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

Fe-Mg interdiffusion rates in clinopyroxene: Experimental data and implications for Fe-Mg exchange geothermometers

Chemical interdiffusion of Fe-Mg along the c-axis [001] in natural diopside crystals (XDi = 0.93) was experimentally studied at ambient pressure, at temperatures ranging from 800 to 1,200 °C and oxygen fugacities from 10-11 to 10-17 bar. Diffusion couples were prepared by ablating an olivine (XFo = 0.3) target to deposit a thin film (20-100 nm) onto a polished surface of a natural, oriented diopside crystal using the pulsed laser deposition technique. After diffusion anneals, compositional depth profiles at the near surface region (~400 nm) were measured using Rutherford backscattering spectroscopy. In the experimental temperature and compositional range, no strong dependence of DFe-Mg on composition of clinopyroxene (Fe/Mg ratio between Di93-Di65) or oxygen fugacity could be detected within the resolution of the study. The lack of fO2-dependence may be related to the relatively high Al content of the crystals used in this study. Diffusion coefficients, DFe-Mg, can be described by a single Arrhenius relation with (Formula presented). DFe-Mg in clinopyroxene appears to be faster than diffusion involving Ca-species (e.g., DCa-Mg) while it is slower than DFe-Mg in other common mafic minerals (spinel, olivine, garnet, and orthopyroxene). As a consequence, diffusion in clinopyroxene may be the rate-limiting process for the freezing of many geothermometers, and compositional zoning in clinopyroxene may preserve records of a higher (compared to that preserved in other coexisting mafic minerals) temperature segment of the thermal history of a rock. In the absence of pervasive recrystallization, clinopyroxene grains will retain compositions from peak temperatures at their cores in most geological and planetary settings where peak temperatures did not exceed ~1,100 °C (e.g., resetting may be expected in slowly cooled mantle rocks, many plutonic mafic rocks, or ultra-high temperature metamorphic rocks)

Strength of magnesium oxide under high pressure:13; evidence for the grain-size dependence

X-ray diffraction patterns from magnesium oxide compressed in a diamond anvil cell up to 55 GPa have been recorded and13; the differential stress (a measure of compressive Strength) and grain-size (crystallite size) determined as a function of pressure from the line-width analysis. The strength agrees well with the uniaxial stress component (another measure of compressive strength) derived earlier from the line-shift data. The strength increases while the crystallite size decreases steeply as the pressure is raised from ambient to w10 GPa. The increase in strength is much smaller at higher pressures. The strength-pressure13; data are explained by combining the grain-size dependence of strength and the shear-modulus scaling law. The dependence of strength on grain-size has not been considered in the past in the discussion of high-pressure strength data

Compression behavior of NbC under nonhydrostatic conditions to 57 GPa 13; 13;

The analysis of the diffraction data under nonhydrostatic stress condition created in a diamond anvil cell suggests that the compressive strength of NbC crystallites is at least 24 xB1; 4 GPa at a confining pressure of 57 xB1; 0.5 GPa. When using Al as a pressure transmitting medium in the diamond anvil cell pressure conditions become nearly hydrostatic. The bulk modulus derived from the compression data under quasi-hydrostatic pressure condition gives K0 = 274 xB1; 3 GPa when fixing . This value agrees well with the value obtained from the first-principles numerical calculation

Strength of iron under pressure up to 55 GPa from X-ray diffraction line-width analysis13;

The X-ray diffraction patterns are recorded from polycrystalline iron sample compressed in a diamond anvil cell up to 55 GPa. The maximum micro-stress in the sample, a measure of compressive strength, is derived from the line-width analysis. The strength of iron in the body centered cubic (BCC)-phase is 1.1amp;plusmn;0.2 GPa. This value is in good agreement with the strength derived from the hardness vs. grain-size data. The strength increases steeply during the BCC-hexagonal closed packed (HCP) transition. The strength-pressure data for HCP-iron fit the relation amp;sigma; = 2.9 + 0.028 P, where amp;sigma; and P are the compressive strength and pressure in GPa, respectively. The present results agree well with those obtained from the line shift analysis carried out in earlier studies

