Novel experimental setup for megahertz X-ray diffraction in a diamond anvil cell at the High Energy Density (HED) instrument of the European X-ray Free-Electron Laser (EuXFEL).

The high-precision X-ray diffraction setup for work with diamond anvil cells (DACs) in interaction chamber 2 (IC2) of the High Energy Density instrument of the European X-ray Free-Electron Laser is described. This includes beamline optics, sample positioning and detector systems located in the multipurpose vacuum chamber. Concepts for pump-probe X-ray diffraction experiments in the DAC are described and their implementation demonstrated during the First User Community Assisted Commissioning experiment. X-ray heating and diffraction of Bi under pressure, obtained using 20 fs X-ray pulses at 17.8 keV and 2.2 MHz repetition, is illustrated through splitting of diffraction peaks, and interpreted employing finite element modeling of the sample chamber in the DAC.

Ferrous Iron Under Oxygen-Rich Conditions in the Deep Mantle.

Recent experiments have demonstrated the existence of previously unknown iron oxides at high pressure and temperature including newly discovered pyrite-type FeO2 and FeO2Hx phases stable at deep terrestrial lower mantle pressures and temperatures. In the present study, we probed the iron oxidation state in high-pressure transformation products of Fe3+OOH goethite by in situ X-ray absorption spectroscopy in laser-heated diamond-anvil cell. At pressures and temperatures of ~91 GPa and 1,500-2,350 K, respectively, that is, in the previously reported stability field of FeO2Hx, a measured shift of -3.3 ± 0.1 eV of the Fe K-edge demonstrates that iron has turned from Fe3+ to Fe2+. We interpret this reductive valence change of iron by a concomitant oxidation of oxygen atoms from O2- to O-, in agreement with previous suggestions based on the structures of pyrite-type FeO2 and FeO2Hx phases. Such peculiar chemistry could drastically change our view of crystal chemistry in deep planetary interiors.

High-speed tomography under extreme conditions at the PSICHE beamline of the SOLEIL Synchrotron.

In situ microtomography at high pressure and temperature has developed rapidly in the last decade, driven by the development of new high-pressure apparatus. It is now routinely possible to characterize material under high pressure with acquisition times for tomograms of the order of tens of minutes. Here, advantage was taken of the possibility to combine the use of a pink beam projected through a standard Paris-Edinburgh press in order to demonstrate the possibility to perform high-speed synchrotron X-ray tomography at high pressure and temperature allowing complete high-resolution tomograms to be acquired in about 10 s. This gives direct visualization to rapidly evolving or unstable systems, such as flowing liquids or reacting components, and avoids assumptions in the interpretation of quenched samples. Using algebraic reconstruction techniques allows the missing angle artefacts that result from the columns of the press to be minimized.

Sound velocities and thermodynamical properties of hcp iron at high pressure and temperature.

Sound velocities and thermodynamical properties of hcp iron have been computed usingab initiocalculations over an extended density and temperature range, encompassing the conditions directly relevant for the Earth's inner core. At room temperature, and up to 350 GPa, an excellent agreement is obtained between present results and experimental data for many thermodynamical quantities: phonon density of states, vibrational entropy, heat capacity, Grüneisen parameter and thermal expansion. With increasing temperature, along an isochore, we observe a strong decrease of the phonon frequencies, demonstrating that intrinsic anharmonic effects cannot be neglected. We also carefully compare previous theoretical data for the sound velocities and try to explain the discrepancies observed with experiments. Finally, we propose a temperature dependant Birch's law that we compare with previous experimental work.

The Fe-FeSi phase diagram at Mercury's core conditions.

Mercury's metallic core is expected to have formed under highly reducing conditions, resulting in the presence of significant quantities of silicon alloyed to iron. Here we present the phase diagram of the Fe-FeSi system, reconstructed from in situ X-ray diffraction measurements at pressure and temperature conditions spanning over those expected for Mercury's core, and ex situ chemical analysis of recovered samples. Under high pressure, we do not observe a miscibility gap between the cubic fcc and B2 structures, but rather the formation of a re-entrant bcc phase at temperatures close to melting. Upon melting, the investigated alloys are observed to evolve towards two distinct Fe-rich and Fe-poor liquid compositions at pressures below 35-38 GPa. The evolution of the phase diagram with pressure and temperature prescribes a range of possible core crystallization regimes, with strong dependence on the Si abundance of the core.

Melting of iron at Earth's inner core boundary based on fast X-ray diffraction.

Earth's core is structured in a solid inner core, mainly composed of iron, and a liquid outer core. The temperature at the inner core boundary is expected to be close to the melting point of iron at 330 gigapascal (GPa). Despite intensive experimental and theoretical efforts, there is little consensus on the melting behavior of iron at these extreme pressures and temperatures. We present static laser-heated diamond anvil cell experiments up to 200 GPa using synchrotron-based fast x-ray diffraction as a primary melting diagnostic. When extrapolating to higher pressures, we conclude that the melting temperature of iron at the inner core boundary is 6230 ± 500 kelvin. This estimation favors a high heat flux at the core-mantle boundary with a possible partial melting of the mantle.

High efficiency multichannel collimator for structural studies of liquids and low-Z materials at high pressures and temperatures.

A high efficiency multichannel collimator (MCC) device has been developed at the high pressure beamline ID27 of the European Synchrotron Radiation Facility to drastically reduce the x-ray background from the sample environment in the Paris-Edinburgh press. The main technical difficulty, which resides in the minimum slits size achievable using the classical mono-bloc design, has been resolved using an original concept based on a set of independent slits. Then, a very small slit size of 50 µm was manufactured resulting in a great improvement of the signal to background ratio. In addition, the transfer function of the MCC has been measured using the x-ray diffusion signal of a metal doped glass and efficiently applied to correct the raw data. The potential of this new device is illustrated in two challenging examples: iron-sulfur liquid structures and C(60) polymerization process at high pressure and high temperature.