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
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.
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 Washingtons Geophysical Laboratory.
With this technique, you can actually
probe what is going on at high pressure in compressed graphite.
Wendy Mao, University of Chicago
Carnegie Institution of Washingtons 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 doesnt
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 thats why only those three carbons can easily form a
sigma bond. The other three just remain pi bonds because theres no adjacent
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
Geotimes contributing writer
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