Phase transitions at high pressure are typically expressed as changes in the material’s crystal structure and elastic parameters. This work describes the Fe(II) spin collapse in the ferropericlase system through examining structure factors and through modeling of compressibility data. Detailing electron density distribution changes with compression allows for a comprehensive picture of high-pressure chemistry underpinning the theory in first-principles calculations, giving detailed insight into not only the confidence of measured properties but the electronic landscape leading to essential differences between deep-Earth materials. Experimental charge density changes obtained from single-crystal x-ray diffraction are compared to atomic orbital theory for d-electron spin collapse in high-pressure (Fe,Mg)O, characterized by the spin pairing of the two high energy lone-spin electrons in the Fe valence to a spin-paired state in lower-energy orbitals. Expected changes in bonding can be compared with ratios of local electron density intensities relative to those from atom centers. When normalizing to the oxygen site, there is rough agreement between charge transfer around the cation site (5 percent local charge density changes) and expectation of 6 percent charge transfer for 53 percent Fe stoichiometry (two electrons from Fe divided by the total charge of the cation site). However, the change in charge density in the cation center is roughly twice as large (compared to oxygen) as expected from electron accounting, potentially related to expected electron donation from oxygen. Modeling of compression by way of normalized pressure (F) as a function of finite strain (f) provides an architecture for evaluating the phase transition over a suite of compositions. Through this, the volume collapse of the Fe(II) cation is found and the bulk modulus of both the low- and high- pressure phases is determined. Results are reported as a function of Fe content and display the strengths and limitations of this approach for the data examined.

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.

The goal of this thesis is to help improve models of the evolution of cores of the Earth and other planets, and to improve understanding of melting transitions of metals in general.

First, I present laboratory studies of high-pressure melting and near-melting phase transitions of two metals. The &epsilon-to-B2 phase boundary of FeSi is constrained to 30 ± 2 GPa with no measurable pressure-dependence from 1200 ± 200 to 2300 plusmn 200 K using x-ray diffraction in laser heated diamond anvil cells. The miscibility of Si in crystalline Fe likely increases at this transition due to the increasing effective ionic radius of Si, evidenced by the coordination change documented here. The result is that silicon is even more miscible in iron in the cores of Mercury and Mars than shown previously. Solid-solid transitions are also documented in AuGa_{2 from cubic (fluorite-type) to denser phases above 5.5 GPa and 600 K, in close proximity to the reversal in melting curve from negative slope to positive slope, which is also documented here. The change in melting curve therefore seems to be primarily driven by the crystallographic transitions and not the electronic transitions thought to occur at low temperatures. All transitions described here are reversed in the experiments, revealing hysteresis that ranges from 90 K to less than 15 K, and from 7 GPa to less than 2 GPa. This complexity, along with other complexities seen here and in other studies, suggest the need for new experimental techniques to make unambiguous measurements of a variety of equilibrium properties at melting and near melting.}

To improve future laboratory studies of melting at high pressure, I analyze several varieties of dynamic heating experiments. Laser heating experiments on metals in diamond anvil cells are shown to be at least 5 times less sensitive (and sometimes > 100 times less sensitive) to the latent heat of melting than suggested by published experimental data from pulsed-heating and continuous-heating experiments. Rather, experimentally detected plateaus in temperature likely result from changes in reflectivity of the laser absorber. To reveal a material's energetic properties (latent heat or heat capacity) in the highly conductive environment of diamond cells, heating frequencies >100 kHz should be used, and heat should be deposited uniformly through the material. Specifically, an ``adiabaticity parameter'' is presented in Chapter 4 to guide experiments seeking to measure temperature plateaus that reveal the latent heats of first order phase transitions. Focusing on heat capacity alone, two experimental possibilities are described in Chapter 5: relative measures of heat capacity of metallic samples using modulated laser heating at 1 MHz to 1 GHz, and absolute measure of heat capacity using Joule-heating of metallic samples at 1 to 100 MHz frequency. Finally, Chapter 6 shows that a specific experimental design for Joule-heating is feasible: a realistic electrical circuit using two amplifiers and a Wheatstone bridge can couple electrical current into a diamond-cell-sized metal sample and output 20 &mu V residual voltage oscillations induced by the sample's 1 MHz temperature oscillations, allowing measurement of the sample's heat capacity with 11% contribution from the insulation.

