Research Opportunities

Collaborations

Our laboratory provides an ideal framework for developing new, complementary techniques to study physical and chemical properties of materials at high P-T conditions. We are interested in high-pressure projects in geophysics, planetary physics and condensed matter physics. We welcome enquiries on collaborative research projects. In particular we would be happy to discuss hosting research fellowships awarded to individuals from NATO, EU or any other international organization.

Graduate Study (Postgraduate) Research Opportunities

Supervisor: Dr. Andrew P. Jephcoat, email: andrew@earth.ox.ac.uk

Scholarships for international students

High-pressure partitioning experiments in the diamond-anvil cell

High-pressure and high-temperature metal-silicate interactions are important for understanding the separation of the Earth's core and the present nature of the core-mantle boundary (P = 130 GPa, 130 Mbar, depth = 2900 km). High-pressure partitioning of elements between metal and silicate melts (M-Sil) would constrain the nature of the light elements present in the core. Moreover, understanding these M-Sil interactions may help in understanding the nature of the seismically anomalous D" layer above the core-mantle boundary. Some major ongoing questions about the nature of the Earth's core include (i) whether it's formation was in equilibrium with the mantle and occurred as segregation or metal melts from a magma ocean on a full-sized, hot, proto-Earth, or more gradually from smaller planetesimals at lower pressures and temperatures, (ii) whether there has been subsequent interaction between core and mantle, and (iii), whether there is now disequilibrium at the CMB. Recent work suggests that both temperature and pressure could be involved in the "excess siderophile" problem in the mantle by changing the effective partitioning relations between metal and silicate melt, or indeed between silicate melt and metal/metal-sulphide phases, although there are many other parameters such as melt composition, amount of ferric iron (oxygen fugacity) that may have a dominant effect. Much of the available data so far has been obtained from experiments in multi-anvil high-pressure experiments, with relatively few measurements performed in the diamond-anvil cell. In this project, we plan to develop techniques for making partitioning experiments in the laser-heated diamond-anvil cell (DAC) together with methods for the microanalysis of quenched samples. A primary goal would be to first confirm the results against the multianvil experiments (up to Earth's transition zone pressures: 13–20 GPa, 400–600 km deep) and then to extend the pressure range in order to search for an explicit pressure effect on the partition coefficients of major/minor transition metals, potassium and trace metals among silicate and metal/metal-sulphide melts. The project will require use of infra-red lasers coupled to the DAC and considerable work confirming that equilibrium thermodynamic conditions can be generated on samples 0.02–0.05 mm in size. Analytical techniques for the fine-scale quench textures generated will be developed and aided by new electron microprobe and ion-probe (micro-SIMS) instruments.

High-pressure studies of hydrogen (water) in deep-Earth minerals: Neutron and Light scattering experiments

Knowledge of the pressure dependence of the structural behaviour of OH and hydrogen-bonding in minerals is of importance in understanding their crystal chemistry, stability and elastic properties (e.g. compressibilities). It is well-known that the OH environment and hydrogen-bonding topology can have a major effect upon mineral compressibilities (e.g. compare chlorite and brucite). However, the study of OH behaviour in geological materials at high pressure is still in its infancy and much important information is to be gained from high-pressure studies using spectroscopic and diffraction methods. Together these methods are a powerful combination and give highly complementary information. Developing widely-applicable quantitative correlations between structural parameters (as measured by x-ray or neutron from diffraction) and spectroscopic signatures is an important objective of high-pressure research in the mineral sciences. Vibrational spectroscopies, such as infrared and Raman, are excellent probes of the local environment of OH and hydrogen bonding. Micro-Raman spectroscopy, in conjunction with the diamond-anvil cell, can be used to study the high-pressure properties of minerals having a range of OH and hydrogen-bonding environments and topologies for which good ambient and low-pressure structural data already exist. A number of these minerals (e.g. chlorite, hydrous-B analogues, humites, hydrous pyroxenes) are relevant to mantle mineralogy and geophysics, while other types will allow novel topologies to be explored. The work will address a range of minerals including those synthesised in high-pressure multi-anvil apparatus at depths as great as the Earth's transition zone (410–670 km deep, 18–30 GPa) where water may play an important role in phase relations.
Supervisors: Dr. Andrew P. Jephcoat and Dr. Mark D. Welch (Division of Mineral Sciences & Systematics, Department of Mineralogy, The Natural History Museum, London)

Experimental studies of molecular systems at extreme (planetary) pressure and temperature conditions

