title: Planetary Materials Studied by Time-Resolved X-ray Diffraction under Compression in Dynamic Diamond Anvil Cells
Creators: name: Méndez, Alba S. J.
Publisher: name: University of Bayreuth
Year: 2021
Subjects: subject: planetary materials
subject: water ice
subject: ferropericlase
subject: diamond anvil cells
subject: x-ray diffraction
subject: proton order-disorder transitions
subject: spin transition
subject: bulk modulus Mineral physics
subject: planetary geology
subject: extreme conditions research
subject: 550

description: Investigating the physical properties of minerals at extreme pressures and temperatures is crucial for understanding the interior structure of planetary bodies and unravelling their formation and evolution. Earth’s interior is characterized by hosting an iron-rich core enveloped by a mantle with a composition dominated by MgSiO3 and (Mg,Fe)O. One of the aspects that make Earth different from other terrestrial bodies in our Solar System is the presence of liquid H2O. Out of the Solar System, Earth-like exoplanets have caught attention as their composition might be analogous to Earth. Depending on the materials available during their formation, Earth-like exoplanets could have accumulated larger amounts of H2O as compared to Earth (ocean planets). Current models for ocean planets assume an interior structure analogous to Earth, with an additional layer overlying the silicate mantle: an icy mantle of high-pressure ice VII/ice X at the bottom of a liquid ocean. Experimental data on the phase diagram and physical properties of these candidate mantle minerals at high-pressure/-temperature are essential to constrain current planetary interior models. Here, a novel experimental approach was implemented to investigate the compression behavior of planetary materials at extreme conditions. A piezo-driven dynamic Diamond Anvil Cell (dDAC) was employed to generate high pressures. Time-resolved X-ray diffraction was used to characterize the evolution of unit cell volumes (V) with pressure (P). The combination of the dDAC with extremely sensitive X-ray detectors allowed for the collection of diffraction images with high temporal resolution while the sample is being compressed. The uniquely dense sampling of pressure and volume in our data allowed to directly determine changes in the bulk modulus by local differentiation of V(P) without having to rely on an equation-of-state formulation. A resistively-heated dDAC (RHdDAC) was implemented for conducting experiments at simultaneous high-P/T. Using this approach, the compression behavior of two planetary materials, H2O ice and (Mg,Fe)O ferropericlase, was investigated in this thesis. H2O crystallizes as ice VII at a pressure of about 2 GPa, a molecular solid characterized by a body centered cubic (bcc) arrangement of oxygens. Increasing pressure triggers the dissociation of H2O molecules towards the formation of ice X, an atomic solid also presenting a bcc crystal lattice. The ice VII – ice X transition proceeds through the symmetrization of the hydrogen-bonds and the formation of intermediate stages without a change in crystallographic symmetry. The continuous nature of this transformation makes its characterization very complicated and no consensus on the transition pressure range has emerged in the literature. Moreover, only a few studies have measured the compression behavior of H2O across the ice VII – ice X transition, and whether or not the H-bond symmetrization has any effect remains unclear. Here, the ice VII – ice X transition was studied in the dDAC up to pressures of about 180 GPa while collecting time-resolved X-ray diffraction. The bulk modulus of H2O ice was directly derived from quasi-continuous V(P) data showing three changes in compression behavior in the ranges of 35-45, 50-55 and 90-110 GPa. The observed changes are in perfect agreement with previous computational results, showing that the H-bond symmetrization influences the high-pressure compressibility of H2O. Preliminary high-temperature data were collected in the RHdDAC across the ice VII –ice X transition, covering a large portion of the phase diagram, to show the potential of the RHdDAC to study thermal effects on the compression behavior of H2O ice. (Mg,Fe)O ferropericlase undergoes changes in compression behavior as a consequence of a change in the spin state of iron at high pressure. The iron spin crossover is a gradual transformation accompanied by an elastic softening of the bulk modulus of ferropericlase starting at 40 GPa at room temperature. Several experimental and computational studies have shown that the pressure range over which the spin crossover occurs and the magnitude of the elastic softening are sensitive to iron content and stress conditions. However, the reported transition ranges differ considerably. Furthermore, although it has been suggested that iron-iron interactions may cause a broadening of the transition pressure range, these effects have not been investigated. Here, the compression behavior (Mg,Fe)O was investigated in the dDAC up to pressures of about 100 GPa while collecting time-resolved X-ray diffraction. Complementary ab-initio calculations were performed to investigate the effect of iron-iron interactions on the elastic softening across the spin crossover. Both experiments and computations are in perfect agreement, showing a broad and asymmetric softening of the bulk modulus of (MgFe)O at pressures between about 40 and 80 GPa, with possible implications for the seismic signature of the spin crossover in the lower mantle. First high-T data were collected in the RHdDAC up to temperatures of 1400 K showing the potential of the RHdDAC to explore thermal effects on the compression behavior of (Mg,Fe)O at high pressure.