Compressed under ambient temperature, graphite undergoes a transition at approximately 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.

The compression of Au has been measured at room temperature to 70 GPa (700 kbar) using x-ray diffraction through a diamond-anvil cell and the ruby-fluorescence pressure scale. Based on these data, the isothermal bulk modulus and its pressure derivative at zero pressure are K0T =167 (±11) GPa, and K′0T=5.5 (±0.8). These results are in excellent agreement with ultrasonic measurements of the elastic constants as well as an equation of state based on shock-wave data. Hence, this study represents an independent experimental confirmation of both the ruby fluorescence pressure scale, and the predicted equation of state of the proposed Au pressure calibration standard. We derive a thermal equation of state for gold by inverting all equation-of-state data simultaneously. From this, we extend the gold pressure-calibration standard to cover the range 0–200 GPa in pressure and 300–3000 K in temperature.