The thermal models of Joule heating in diamond cells are validated by laboratory data of the heat capacity of a nickel foil pressed between thin glass pieces glued to a diamond: measured heat capacities decrease from 100s of % above the actual heat capacity of a 6 &mu m-thick nickel sample at &le 1 kHz, to within ~ 20% of the actual heat capacity at 30 kHz.

The physical and chemical properties of materials are profoundly affected by high

pressure and temperature and differ significantly from the behavior commonly observed

at ambient conditions. Understanding the interior dynamics and evolution of planetary

bodies thus requires measurement of the equations of state of materials at these extreme conditions. Experiments employing laser-driven shock compression now permit

exploration of phase space spanning orders of magnitude in pressure and temperature.

Results are presented here from a suite of laser-driven shock experiments on three

major mineral phases of significance to the terrestrial mantle: SiO₂, MgO and MgSiO₃. New optical diagnostics, including an absolutely calibrated streaked optical pyrometry system were developed to measure shock temperatures from ∼ 4000-60,000 K. This system was applied to observe high-pressure phase transitions and melting at previously unexplored conditions.

Experiments on two polymorphs of SiO₂ are used to validate experimental technique and pyrometry calibration and are compared to previous results. Data on MgO and MgSiO₃ constrain controversial predications for the ultra-high pressure melt curves and support melting temperatures at the Earth's core-mantle boundary higher than most previous predictions. In the case of MgSiO₃, the first observations of a distinct liquid-liquid phase transformation in a silicate material are presented. Experiments on amorphous and crystalline MgSiO₃ starting materials show evidence of a transition to a high-pressure liquid phase approximately 10% denser than the low-pressure counterpart. Isochemical liquid-liquid phase separation may represent a previously unrecognized means of geochemical partitioning in early planetary history. Finally, discussions of the transport properties of each material are given and it is found that all three transform to metallic liquids upon melting with high thermal and electrical conductivity, suggesting the possibility of enhanced electromagnetic coupling across the core-mantle boundary in the molten state.

The high&ndashpressure behavior of the elastic properties of materials is important for condensed matter and materials physics, and planetary and geosciences, as it determines the mechanical deformations and stability of solids under external stresses. The high&ndashpressure elasticity of relevant minerals is of substantial geophysical importance for the Earth's interior since they relate to (1) velocities of seismic waves and (2) the change in density that occurs when minerals are under pressure. Absence of samples from the deep interior prompts comparisons between seismological observations and the elastic properties of candidate minerals as a way to extract information regarding the composition and mineralogy of the Earth.

Implementing a new method for measuring compressional&ndash and shear&ndashwave velocities, Vp and Vs, based on Brillouin spectroscopy of polycrystalline materials at high pressures, we examine four case studies: 1) soda-lime glass, 2) NaCl, 3) MgO, and 4) Kilauea basalt glass. Our results on multigrain soda&ndashlime glass show that even with an index differing by 3 percent, an oil medium removes the effects of multiple scattering from the Brillouin spectra of our glass powders; similarly, high&ndashpressure Brillouin spectra from transparent polycrystalline samples should, in many cases, be free from optical&ndashscattering effects. We measure seismic wave velocities of polycrystalline NaCl (halite) to 30.5 GPa at room temperature, finding the bulk and shear moduli and pressure derivatives to be: K_T = 23.75 (± 0.08) GPa, K_T&rsquo = 5.32 (±0.1), G = 14.9 (± 0.5) GPa, G&rsquo = 2.6 (± 0.5). The resulting equation of state agrees well with previous x&ndashray diffraction measurements, illustrating the suitability of high pressure Brillouin scattering to characterize the elasticity of polycrystalline materials. Brillouin scattering of non-hydrostatically compressed and decompressed polycrystalline MgO (periclase) to 60 GPa documents shear&ndash and compressional&ndashwave velocities &sim20% lower than the Voigt&ndashReuss&ndashHill average values calculated from measurements on hydrostatically compressed single crystals. Calculations on the effects of non&ndashhydrostaticity reveal that the sound wave velocities can be lowered, however, the calculated magnitude cannot explain our observations. We discuss possible additional causes for these anomalously low velocity trends, including grain size, grain boundary effects, preferred orientation, and impurities. High pressure Brillouin spectroscopy of natural Kilauea basalt glass not only provides measurements of elastic moduli and density, but we also find a reversible structure change over 0 &ndash 22 GPa. As glass at high pressure is used as a proxy for melt or magma at high pressure, this ultimately documents a structural change in the magma and has implications for the buoyancy of silicate melts in terrestrial planets.

