In situ X-ray diffraction of key high-pressure materials under dynamic compression

Abstract

Dynamic compression experiments are short timescale, high strain rate events that can be used to generate conditions of extreme temperature and pressure out of reach of traditional static compression techniques. They have recently been paired with high-brilliance X-ray light sources to allow for direct structural measurements of compressed material via in situ X-ray diffraction. In this dissertation, I describe experiments that pair various compression techniques with high-brilliance X-ray sources to characterize the in situ density and high-pressure polymorphism of gold, an important high-pressure standard, soda-lime glass, a widely used silicate glass, and forsterite (Mg2SiO4), which is representative of the interior compositions of rocky exoplanets. Several shock-ramp compression experiments were conducted on gold through 713 GPa at the Omega laser facility using in situ X-ray diffraction to test predictions of high-pressure polymorphism. No evidence for a phase transition to predicted hcp or bcc phases was observed through the pressure range of the study. The structure of soda-lime glass under shock and static loading was investigated using in situ X-ray diffraction at the Advanced Photon Source and Raman spectroscopy. A structural change was identified at ~10 GPa in the static X-ray diffraction and Raman spectroscopy data corresponding to conditions where permanent densification is observed. No evidence for bulk crystallization was observed under shock loading or room temperature static compression through 72 GPa. Finally, forsterite was shock-ramp compressed to peak pressures between 303 and 996 GPa at the National Ignition Facility and probed using in situ X-ray diffraction. At 303 GPa, diffraction data were consistent with transformation to an orthorhombic post-spinel phase such as forsterite-III. Evidence for a theoretically predicted, but experimentally novel, 8-coordinated silicate phase was observed in the X-ray diffraction data between 895 and 996 GPa. This new phase may be a major constituent of the deep interiors of large rocky exoplanets. This thesis demonstrates the ability of in situ X-ray diffraction to provide direct constraints on the lattice-level structures of dynamically compressed materials across a variety of bonding environments and degrees of ordering as well as the broad range of responses that these materials have to high strain-rate conditions.