A. E. Gleason

A BEAMLINE FOR HIGH PRESSURE STUDIES AT THE ADVANCED LIGHT SOURCE WITH A SUPERCONDUCTING BENDING MAGNET AS THE SOURCE

A new facility for high-pressure diffraction and spectroscopy using diamond anvil high-pressure cells has been built at the Advanced Light Source on beamline 12.2.2. This beamline benefits from the hard X-radiation generated by a 6 T superconducting bending magnet (superbend). Useful X-ray flux is available between 5 keV and 35 keV. The radiation is transferred from the superbend to the experimental enclosure by the brightness-preserving optics of the beamline. These optics are comprised of a plane parabola collimating mirror, followed by a Kohzu monochromator vessel with Si(111) crystals (E/DeltaE approximately equal 7000) and W/B4C multilayers (E/DeltaE approximately equal 100), and then a toroidal focusing mirror with variable focusing distance. The experimental enclosure contains an automated beam-positioning system, a set of slits, ion chambers, the sample positioning goniometry and area detector (CCD or image-plate detector). Future developments aim at the installation of a second endstation dedicated to in situ laser heating and a dedicated high-pressure single-crystal station, applying both monochromatic and polychromatic techniques.

Pressure-temperature stability studies of FeOOH using x-ray diffraction

Abstract The Mie-Grüneisen formalism is used to fit a Birch-Murnaghan equation of state to high-temperature (T), high-pressure (P) X-ray diffraction unit-cell volume (V) measurements on synthetic goethite (α-FeOOH) to combined conditions of T = 23-250 °C and P = 0-29.4 GPa. We find the zero-pressure thermal expansion coefficient of goethite to be α0 = 2.3 (±0.6) × 10-5 K-1 over this temperature range. Our data yield zero-pressure compressional parameters: V0 = 138.75 (±0.02) Å3, bulk modulus K0 = 140.3 (±3.7) GPa, pressure derivative K0′ = 4.6 (±0.4), Grüneisen parameter γ0 = 0.91 (±0.07), and Debye temperature Θ0 = 740 (±5) K. We identify decomposition conditions for 2α-FeOOH → α-Fe2O3 + H2O at 1-8 GPa and 100-400 °C, and the polymorphic transition from α-FeOOH (Pbnm) to ε-FeOOH (P21mn). The non-quenchable, high-pressure ε-FeOOH phase P-V data are fitted to a second-order (Birch) equation of state yielding, K0 = 158 (±5) GPa and V0 = 66.3 (±0.5) Å3.

Pressure-temperature studies of talc plus water using X-ray diffraction

Abstract X-ray diffraction measurements of natural talc plus water at combined pressures and temperatures of 0-15 GPa and 23-400 °C reveal the presence of a structural change that could be interpreted as a new high-pressure phase at 4.0 (±0.5) GPa, and raise the possibility that the newly inferred phase transition takes place in cold subducting slabs as a precursor to appearance of the 10 Å phase of talc.