INELASTIC X-RAY SCATTERING AND X-RAY DIFFRACTION STUDY OF GRAPHITE AT HIGH PRESSURE

Near K-edge features measured by x-ray absorption or electron energy loss spectroscopy provide rich information about the chemical bonding and have been used as quantitative tools to investigate a variety of carbon bonding in organic and inorganic materials. However, the low-energy x-ray and electron probes require vacuum environment, and are completely inaccessible, blocked by the pressure vessels for high-pressure studies, as the soft x-ray and electron beams. Synchrotron inelastic x-ray scattering (IXS) (also called x-ray Raman) spectroscopy, is capable of penetrating high pressure vessels and probing the bonding of samples at high-pressure in-situ. Although this technique was demonstrated almost forty years ago, it has only recently been successfully applied to ambient pressure studies. Compressed under ambient temperature, graphite undergoes a transition at approximately 17 GPa. 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 while the other half remain as pi-bonds in the high-pressure form. The x-ray diffraction pattern of the high-pressure form, using He as a pressure transmitting medium, is consistent with a distorted graphite structure in which bridging carbon atoms between graphite layers pair and form sigma-bonds, while the non-bridging carbon atoms remain unpaired with dangling pi-bonds. The high-pressure form is superhard, capable of indenting cubic diamond single crystals.