We report the results of an EXAFS (extended x-ray absorption fine structure) study of Fe3+, Ni2+, and Cu2+ aqueous solutions under high pressures. EXAFS experiments were performed using synchrotron radiation at room temperature and up to pressures of about 1.2 GPa using a diamond anvil cell. Data analysis has been performed using advanced multiple-scattering simulations, and information about the evolution of the first hydration shell around the metal ions has been obtained. It is shown that Fe3+ and Ni2+ solutions retain a local octahedral structure up to the highest pressure, while Cu2+ solutions show a predominant distorted pyramidal fivefold structure with two oxygen distances. The first-neighbor metal-oxygen distances show a different behavior with pressure in the three solutions, being gradually shortened for Ni2+ solutions or elongated in Fe3+ solutions (by ∼-0.01 and ∼0.02 Å respectively), while in Cu2+ solutions, the difference between average equatorial and axial Cu-O distances is gradually reduced. The present results show that pressure does not act as a simple isotropic perturbation on ionic hydration, which is found to be dependent on the bonding mechanisms and ligand-field anisotropy of transition-metal ions.

The Fe–Ir binary alloy has been calibrated up to 100 GPa as a part of a thermodynamic model (oxygen fugacity (f_O_2) sensor system) to monitor the redox conditions during high–pressure and high–temperature petrology experiments. The existing Fe–Ir activity–composition relations at 1 bar and 473–2873 K have been updated by including the pressure dependence on the activity–composition relations. We calibrated the volume dependent interaction parameter W^V up to 61 GPa and extrapolated the Margules activity model (i.e., W_Fe-Ir^G and W_Ir-Fe^G ) up to 100 GPa by combining the experimentally determined compressibility and compositional data of the alloy, with those of the end member phases in a thermodynamic model. We apply the newly calibrated redox sensor to oxygen fugacity determination during laser heated diamond anvil cell (LHDAC) experiments. In situ LHDAC experiments were performed up to 61 GPa and 2000 K. We tracked the f_O_2 conditions in the DAC by reacting the powdered mixture during laser heating to form a Fe–Ir alloy (sliding redox sensor) and using high–resolution synchrotron Mössbauer Source spectroscopy, powder X–ray diffraction, X–ray absorption near–edge structure spectroscopy, analytical transmission electron microscopy, and chemical analyses down to the nanoscale. The inferred oxygen fugacities measured at the specific P–T conditions of the experiments are 1 log unit below the iron–wüstite buffer, which is lower than estimates obtained with the existing model and compared to multi anvil experiments performed with the same starting mixture at similar P–T conditions.

The hydrous Ca-Al silicates lawsonite and epidote group minerals (EGMs) are key phases in subduction-zone H2O and element cycling. In high-pressure-low-temperature metamorphic rocks, Fe in both minerals is typically assumed to be entirely Fe3+, which substitutes for Al in octahedral sites as a major component in most EGMs and as a minor component in lawsonite and zoisite. New Fe micro-X-ray absorption near-edge spectroscopy (mu-XANES) analyses show substantial Fe2+ in lawsonite in blueschist from New Caledonia and zoisite from an unknown locality. Analysed Fe-rich EGMs (epidote, clinozoisite) contain primarily Fe3+. Lawsonite and some EGMs in subducted oceanic crust may contain more Fe2+ than is currently known, with possible implications for understanding subduction redox processes and conditions and why they vary in different subduction zones.

Isotopic signatures of heavy noble gases in the Earth's mantle contain a major component recycled by subduction. The experimental and field studies reported in the literature show increasing evidence that serpentine minerals can hold large quantities of noble gases, potentially serving as their primary vectors to depth. However, at present, their retention mechanism in these minerals is not fully understood. Additionally, noble gas solubilities from field and experimental studies show large differences in terms of elemental concentrations. Here, we performed crystal chemical modeling to evaluate the incorporation mechanism of noble gases and their solubilities in serpentine minerals along subduction zone geotherms. To this end, we determined the thermal equation of state of xenon using in situ X-ray diffraction and absorption up to 60 GPa and 728 K. In this range, the xenon equation of state is well-adjusted using the Mie-Gr & uuml;neisen-Debye formalism with relevant fitting parameters. We show that the experimentally observed solubility trend, which follows the order Ne < He < Ar < Kr < Xe, can be explained by the incorporation of noble gases at two distinct crystallographic sites. The light noble gases He and Ne are most likely retained at the van der Waals hydrogen-oxygen bond position between the layers, while the heavy and larger noble gases enter the voids between the six-membered SiO4 rings. It should be noted that octahedral sites can potentially host xenon, but cannot accommodate argon and krypton. Indeed, this would require unrealistic flexibility of the crystal lattice. Our models extended to mantle wedge conditions predict decreasing solubilities, particularly for light noble gases, in agreement with observations from natural samples. Compared to the noble gas concentrations determined experimentally in serpentine, natural concentrations are much higher and very variable. Our solubility model confirms that equilibrium processes cannot explain these observations. We therefore suggest that the high and variable noble gas concentrations found in natural samples must be due to hybrid hydration processes in ultramafic rocks that involve different degrees of water activities.

