X-Ray Fine Structure of Solids Under High Pressure.

This thesis examines the application of X-Ray -Absorption Fine-Structure (XAFS) measurements to problems in high pressure physics. It examines the content of quantitative information of high pressure XAFS spectra and the relation of the XAFS parameters extracted to known physical parameters, especially the bulk modulus and interatomic potential parameters. The materials investigated are copper, amorphous germanium, sodium bromide, potassium bromide and rubidium chloride at pressures ranging from 0 to 10 GPa (100 kbar). The difficulties of XAFS measurements under high pressure are addressed, above all the need to use the XAFS of selected materials as pressure calibrants. Great care must be exercised both in the theoretical formulation of XAFS and in the data analysis to obtain pressure scales with sufficient accuracy. It is found that the XAFS phase information of both copper and the alkali-metal halides can be used for that purpose. The fact that XAFS is sensitive to the short-range order allows the XAFS phase of amorphous germanium to be used for the first determination of the bulk modulus of the nearest-neighbor bond. It is found to be 30% larger than in crystalline germanium. Fair agreement is obtained with model calculations using an interatomic pair and three -body potential energy formulation. Extensive efforts have been made to relate the second cumulant, i.e. the variance, of the nearest-neighbor distance to known quantities. After application of some restrictive assumptions the second cumulant of copper is found to be related to the first pressure derivative of the pressure-volume relation. For the alkali-metal halides the second cumulant is calculated from first principles using various empirical interatomic pair and three-body potential models. The integration is evaluated with the Monte Carlo technique. In all cases good to excellent agreement is found between experimental data and model calculations. The appendix contains a compilation of all isothermal equations of state with pressure being the independent and volume the dependent variable. Two new such empirical equations are invented, one of which produces the smallest goodness-of-fit of all equations investigated.