Energy as well as the National Science Foundation and the W. M. Keck Foundation and made possible by the Advanced Photon Source at Argonne National Laboratory, also found that cold-compressed graphite can be hard enough to scratch diamond. “Once we released the pressure,” Mao says, “we realized it had scratched the diamond anvil surface.”

The new material has many potential industrial applications, for example as a structural component or perhaps, Mao says, for use in high-pressure scientific instruments.

Sara Pratt
Geotimes contributing writer


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