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Earth's core-mantle boundary (CMB) is the most extreme interface of the planet's interior. It regulates the flow of heat out of the core and in doing so influences the two internal engines of our dynamic habitable planet: convection in the solid mantle and the magnetic geodynamo in the core. Seismic observations of the CMB have revealed a complex landscape of heterogeneous multi-scale structures that likely play key roles in Earth's internal dynamics and may hold memory of Earth's ancient past. Many details of the compositions and properties of these structures, however, are essentially unknown. In this thesis, I deploy a suite of experimental techniques and interdisciplinary approaches to constrain the temperature and phase relations of the CMB, properties that affect dynamics of the mantle and core. In particular, I focus my study on ultralow velocity zones (ULVZs) - the most extreme and perhaps least well understood structures in the lowermost mantle. I first quantitatively show that these structures, originally posited to be areas of partial melt, can be well explained as solid FeO-rich formations given seismic, geodynamic, and mineralogical constraints. To further explore the viability of such solid FeO-rich structures, I develop a multi-technique approach combining two in-situ synchrotron-based methods, one sensitive to crystal structure and another to atomic dynamics, to study the high-pressure melting of iron-bearing materials. With this approach, I place new constraints on the core-mantle boundary temperature by measuring the melting temperature of a candidate core-forming alloy (Fe0.8Ni0.1Si0.1) at high pressures, finding that the addition of silicon to an Fe0.9Ni0.1 core can reduce CMB temperatures to ~3500 K. I then measure the melting of Fe0.94O, finding a melting temperature of 4140 ± 110 K at CMB pressure, demonstrating the stability of solid FeO-rich ULVZs in the lowermost ma

Hydrated sulfates have been identified and studied in a wide variety of environments on Earth, Mars, and the icy satellites of the solar system. The subsurface presence of hydrous sulfur-bearing phases to any extent necessitates a better understanding of their thermodynamic and elastic properties at pressure. End-member experimental and computational data are lacking and are needed to accurately model hydrous, sulfur-bearing planetary interiors. In this work, high-pressure X-ray diffraction (XRD) and synchrotron Fourier-transform infrared (FTIR) measurements were conducted on szomolnokite (FeSO₄·H₂O) up to ~83 and 24 GPa, respectively. This study finds a monoclinic-triclinic (C2/c to P1̅) structural phase transition occurring in szomolnokite between 5.0(1) and 6.6(1) GPa and a previously unknown triclinic-monoclinic (P1̅ to P2₁) structural transition occurring between 12.7(3) and 16.8(3) GPa. The high-pressure transition was identified by the appearance of distinct reflections in the XRD patterns that cannot be attributed to a second phase related to the dissociation of the P1̅ phase, and it is further characterized by increased H₂O bonding within the structure. We fit third-order Birch-Murnaghan equations of state for each of the three phases identified in our data and refit published data to compare the elastic parameters of szomolnokite, kieserite (MgSO4·H2O), and blödite (Na₂Mg(SO₄)₂·4H₂O). At ambient pressure, szomolnokite is less compressible than blödite and more than kieserite, but by 7 GPa both szomolnokite and kieserite have approximately the same bulk modulus, while blödite’s remains lower than both phases up to 20 GPa. These results indicate the stability of szomolnokite’s high-pressure monoclinic phase and the retention of water within the structure up to pressures found in planetary deep interiors.