An in situ angle-dispersive x-ray diffraction study was undertaken of iron in a laser-heated, diamond-anvil cell up to 2375 kelvin and between 30 and 100 gigapascals in Al2O3- and SiO2-pressure media. The resolution and reliability of diffraction peak intensities allow quantitative assessment of a structural model. The results confirm that iron undergoes a phase transformation at high pressures and temperatures. The space group isPbcm for an orthorhombic lattice, and the atomic topology is close to that of ɛ hexagonal close-packed iron.

https://science.sciencemag.org/content/sci/278/5339/831.full.pdf

The Orthorhombic Structure of Iron: An in Situ Study at High-Temperature and High-Pressure

The crystal structure of a high-pressure Fe 3 O 4 phase was determined by in situ X-ray diffraction measurements at high pressure and temperature, using an imaging plate detector and monochromatic synchrotron X-radiation. The high-pressure phase has the Pbcm space group (CaMn 2 O 4 -type structure) with cell parameters a = 2.7992(3) Aa, b = 9.4097(15) Aa, and c = 9.4832(9) Aa at 23.96 GPa and 823 K. Fe (super 3+) occupies an octahedral site and Fe (super 2+) is in an eightfold-coordinated site described as a bicapped trigonal prism. The high-pressure CaMn 2 O 4 -type Fe 3 O 4 phase is about 6.5% more dense than the spinel form at 24 GPa.

In situ structure determination of the high-pressure phase of Fe3O4

The (Mg,Fe) partition coefficients between Al-(Mg,Fe)SiO 3 perovskite (Pv) and (Mg,Fe)O magnesiowustite (Mw) were inferred from Pv and Mw volumes measured by X-ray diffraction (European Synchrotron Radiation Facility, Grenoble), from 22 to 120 GPa, after laser annealing up to 2300 K. The (Mg,Fe) partition coefficient is found to decrease with increasing pressure and temperature (at moderate pressures) and with the additions of Al 2 O 3 . More iron is found in perovskite than in magnesiowustite at the highest pressures and temperatures. Artifacts possibly encountered during the calculation are discussed. Perovskite was found stable up to 120 GPa and 2200 K, with iron contents (Fe/(Mg+Fe)) up to 25%. The effects of Fe and Al 2 O 3 on the orthorhombic distortion remain reduced.

/pdf/evaluation-of-mg-fe-partitioning-between-silicate-perovskite-3xvic1ch30.pdf

Evaluation of (Mg,Fe) partitioning between silicate perovskite and magnesiowustite up to 120 GPa and 2300 K

Our knowledge of the structure of the Earth's interior has been obtained by analysing seismic waves that travel in the Earth, and the reference Earth global models used by geophysicists are essentially seismological. Depth profiles of the seismic waves velocities reveal that the deep Earth is divided in several shells, separated by velocity and density discontinuities. The main discontinuity located at a depth of 2900 km corresponds to the transition between the mantle and the core. The Earth's mantle can be further divided into the upper mantle and the lower mantle, with a transition zone characterised by two prominent increases in velocities observed at 410- and 660-km depths. This article will be focused on the mineral phases of the Earth's mantle. The interpretation of seismological models in terms of chemical composition and temperature relies on the knowledge of the nature, structure and elastic properties of the candidate materials. We will describe to what extent recent advances in experimental mineral physics and X-ray diffraction have yielded essential knowledge on the structure and high-pressure high-temperature behaviour of pertinent materials, and major improvements in our understanding of the chemical and mineralogical composition of the Earth's mantle.

Mineral phases of the Earth´s mantle

The last decade has seen the development of very bright third generationx-ray synchrotron sources that have significantly improved x-ray diffraction experiments at high pressure and high temperature. In the same time, focusing optics as well as detectors have been improved in such a way that x-ray diffraction experiments can be routinely carried out on samples in laser-heated diamond anvil-cell, i.e., under extreme static pressure and temperature conditions. It is now possible to obtain from a laser heated sample in a diamond anvil cell(DAC) very high quality powder patterns, needed for accurate in situ phase boundary and pressure-volume-temperature equation of state determinations, as well as for structural refinements. The setup for in situx-ray investigation at simultaneous high pressure and temperature is complex, since perfect alignment of x-ray path, infrared-laser hot spot, and optical temperature measurement is required. It provides, however, the most accurate information on the structural behavior with step by step increases of pressure and temperature. In addition, a range of experiments can also be realized at room temperature after laser annealing of DAC samples, which has been shown very efficient in releasing stresses build on compression, and/or overcoming phase transitions kinetic barriers.

Synchrotron radiation and laser heating in a diamond anvil cell

The European Synchrotron Radiation Facility (ESRF) is the first high-energy, high-brilliance synchrotron radiation source in operation today and it is ideally suited to high pressure research. Whit...

Optics and beamlines for high-pressure research at the european synchrotron radiation facility

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Optics and beamlines for high-pressure research at the european synchrotron radiation facility