X-ray diffraction from materials under extreme pressures (Invited Paper)

Diamond anvil cells have been used to generate a wide range of pressures, from 0.1 GPa to over 400 GPa (for reference, the center of Earth is about 360 GPa). Samples are squeezed between two diamond anvils and studied using infrared, visible, and X-ray probes. In the past year a new synchrotron beam line has become available at CHESS for the general user for diamond anvil cell work using X-rays. This has opened up new areas of research as the experimenters need only to bring a sample in a diamond anvil cell and can leave with the X-ray data mostly analyzed. Current X-ray diffraction work at CHESS on materials subjected to static pressures up to 400 GPa are reviewed. Both energy dispersive and Laue diffraction techniques have been applied to phase transition, equation of state, and state of stress problems. Since most samples at very high pressures are powders, energy dispersive diffraction is most often used. An example of this is xenon which turns metallic at 150 GPa. Since the plasma frequency of xenon is in the infrared and because of the presence of an absorption band at 2 eV, xenon is a transparent blue metal at this pressure. The energy dispersive diffraction data provided the structural and equation of state information needed to understand the physics of the problem. An example of Laue diffraction using diamond anvil cell is the study of the state of stress of diamond anvils themselves. In an ongoing experiment at CHESS, the tips of highly stressed diamonds are analyzed by studying the energy distribution of various Laue spots using a solid-state detector.