In situ x-ray diffraction of fast compressed iron: Analysis of strains and stress under non-hydrostatic pressure

Series of high-pressure x-ray diffraction patterns of iron and its high-pressure polymorphs were collected with 0.1–0.2-s exposure time utilizing a membrane diamond anvil cell (DAC) for compression at various loading and unloading rates to a maximum pressure of 70 GPa. Strain rates of 10−2 s−1 at a maximum pressurization rate of 4.1 GPa/s were achieved in non-hydrostatic compression of hcp Fe. Linewidth analysis was used to retrieve strain and uniaxial stress of Fe as a function of pressure upon both compression and decompression. Analysis of the lattice parameters ratio c/a of hcp Fe indicates the presence of complex non-hydrostatic stress states, which developed as a function of strain rate, relaxation time, and various levels of hydrostaticity. Our results emphasize the importance of a controlled pressurization in DACs because the experimental loading rate strongly influences the stress state of the sample, particularly on decompression. Our time-resolved x-ray diffraction of the phase transition from bcc Fe to hcp Fe reveals residual grains of bcc Fe capable of surviving to very high pressures (>35 GPa) for a few minutes after the transition

Compression behavior of TaC0.98 under nonhydrostatic13; and quasi-hydrostatic pressures up to 76 GPa

Powder samples of TaC0.98 sandwiched between aluminum disks were placed in a rhenium gasket and compressed in a diamond anvil cell. The X-ray diffraction patterns were recorded under pressures up to 50 GPa using synchrotron radiation. The presence of aluminum in the cell rendered the sample pressure nearly hydrostatic and also served as the pressure standard. In another set experiments,TaC0.98 powder mixed with small quantity of platinum powder was placed in stainless steel gasket and compressed between the anvils. The X-ray diffraction patterns were recorded up to 76 GPa. In absence of any pressure-transmitting medium, the stress state of the sample was expected to be highly nonhydrostatic. The diffraction data were analyzed using lattice strain theory to estimate, the difference between the axial and radial stress components in the sample. The magnitudes of t suggest that the lower limit13; of compressive strength of TaC0.98 increases with increasing pressure and reaches -11 GPa at 76 GPa pressure. No phase transformation was observed up to the highest pressure. The bulk modulus and its pressure derivative derived from the volume-compression-pressure data are 345(9) GPa and 4.0(4), respectively

Compression behavior of VC0:85 up to 53 GPa

Samples of VC0:85 sandwiched between aluminum disks were compressed in a diamond anvil cell and X-ray diffraction patterns recorded at high pressures up to 53 GPa using synchrotron radiation. The presence of aluminum in the cell rendered the sample pressure nearly hydrostatic and also served as the pressure standard. No phase transformation was observed up to the highest pressure. The measured unit cell volume versus pressure data gave 258 xB1; 11 GPa and 4.5 xB1; 0.6 for the bulk modulus and the pressure derivative, respectively

Oxidized iron in garnets from the mantle transition zone

The oxidation state of iron in Earth’s mantle is well known to depths of approximately 200 km, but has not been characterized in samples from the lowermost upper mantle (200–410 km depth) or the transition zone (410–660 km depth). Natural samples from the deep (>200 km) mantle are extremely rare, and are usually only found as inclusions in diamonds. Here we use synchrotron Mössbauer source spectroscopy complemented by single-crystal X-ray diffraction to measure the oxidation state of Fe in inclusions of ultra-high pressure majoritic garnet in diamond. The garnets show a pronounced increase in oxidation state with depth, with Fe3+/(Fe3++ Fe2+) increasing from 0.08 at approximately 240 km depth to 0.30 at approximately 500 km depth. The latter majorites, which come from pyroxenitic bulk compositions, are twice as rich in Fe3+ as the most oxidized garnets from the shallow mantle. Corresponding oxygen fugacities are above the upper stability limit of Fe metal. This implies that the increase in oxidation state is unconnected to disproportionation of Fe2+ to Fe3+ plus Fe0. Instead, the Fe3+ increase with depth is consistent with the hypothesis that carbonated fluids or melts are the oxidizing agents responsible for the high Fe3+ contents of the inclusions