Diffraction studies at high pressures provide an opportunity to probe the behaviour of the chemical bonding of solids as a function of decreasing inter-atomic separation, without the complications introduced by changing chemistry. In this contribution to the workshop, the methods available for performing high-pressure powder diffraction are briefly introduced along with a number of important cautions for the experimentalist new to high-pressure diffraction.
In general, powder diffraction methods at high pressures yield data that is of lower quality than that which is obtainable from the same sample measured at ambient conditions on the same instrument. The reasons for this are several. High-pressure apparatus generally only allow small sample volumes while the containment of the sample absorbs both the incident and diffracted beams, all of which reduce the intensities of the diffracted beams. Secondly, the containment of the sample can give rise to scattered radiation that appears as background in the detector, either by diffraction from pressure-cell components (especially the gasket) or other scattering processes including Compton scattering from the diamonds of diamond-anvil cells (DACs). The lower signal levels can only be overcome by longer counting times and more intense radiation sources. The higher levels of background are best addressed by appropriate shielding and/or collimation at the time of the experiment.
Thus, the most important differences between performing a powder diffraction experiment at high pressure and one at ambient conditions are in the data collection process. Some important points are reviewed here in some detail. Then, a few brief remarks are made about the refinement of high-pressure powder diffraction data, which is essentially no different from refinement of data collected at ambient conditions. Finally, a brief review of how to analyse pressure-volume data in the form of Equations of State (EoS) is presented, along with a program to do the necessary least-squares fitting.