HIGH-PRESSURE STUDIES OF ANALOGS WITH APPLICATIONS TO MATERIALS SCIENCE AND GEOSCIENCE

Abstract

Analog materials can provide insights to understanding inaccessible phase transitions and novel material properties. The work presented here examines both structural and chemical variation of analogs in order to probe the physics and chemistry of materials at high pressure and temperature conditions. The experiments reported here used the diamond anvil cell to achieve pressures up to 1.5 Mbar and temperatures as high as 2500 K, in conjunction with synchrotron x-ray diffraction and Raman spectroscopy. The work is divided into three major projects.

Polymorphism in AX2-type compounds was explored using PbF2 as an archetype for phases with highly coordinated cations. These materials are of interest due to their potential technical applications and, in the case of SiO2, their geophysical relevance. Compression studies at room temperature revealed an unusual isosymmetric phase transition in PbF2, and the combination of experimental and theoretical approaches explored the mechanism of the phase transformation that had hitherto not been understood. High-pressure-temperature experiments led to the discovery of a new phase transition in this material, thus furthering our understanding of phase transformation pathways in AX2 materials.

Garnets are important compounds in geoscience and materials science. The high-pressure behavior of Y3Fe5O12 has been controversial due to conflicting reports regarding its high-pressure polymorphs. Here I show that the high-pressure phase is a perovskite with a (Y0.75Fe0.25)FeO3 composition. In addition, I also identify a spin transition in the octahedral Fe3+ site at 45-51 GPa. Comparison with other perovskite-structured orthoferrites shows that the volume discontinuity associated with the spin transition is controlled by the size of the cation occupying the distorted dodecahedral site.

Silicate perovskites and post-perovskites are the dominant mineral phases in the Earth’s lower mantle. The effect of incorporation of Fe2+ on the perovskite and post-perovskite structures, and on the phase transition between them, is investigated using the analog system (Mg,Fe)GeO3. At high pressures and temperatures, single-phase perovskites and post-perovskites were be synthesized for Mg-rich compositions with Mg# ≥ 78. Iron is found to lower the perovskite-post-perovskite phase boundary, and to affect a modest decrease of bulk modulus and a modest increase in volume of perovskites.