Here we present the new experimental stations devoted to the studies of matter under extreme conditions at the X-ray absorption beamlines BM23 and ID24-DCM that were recently refurbished within the ESRF - Extremely Brilliant Source (EBS) upgrade program. In comparison with the stations before the EBS upgrade, they exhibit outstanding performances in terms of sample positioning capabilities, acceptance of multi-detection systems and complex sample environments. In addition, significant improvements regarding the photon flux and focusing capabilities down to the submicron size have been achieved. These stations are now coupled with the new ESRF double crystal monochromators that exhibit an exceptional beam position and energy stability and that permit quick micro-EXAFS measurements down to one EXAFS/second, and hyperspectral EXAFS mapping. In this contribution, we discuss the choices regarding the sample and detector stages and illustrate the potential of the new setups for extreme conditions studies based on selected preliminary results.

The structural changes in PrNiO3 around the Ni atoms are studied by XAS. The spin–phonon coupling and the softening of the lattice vibrations are observed. EXAFS is suitable for exploring the coupling between the spins and phonons.

The martensitic transformation is a fundamental physical phenomenon at the origin of important industrial applications.However, the underlying microscopic mechanism, which is of critical importance to explain the outstanding mechanical properties of martensitic materials, is still not fully understood.This is because for most martensitic materials the transformation is a fast process that makes in situ studies extremely challenging.Noble solids krypton and xenon undergo a progressive pressure induced fcc to hcp martensitic transition with a very wide coexistence domain.Here, we took advantage of this unique feature to study the detailed mechanism of the transformation by employing in situ X-ray diffraction and absorption.We evidenced a four stages mechanism where the lattice mismatch between the fcc and hcp forms plays a key role in the generation of strain.We also determined precisely the effect of the transformation on the compression behavior of these materials.

Received 8 July 2021Accepted 26 January 2022DOI:https://doi.org/10.1103/PhysRevLett.128.099701© 2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectronic structureFirst-principles calculationsMetal-insulator transitionPhysical SystemsOxidesStrongly correlated systemsTechniquesDensity functional theoryHybrid functionalsCondensed Matter, Materials & Applied Physics

Extreme pressures and temperatures are known to drastically affect the chemistry of iron oxides, resulting in numerous compounds forming homologous series nFeOmFe_{2}O_{3} and the appearance of FeO_{2}. Here, based on the results of in situ single-crystal x-ray diffraction, Mössbauer spectroscopy, x-ray absorption spectroscopy, and density-functional theory+dynamical mean-field theory calculations, we demonstrate that iron in high-pressure cubic FeO_{2} and isostructural FeO_{2}H_{0.5} is ferric (Fe^{3+}), and oxygen has a formal valence less than 2. Reduction of oxygen valence from 2, common for oxides, down to 1.5 can be explained by a formation of a localized hole at oxygen sites.

The discovery of superconductivity above 250 K at high pressure in LaH10 and the prediction of overcoming the room temperature threshold for superconductivity in YH10 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 YH3 under pressure up to 180 GPa. The combination of the experimental methods allowed us to implement a multiscale length study of YH3: XAFS (short-range), Raman scattering (medium-range) and XRD (long-range). XANES data evidence a strong effect of hydrogen on the density of 4d yttrium states that increases with pressure and EXAFS data evidence a strong anharmonicity, manifested as yttrium atom vibrations in a double-well potential.

Nickel is the second most abundant element in the Earth's core. However, the properties of Fe‐Ni alloys are still poorly constrained under planetary cores conditions, in particular concerning the effect of Ni on the melting curve of Fe. Here we show that Ni alloying up to 36 wt% does not affect the melting curve of Fe up to 100 GPa. However, Ni strongly modifies the hexagonal‐closed‐packed/face‐centered‐cubic (hcp/fcc) phase boundary, pushing the hcp/fcc/liquid triple point of Fe‐20wt%Ni to higher pressures and temperatures. Our results allow constraining the triple point for Fe‐10wt%Ni, a composition relevant for the Earth interior, and point out a decrease of the melting temperature at core‐mantle boundary by 400 K with respect to pure Fe. A lower amount of light elements than previously predicted is thus required to reduce the crystallization temperature of core materials below that of a peridotitic lower mantle, in better agreement with geochemical observations.