The giant planets represent a large natural reservoir of simple molecular systems at ultrahigh pressures. The most abundant of these constituents are thought to be hydrogen and helium as well as the "ices" ammonia, methane and water for Uranus and Neptune. H and He are subjected to conditions ranging from 1 bar and 160 K to 450 GPa (45 Mbar) and 24000 K in Jupiter; and, in Saturn, to near 10 Mbar and 10000K. The "ices" in Uranus and Neptune are subjected to pressures between 0.2 -7 Mbar and 2000 to 8000 K. Compound formation and solid solution in these systems, not observed at ambient conditions, as well as phenomena like polymerization and molecular dissociation under the more extreme conditions take place in planetary interiors. Light scattering measurements of Raman-active phonons can provide a direct probe of bonding and are a powerful tool for phase-diagram studies. Raman spectroscopic measurements are necessary to evaluate the inter- and intramolecular interactions as well as P-T studies of pure compounds and mixtures, which might have impacts on our current models of the interior of the outer planets. Selected simple molecular systems including mixtures, relevant to the outer planets, will be studied in the laboratory in the 100 GPa regime using high-pressure diamond-anvil cell techniques in connection with laser-heating to ~3000 K and Raman spectroscopy. High temperatures will be essential in the detection of new phases as well as in synthesizing new materials by lowering the high kinetic barriers often present at low temperatures. The experimental work is useful because for many simple molecular systems, comparison with theoretical results indicates that we lack a sufficiently accurate description of the interaction potentials and a quantitative understanding of behaviour at ultrahigh pressures.

Melting curves of rare gas solids

The rare gases are important trace elements in the Earth and provide geochemical constraints on processes ranging from atmosphere evolution to Earth formation and convection within the solid mantle. In the condensed phase at high pressures the light rare gases (He, Ne) have low melting points and are soft solids. They are frequently used to provide an inert sample environment in laboratory experiments with the diamond-anvil cell that can subject samples to ultrahigh pressures up to 106 atmospheres pressure. The heavier rare gases (Ar, Kr, Xe) appear from recent experiments to form dense, high-melting point solids under these conditions comparable to the mineral phases of the Earth's deep interior and may affect the rate at which they are degassed from planetary interiors. The project will use atomistic modelling techniques including first-principles electronic structure methods and ab-initio and classical model-based molecular-dynamics simulation to calculate the pressure-volume relations and melting point curves to pressures in the 100 GPa range for the suite of rare gas elements. A comparison of the theoretically predicted properties with those measured under pressure makes a good test of these first-principles methods and can extend knowledge of material properties into a pressure and temperature range not yet reached in experiments. This project will suit a student with a strong physics or physical chemistry education and will provide training in leading edge quantum-mechanical and classical simulation methods. The successful applicant will interact closely with experimental as well as theoretical researchers at the forefront of deep-Earth physics.

Synchrotron X-ray diffraction experiments on high-pressure perovskites of the Earth's lower mantle

Transitions to the dense phases of the lower mantle below 670 km have recently been shown to depend strongly on the presence of minor cations such as aluminium. The presence of Aluminium for example appears to stabilize high concentrations of Fe3+ in the perovskite structure. Mössbauer spectra suggest that Fe3+/Fe is as high as 50% and it has also been suggested that garnet and perovskite coexist up to 65 GPa in this system, but the detailed properties remain controversial. The project will involve synthesis of high-pressure perovskites at the Bayreuth Geoinstitut in Germany with varying levels of Fe3+ present and experiments at Grenoble with high-resolution synchrotron x-ray diffraction and analysis with Rietveld (profile) refinement techniques to assess the role of Fe3+ in aluminous and other perovskites. We have an established track record at various beamlines at the European Synchrotron Radiation Facility (ESRF), Grenoble. Beamline BM16 at ESRF has been optimized for high-resolution powder diffraction that could readily be used for the Rietveld analysis of quenched, high-pressure mineral phases. These experiments will provide a detailed knowledge of the crystal chemistry in a quaternary system relevant to new compositional models of the Earth's lower mantle. Further, because the concentration of Fe3+ is significant, the techniques of anomalous scattering with a incident beam wavelength near the iron absorption edge, could be used to distinguish unambiguously the site occupancy of Fe2+ and Fe3+ in the structure and can be compared directly with existing Mössbauer results. The high x-ray brilliance at ESRF is expected to make these experiments possible for the first time including in situ high resolution, high-pressure powder diffraction experiments with the diamond-anvil cell.