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Science 17 October 2003:
Vol. 302. no. 5644, pp. 425 - 427
DOI: 10.1126/science.1089713
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Reports

Bonding Changes in Compressed Superhard Graphite

Wendy L. Mao,1,4* Ho-kwang Mao,1,4 Peter J. Eng,2,3 Thomas P. Trainor,2,7 Matthew Newville,2 Chi-chang Kao,5 Dion L. Heinz,1,3 Jinfu Shu,4 Yue Meng,6 Russell J. Hemley4

Compressed under ambient temperature, graphite undergoes a transition at ~17 gigapascals. The near K-edge spectroscopy of carbon using synchrotron x-ray inelastic scattering reveals that half of the {pi}-bonds between graphite layers convert to {sigma}-bonds, whereas the other half remain as {pi}-bonds in the high-pressure form. The x-ray diffraction pattern of the high-pressure form is consistent with a distorted graphite structure in which bridging carbon atoms between graphite layers pair and form {sigma}-bonds, whereas the nonbridging carbon atoms remain unpaired with {pi}-bonds. The high-pressure form is superhard, capable of indenting cubic-diamond single crystals.

1 Department of the Geophysical Sciences, University of Chicago, Chicago, IL 60637, USA.
2 Consortium for Advanced Radiation Sources, University of Chicago, Chicago, IL 60637, USA.
3 James Franck Institute, University of Chicago, Chicago, IL 60637, USA.
4 Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA.
5 National Synchrotron Light Source, Brookhaven National Laboratory, Upton, NY 11973, USA.
6 High Pressure Collaborative Access Team, Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA.
7 Department of Chemistry and Biochemistry, University of Alaska, Fairbanks, AK 99775, USA.

* To whom correspondence should be addressed. E-mail: wmao{at}chicago.edu

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