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.

Nano-polycrystalline diamonds (NPDs) have become fundamental tools for cutting-edge X-ray absorption spectroscopy (XAS) studies at high P/T conditions that opened up new research directions by overcoming previous limitations. Indeed, NPDs yield a continuous and weak X-ray background signal which enables the collection of high-quality XAS data of materials compressed in diamond anvil cells. This is a critical advantage over the classically used single-crystal diamonds that generate strong parasitic signals (glitches) which render the analysis of XAS data in many cases impossible. In this contribution we give an overview of the impact and the scientific opportunities that NPDs opened up for extreme condition XAS spectroscopy at the European Synchrotron Radiation Facility and discuss future needs.

AbstractRecent 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.

BaFe2Se3 is a potential superconductor material exhibiting transition at 11 K and ambient pressure. Here we extended the structural and performed electrical resistivity measurements on this compound up to 51 GPa and 20 GPa, respectively, in order to distinguish if the superconductivity in this sample is intrinsic to the BaFe2Se3 phase or if it is originating from minor FeSe impurities that show a similar superconductive transition temperature. The electrical resistance measurements as a function of pressure show that at 5 GPa the superconducting transition is observed at around 10 K, similar to the one previously observed for this sample at ambient pressure. This indicates that the superconductivity in this sample is most likely intrinsic to the BaFe2Se3 phase and not to FeSe with Tc  >  20 K at these pressures. Further increase in pressure suppressed the superconductive signal and the sample remained in an insulating state up to the maximum achieved pressure of 20 GPa. Single-crystal and powder x-ray diffraction measurements revealed two structural transformations in BaFe2Se3: a second order transition above 3.5 GPa from Pnma (CsAg2I3-type structure) to Cmcm (CsCu2Cl3-type structure) and a first order transformation at 16.6 GPa. Here, γ-BaFe2Se3 transforms into δ-BaFe2Se3 (Cmcm, CsCu2Cl3-type average structure) via a first order phase transition mechanism. This transition is characterized by a significant shortening of the b lattice parameter of γ-BaFe2Se3 (17%) and accompanied by an anisotropic expansion in the orthogonal ac plane at the transition point.

C. Donnerer,1 M. Moretti Sala,2 S. Pascarelli,2 A. D. Rosa,2 S. N. Andreev,3 V. V. Mazurenko,3 T. Irifune,4 E. C. Hunter,5,* R. S. Perry,1 and D. F. McMorrow1 1London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom 2ESRF–The European Synchrotron, 71 Avenue des Martyrs, 38000 Grenoble, France 3Theoretical Physics and Applied Mathematics Department, Ural Federal University, 620002 Ekaterinburg, Russia 4Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama 790-8577, Japan 5SUPA, School of Physics and Astronomy, and Centre for Science at Extreme Conditions, The University of Edinburgh, Mayfield Road, Edinburgh EH9 3JZ, United Kingdom

Knowledge on the temperature distribution in high pressure and temperature devices is important for the interpretation of data and can often not be obtained from experiments. Here, we report on the thermal characteristics of the most employed Paris-Edinburgh press assemblies for in-situ X-ray purposes using finite element calculations. The maximal horizontal T variations in the sample are found to be small and amount maximal 50K at 2500K. Temperature differences between the sample and the calibrant material are in the same order of magnitude only if the latter is placed well centred on the sample capsule. The present (pure thermal) calculations can only partly reproduce the discrepancies between experimentally determined input power to T relations (1000K at 350W) indicating that different deformation behaviours of assemblies may play a crucial role. Based on the obtained results we present an optimized assembly in terms of sample temperature and gradients.

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 similar to 20 GPa when the proportion of the hcp form reaches similar to 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.

At P/T conditions of the D” layer, bridgmanite transforms into its high-pressure phase of (Mg,Fe)SiO3 postperovskite (pPv). Observations of seismic anisotropy in D” are inferred to arise from textures and microstructures within pPv. Specifically, mantle flow is though to cause pPv to deform, creating lattice-preferred orientations. However, debates emerged in the literature whether experimentally observed textures were induced by plastic deformation of the sample or by phase transformation from a previous phase and whether this could explain the observed patterns of anisotropy in the lowermost mantle.