BM23 is the general‐purpose EXAFS bending‐magnet beamline at the ESRF, replacing the former BM29 beamline in the framework of the ESRF upgrade. Its mission is to serve the whole XAS user community by ...providing access to a basic service in addition to the many specialized instruments available at the ESRF. BM23 offers high signal‐to‐noise ratio EXAFS in a large energy range (5–75 keV), continuous energy scanning for quick‐EXAFS on the second timescale and a micro‐XAS station delivering a spot size of 4 µm × 4 µm FWHM. It is a user‐friendly facility featuring a high degree of automation, online EXAFS data reduction and a flexible sample environment.
The European Synchrotron Radiation Facility has recently made available to the user community a facility totally dedicated to Time‐resolved and Extreme‐conditions X‐ray Absorption Spectroscopy – ...TEXAS. Based on an upgrade of the former energy‐dispersive XAS beamline ID24, it provides a unique experimental tool combining unprecedented brilliance (up to 1014 photons s−1 on a 4 µm × 4 µm FWHM spot) and detection speed for a full EXAFS spectrum (100 ps per spectrum). The science mission includes studies of processes down to the nanosecond timescale, and investigations of matter at extreme pressure (500 GPa), temperature (10000 K) and magnetic field (30 T). The core activities of the beamline are centered on new experiments dedicated to the investigation of extreme states of matter that can be maintained only for very short periods of time. Here the infrastructure, optical scheme, detection systems and sample environments used to enable the mission‐critical performance are described, and examples of first results on the investigation of the electronic and local structure in melts at pressure and temperature conditions relevant to the Earth's interior and in laser‐shocked matter are given.
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.
Plain Language Summary
Iron oxides are important end‐members of the complex materials that constitute the Earth's interior. Among them, FeO and Fe2O3 have long been considered as the main end‐members of the ferrous (Fe2+) and ferric (Fe3+) states of iron, respectively. All geochemical models assume that high oxygen concentrations are systematically associated to the formation of ferric iron in minerals. The recent discovery of O22− peroxide ions in a phase of chemical formula FeO2Hx stable under high‐pressure and high‐temperature conditions challenges this general concept. However, up to now, the valences of iron and oxygen in FeO2Hx have only been indirectly inferred from a structural analogy with pyrite FeS2. Here we compressed goethite (FeOOH), an Fe3+‐bearing mineral, at lower mantle pressure and temperature conditions by using laser‐heated diamond‐anvil cells, and we probed the iron oxidation state upon transformation of FeOOH in the pressure–temperature stability field of FeO2Hx using in situ X‐ray absorption spectroscopy. The data demonstrate that upon this transformation iron has transformed into ferrous Fe2+. Such reduced iron despite high oxygen concentrations suggests that our current views of oxidized and reduced species in the lower mantle of the Earth should be reconsidered.
Key Points
Ferrous iron is evidenced in coexistence with high oxygen concentration modifying our understanding of deep Earth geochemistry
In situ X‐ray absorption spectroscopy shows the reduction of iron into Fe2+ when FeOOH transforms into the pyrite‐structured phase FeO2Hx
Subduction of FeOOH‐type hydrated iron oxides releases zero‐valent hydrogen (H2) instead of mono‐valent hydrogen (H2O) in the lower mantle
Abstract
The discovery of superconductivity above 250 K at high pressure in LaH
10
and the prediction of overcoming the room temperature threshold for superconductivity in YH
10
urge for a better ...understanding of hydrogen interaction mechanisms with the heavy atom sublattice in metal hydrides under high pressure at the atomic scale. Here we use locally sensitive X-ray absorption fine structure spectroscopy (XAFS) to get insight into the nature of phase transitions and the rearrangements of local electronic and crystal structure in archetypal metal hydride YH
3
under pressure up to 180 GPa. The combination of the experimental methods allowed us to implement a multiscale length study of YH
3
: XAFS (short-range), Raman scattering (medium-range) and XRD (long-range). XANES data evidence a strong effect of hydrogen on the density of 4
d
yttrium states that increases with pressure and EXAFS data evidence a strong anharmonicity, manifested as yttrium atom vibrations in a double-well potential.
Solid krypton (Kr) undergoes a pressure-induced martensitic phase transition from a face-centered cubic (fcc) to a hexagonal close-packed (hcp) structure. These two phases coexist in a very wide ...pressure domain inducing important modifications of the bulk properties of the resulting mixed phase system. Here, we report a detailed in situ x-ray diffraction and absorption study of the influence of the fcc-hcp phase transition on the compression behavior of solid krypton in an extended pressure domain up to 140 GPa. The onset of the hcp-fcc transformation was observed in this study at around 2.7 GPa and the coexistence of these two phases up to 140 GPa, the maximum investigated pressure. The appearance of the hcp phase is also evidenced by the pressure-induced broadening and splitting of the first peak in the XANES spectra. We demonstrate that the transition is driven by a continuous nucleation and intergrowth of nanometric hcp stacking faults that evolve in the fcc phase. These hcp stacking faults are unaffected by high-temperature annealing, suggesting that plastic deformation is not at their origin. The apparent small Gibbs free-energy differences between the two structures that decrease upon compression may explain the nucleation of hcp stacking faults and the large coexistence domain of fcc and hcp krypton. We observe a clear anomaly in the equation of state of the fcc solid at ∼20 GPa when the proportion of the hcp form reaches ∼20%. We demonstrate that this anomaly is related to the difference in stiffness between the fcc and hcp phases and propose two distinct equation of states for the low and high-pressure regimes.
