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Perovskite-structured MgSiO3 dominates the mineralogy of Earth's lower mantle. As a result, the physical properties and phase transitions of this mineral are key to understanding anomalous seismic observations of the mantle's lowermost 150-300 km-- the D'' region. Recent literature suggests a post-perovskite phase of MgSiO 3 , experimentally observed at pressures and temperatures consistent with those expected to exist at D'', is responsible for the observed discontinuity in seismic velocity. While characterizing the crystal-chemistry, structure, and elastic properties of these two mineral enigmatic region of the Earth, many conventional experimental apparatus are unable to reproduce these extreme conditions in the laboratory. Thus, measurements of the solubility of trace elements, the elastic changes with pressure and temperature, and the Clapeyron slope between perovskite and post-perovskite phases are in desperate need; however difficult or impossible to perform on MgSiO 3 directly. This dissertation addresses structure changes at high pressure and temperature occurring in materials analogous to MgSiO 3 with perovskite and post-perovskite structure, considering that conclusions drawn from this research will prove useful to a subsequent understanding of the elastic, rheological, and crystal-chemical properties of MgSiO3 . Neighborite (NaMgF3 ) is isostructural to orthorhombic ( Pbnm ) MgSiO3 perovskite. On the basis of X-ray diffraction data, previous research by Yusheng Zhao has shown that increasing temperature, or potassium substitution for sodium in the structure, drives an evolution in the average structure (> 100 Å) towards a perovskite with cubic ( Pm3m ) symmetry. Through utilization of pair-distribution function analysis and nuclear magnetic resonance spectroscopy, experimental techniques sensitive to short-range structure (< 20 Å), we show that when the average structure appears cubic with X-ray diffraction the local structure remains orthorhombic. By measuring the velocity of ultrasonic waves (MHz) through millimeter-sized polycrystalline samples we characterize the elastic behavior of NaMgF 3 in situ at high pressure and temperature as X-ray diffraction shows a transition from orthorhombic to cubic in the average structure. Accompanying constant signal intensity from the acoustic buffer-rod, we observe attenuation before a disappearance of the ultrasonic signal from the sample as pressure-temperature conditions approach the phase transition. While the perovskite/post-perovskite phase transition occurs in MgSiO 3 at 120 GPa, the perovskite/post-perovskite phase transition in NaMgF 3 is driven at much lower pressures. Using X-ray diffraction data collected from NaMgF3 in the diamond anvil cell, we identify the pressure where this material transitions to a post-perovskite structure. Upon heating post-perovskite NaMgF3 , we find crystallization of a new phase(s) in the NaMgF3 system. Through Rietveld structure refinement of perovskite and post-perovskite NaMgF3 , we propose a method for predicting the pressure where a post-perovskite structure becomes more stable than a perovskite structure of a given ABX3 composition. The material CaIrO3 is isostructural to the post-perovskite structure of MgSiO3 (Cmcm ). Utilizing X-ray powder diffraction we identify the compressibility and thermal expansion of the CaIrO 3 unit cell. Via Rietveld refinement of structure models fit to the X-ray diffraction data, we identify structure changes in CaIrO3 at high temperature. Rietveld refinement of structure models fit to time-of-flight neutron diffraction data reveal structure changes occurring at high pressure. Through a synthesis of the data, we propose a method for predicting the Clapeyron slope between perovskite and post-perovskite (CaIrO 3 -type) structures.
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