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Brookhaven Lab scientist helps create a new form of the element carbon

DOE/Brookhaven National Laboratory : 15 May, 2007  (Technical Article)
A new phase of the element carbon, a superhard compressed graphite, has been identified by a research team that includes a scientist from the National Synchrotron Light Source at the U.S. Department of Energy
“The new phase is very hard, it actually left an indentation on diamond,” said NSLS physicist Chi-chang Kao.

Using a method developed at the NSLS known as inelastic x-ray scattering, the research team studied the way carbon bonds in sheets of graphite buckle under very high pressures in a diamond anvil cell, a device that uses the polished faces of two diamonds to apply pressure to a sample. Today, pressures achieved in a diamond anvil cell can reach levels that approach conditions in the center of the Earth.

In the experiment, a beam of x-rays was focused on the sample in the diamond anvil cell through a beryllium gasket that is transparent to x-rays. The energy of the scattered x-rays was then analyzed to very high resolution using crystal optics developed at the NSLS. It is due to this ability to perform inelastic x-ray scattering measurements on samples inside a diamond anvil cell that the researchers were able to reveal the details of the chemical bonding changes under high pressure for the first time.

Specifically, this method allows the researchers to distinguish and quantify different types of carbon bonds in the sample. With this new information, the researchers were able to show conclusively that the structure of the high-pressure graphite is not hexagonal diamond, an intermediate form of carbon that lies between graphite and diamond in terms of hardness. Instead, a new distorted graphite-like structure was proposed. This proposed structure was also supported by studying x-ray diffraction patterns of the material.

This study demonstrates the possibility of performing inelastic x-ray scattering using a diamond anvil cell, which Kao says is a “miniature laboratory” that can simulate deep-Earth pressures and may lead to a better understanding of Earth’s interior. This kind of study can also be used to understand new phases of other elements, opening a door to additional discoveries and potential materials-science breakthroughs.

The experiment was performed at the Advanced Photon Source at Argonne National Laboratory in Argonne, Illinois, primarily because the x-ray brightness needed to perform the experiment was not available at the NSLS. Kao said that a proposed new facility at Brookhaven, dubbed NSLS II, would be much brighter than the current sources and would be ideal for similar future experiments. See more information on NSLS II.

The study’s lead author is Wendy L. Mao of the University of Chicago and the Carnegie Institute of Washington, who collaborated with a team that includes researchers from those institutions, the University of Alaska, and Argonne National Laboratory.

The experiment was funded by the U.S. Department of Energy’s Office of Basic Energy Science within the Office of Science, the DOE’s National Nuclear Security Administration, the National Science Foundation, the U.S. Army Tank-Automotive and Armaments Command, and the W.M. Keck Foundation.
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