Extended x-ray absorption fine structure (EXAFS) at the Cd K edge and diffraction patterns have been measured on CdTe as a function of pressure from 100 kPa (1 bar) to 5 GPa using a cell with ...nano-polycrystalline diamond anvils and an x-ray focussing scanning spectrometer. Three phases-zincblende (ZB), mixed cinnabar-ZB and rocksalt (RS)-are well distinguished in different pressure intervals. The bond compressibility measured by EXAFS in the ZB phase is slightly smaller than the one measured by diffraction and decreases significantly faster when the pressure increases; the difference is attributed to the effect of relative vibrations perpendicular to the Cd-Te bond. The parallel mean square relative displacement (MSRD) decreases, the perpendicular MSRD increases when the pressure increases, leading to an increasing anisotropy of relative atomic vibrations. A constant-temperature bond Grüneisen parameter (GP) has been evaluated for the ZB phase and compared with the constant-pressure bond GP measured in a previous experiment; an attempt is made to connect the bond GPs measured by EXAFS and the more familiar thermodynamic GP and mode GPs; the comparisons suggest the inadequacy of the quasi-harmonic approximation to deal with the local vibrational properties sampled by EXAFS.
•X-ray absorption spectroscopy coupled to the diamond anvil cell technology is a powerful tool to investigate matter at high pressure.•Several X-ray absorption spectroscopy beamlines at synchrotrons ...around the world are equipped to offer XAS and XMCD spectroscopy for high pressure studies.•X-ray magnetic circular dichroism has been employed to investigate pressure induced suppression of ferromagnetism in Fe, Co and Ni.•The availability of high quality data at extreme conditions is very useful to validate existing theoretical models.
The study of the properties of matter under extreme conditions allows reproducing the physical conditions of inaccessible regions of the earth and planets, encountering a geophysical interest. Also it addresses fundamental questions concerning the stability of the crystallographic structure, the magnetic order and the electronic structure thus giving an insight into their own appearance. In this review we illustrate how X-ray absorption spectroscopies, coupled to diamond anvil cell techniques, can be successfully employed at this purpose. As an example, we describe recent advances in the investigation of the ferromagnetic transition metals under high pressure.
The 5d transition metals have attracted specific interest for high-pressure studies due to their extraordinary stability and intriguing electronic properties. In particular, iridium metal has been ...proposed to exhibit a recently discovered pressure-induced electronic transition, the so-called core-level crossing transition at the lowest pressure among all the 5d transition metals. Here, we report an experimental structural characterization of iridium by x-ray probes sensitive to both long- and short-range order in matter. Synchrotron-based powder x-ray diffraction results highlight a large stability range (up to 1.4 Mbar) of the low-pressure phase. The compressibility behaviour was characterized by an accurate determination of the pressure-volume equation of state, with a bulk modulus of 339(3) GPa and its derivative of 5.3(1). X-ray absorption spectroscopy, which probes the local structure and the empty density of electronic states above the Fermi level, was also utilized. The remarkable agreement observed between experimental and calculated spectra validates the reliability of theoretical predictions of the pressure dependence of the electronic structure of iridium in the studied interval of compressions.
Noble gases are important geochemical tracers allowing reconstructing global volatile cycles in Earth's reservoirs. To constrain these fundamental processes, precise data on their partitioning ...behavior at deep Earth conditions are needed. Such data are only available at moderate pressures up to 25 GPa due to experimental challenges. We have investigated the possibility of noble gas storage in the Earth's lower mantle up to 115 GPa. We studied the incorporation of krypton in the second most abundant lower mantle mineral (Mg1-x,Fex)O (ferropericlase) as well as in liquid metal-alloys by performing experiments up to 115 GPa and 3700 K using the laser-heated diamond anvil cell coupled to post-mortem EMPA analysis and X-ray absorption spectroscopy. The results reveal that, at these extreme conditions, up to 3 wt.% of krypton can be stored in (Mg1-x,Fex)O and 3000 ppm in the Fe-rich liquid metal. For both phases the storage capacities increase with pressure (between 40 GPa and 60 GPa) at a constant high temperature of 2300 K. Fpc has never been considered as a NG host, despite being the second most abundant mineral in the Earth's LM. Using recent accurate compressibility data, we demonstrate that a substitution of krypton into the anion site of (Mg1-x,Fex)O in form of neutral oxygen Schottky defects at diluted lower mantle conditions is possible. This noble gas incorporation mechanism is in agreement with a previous study on bridgmanite. We show that (Mg1-x,Fex)O exhibits higher noble gas storage capacities than bridgmanite through the lower mantle using lattice strain modeling and including experimental solubility and thermoelastic data for neon, argon, krypton and xenon. We also demonstrate that both phases exhibit the highest solubilities for argon and krypton. We used the solubility data from lattice strain modeling to predict noble gas abundances stored in the solid lower mantle after magma ocean crystallization. The modeled abundances show apparent similarities with estimates for the deep noble gas reservoir that are based on either 3He abundances in ocean island basalts or radiogenic 40Ar abundances in the bulk Earth. This strongly indicates that the crystalline lower mantle may play an important role as deep noble gas storage reservoir. We propose, based on considerations on noble gas replenishment from the lower mantle to the atmosphere, that the lower mantle can only contribute to a small fraction of the present-day atmospheric noble gases. This suggests that the lower mantle is an un-degassed reservoir.
•Krypton storage capacities of (Mg,Fe)O and metal alloys up to 115 GPa and 3700 K.•Storage capacities of lower mantle minerals are greater than those of metallic melts.•Zero-charged krypton is incorporated in neutral oxygen defects of (Mg,Fe)O.•The lower mantle could be a reservoir for noble gases through geologic times.
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