Two particular experimental approaches have been experiencing increased interest, that of dynamic spectroscopy and that of 2D measurements. The first allows us to obtain a better understanding of the electronic and chemical processes taking place under shock loading while the second provides a detailed view into the microscopic and macroscopic heterogeneities which directly influence material behavior and have been difficult to identify in the bulk material. In order to better understand these behaviors and their origins, two approaches have been utilized. A novel dynamic broadband optical reflectivity diagnostic was developed to probe changes in material properties on the scale of electronic structure and an existing, but only recently developed high resolution velocimetry system was used to observe the aforementioned changes on larger, microstructural scales.

In order to expand understanding of the chemical and mechanical responses of condensed matter to dynamic shock compression two projects were undertaken. The first was the development of a broadband optical reflectivity diagnostic with both time and wavelength resolution. The Shock Wave Optical Reflectivity Diagnostic (SWORD) has enabled us to study the dynamic optical reflectivity in shocked samples over the visible and near-infrared, across a time span of nanoseconds and with a resolution of 0.5 ns and 10 nm. Laser velocimetry was used in tandem with the SWORD to determine kinematic properties of the shocked samples, such as pressure and density. This novel diagnostic has been applied to the semiconductor-to-metal transition in single-crystal germanium and used to observe both a general reflectivity increase on metallization and a wavelength dependent response as a function of pressure.

The second project involved the application of the recently developed two dimensional Velocity Interferometry System for Any Reflector (2D VISAR, alternatively Janus High Resolution Velocimeter, JHRV) to study anisotropic shock wave propagation and dynamic heterogeneous deformation and fracture in diamond. In combination with the aforementioned laser velocimetry (also VISAR), we have obtained velocity histories and two-dimensional velocity maps and images of the shocked target at various time points after breakout. Significant anisotropy in both the elastic and inelastic waves was observed in single-crystal diamond samples. Characteristic length scales for the fracture of polycrystalline diamond samples of varying grain size were also determined.

We report Raman and infrared absorption spectroscopy along with X-ray diffraction for brucite-type beta-Cd(OH)2 to 28 GPa at 300 K. The OH-stretching modes soften with pressure and disappear at 21 GPa with their widths increasing rapidly above 5 GPa, consistent with a gradual disordering of the H sublattice at 5 20 GPa similar to that previously observed for Co(OH)2. Asymmetry in the peak shapes of the OH-stretching modes suggests the existence of diverse disordered sitesfor H atoms in Cd(OH)2 under pressure. Above 15 GPa, the A1g(T) lattice mode shows non-linear behavior and softens to 21 GPa, at which pressure significant changes are observed: new Raman modes appear, two Raman-active lattice modes and the OH-stretching modes of the low-pressure phase disappears, and the positions of some X-ray diffraction lines change abruptly with the appearance of weak new diffraction features. These observations suggest that amorphization of the H sublattice is accompanied by a crystalline-to-crystalline transition at 21 GPa in Cd(OH)2, which has not been previously observed in the brucite-type hydroxides. The Raman spectra of the high-pressure phase of Cd(OH)2 is similar to those of the high-pressure phase of single-crystal Ca(OH)2 of which structure has been tentatively assigned to the Sr(OH)2 type.