UC Berkeley

Structural transition in materials represents one of the most profound changes materials can undergo at high pressure and are an important part of studying planetary interiors. A material’s crystal (or non-crystalline) structure influences its physical properties, including conductivity, elasticity, density and compressibility, and thus changes to that structure have profound effects on those physical properties. Studying these structural transition gives us an understanding of materials under extreme and dynamic conditions, such as deep interiors of planets, and subducting ocean slabs. I studied the insulator to metal transition of PbCl2 and SnCl2, structural analogs to high-pressure SiO2. Both density functional theory and X-ray diffraction shows two displacive phase transitions in the chlorides, as they transition from 9-fold coordinated to 10 and 11-fold coordinated structures. Absorption-edge spectroscopy shows that the bandgaps of PbCl2 and SnCl2 decrease with pressure, and we observe discontinuous changes in band-gap with crystal structural-induced changes in coordination number, suggesting a connection between interatomic geometry and the metallicity of these chlorides. By implementing new techniques for acoustic monitoring, I study the structural transitions resulting from crystalline instabilities in silicon and serpentine. In silicon, I implement a fiber optic based acoustic sensor to expand the possible frequency response for acoustic monitoring to > 100 MHz, providing orders of magnitude better frequency response than previous work. We compressed silicon to 17 GPa, discovering 3 new acoustically active phase transitions on compression and decompression. Emissions range in duration from 10-7-10-3 seconds, with the number of emissions roughly following a power law, similar to crustal earthquakes. In serpentine we monitor acoustic emissions resulting from solid-state amorphization using a 4-sensor acoustic array to get a sense of focal mechanisms. We record acoustic emissions to 26 GPa and find no purely isotropic sources from any of our experiments, replicating findings from natural high-pressure seismicity and setting a new pressure record for determination of focal mechanisms in the laboratory. In both silicon and serpentine, repeating acoustic emissions provide evidence for a self-propagating, transformation-driven process.