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We have synthesized the first hydrous sp 3 -carbonate by laser-heating Ba[CO 3 ], CO 2 and H 2 O in a diamond anvil cell at 40(3) GPa. The crystal structure of Ba[H 4 C 4 O 10 ][H 3 C 4 O 10 ][H 2 CO 3 ][HCO 3 ] was determined by synchrotron single crystal X-ray diffraction. The experiments were complemented by DFT-based calculations. This compound is the first example of a carbonate containing both trigonally-coordinated carbon in [CO 3 ] 2- -groups and tetrahedrally coordinated carbon in [CO 4 ] 4- -groups. The [CO 4 ] 4- -groups polymerize by corner-sharing to form pyramidal [C 4 O 10 ] 4- -groups, which can bind three or four hydrogen atoms. As the pyramidal [C 4 O 10 ] 4- -groups are a constituent of several anhydrous sp 3 -carbonates, we now expect that further high-pressure hydrous carbonates can be obtained.

H 2 O transforms to two forms of superionic (SI) ice at high pressures and temperatures, which contain highly mobile protons within a solid oxygen sublattice. Yet the stability field of both phases remains debated. Here, we present the results of an ultrafast X-ray heating study utilizing MHz pulse trains produced by the European X-ray Free Electron Laser to create high temperature states of H 2 O, which were probed using X-ray diffraction during dynamic cooling. We confirm an isostructural transition during heating in the 26-69 GPa range, consistent with the formation of SI-bcc. In contrast to prior work, SI-fcc was observed exclusively above ~50 GPa, despite evidence of melting at lower pressures. The absence of SI-fcc in lower pressure runs is attributed to short heating timescales and the pressure-temperature path induced by the pump-probe heating scheme in which H 2 O was heated above its melting temperature before the observation of quenched crystalline states, based on the earlier theoretical prediction that SI-bcc nucleates more readily from the fluid than SI-fcc. Our results may have implications for the stability of SI phases in ice-rich planets, for example during dynamic freezing, where the preferential crystallization of SI-bcc may result in distinct physical properties across mantle ice layers., (© 2024. The Author(s).)

Presented and discussed here is the implementation of a software solution that provides prompt X-ray diffraction data analysis during fast dynamic compression experiments conducted within the dynamic diamond anvil cell technique. It includes efficient data collection, streaming of data and metadata to a high-performance cluster (HPC), fast azimuthal data integration on the cluster, and tools for controlling the data processing steps and visualizing the data using the DIOPTAS software package. This data processing pipeline is invaluable for a great number of studies. The potential of the pipeline is illustrated with two examples of data collected on ammonia-water mixtures and multiphase mineral assemblies under high pressure. The pipeline is designed to be generic in nature and could be readily adapted to provide rapid feedback for many other X-ray diffraction techniques, e.g. large-volume press studies, in situ stress/strain studies, phase transformation studies, chemical reactions studied with high-resolution diffraction etc., (© Mikhail Karnevskiy et al. 2024.)

In this study, we conduct extensive high-pressure experiments to investigate phase stability in the cobalt-nitrogen system. Through a combination of synthesis in a laser-heated diamond anvil cell, first-principles calculations, Raman spectroscopy, and single-crystal X-ray diffraction, we establish the stability fields of known high-pressure phases, hexagonal NiAs-type CoN, and marcasite-type CoN 2 within the pressure range of 50-90 GPa. We synthesize and characterize previously unknown nitrides, Co 3 N 2 , Pnma-CoN and two polynitrides, CoN 3 and CoN 5 , within the pressure range of 90-120 GPa. Both polynitrides exhibit novel types of polymeric nitrogen chains and networks. CoN 3 feature branched-type nitrogen trimers (N 3 ) and CoN 5 show π-bonded nitrogen chain. As the nitrogen content in the cobalt nitride increases, the CoN 6 polyhedral frameworks transit from face-sharing (in CoN) to edge-sharing (in CoN 2 and CoN 3 ), and finally to isolated (in CoN 5 ). Our study provides insights into the intricate interplay between structure evolution, bonding arrangements, and high-pressure synthesis in polynitrides, expanding the knowledge for the development of advanced energy materials., (© 2024 Wiley-VCH GmbH.)

Subducting sedimentary layer typically contains water and hydrated clay minerals. The stability of clay minerals under such hydrous subduction environment would therefore constraint the lithology and physical properties of the subducting slab interface. Here we show that pyrophyllite (Al 2 Si 4 O 10 (OH) 2 ), one of the representative clay minerals in the alumina-silica-water (Al 2 O 3 -SiO 2 -H 2 O, ASH) system, breakdowns to contain further hydrated minerals, gibbsite (Al(OH) 3 ) and diaspore (AlO(OH)), when subducts along a water-saturated cold subduction geotherm. Such a hydration breakdown occurs at a depth of ~135 km to uptake water by ~1.8 wt%. Subsequently, dehydration breakdown occurs at ~185 km depth to release back the same amount of water, after which the net crystalline water content is preserved down to ~660 km depth, delivering a net amount of ~5.0 wt% H 2 O in a phase assemblage containing δ-AlOOH and phase Egg (AlSiO 3 (OH)). Our results thus demonstrate the importance of subducting clays to account the delivery of ~22% of water down to the lower mantle., (© 2024. The Author(s).)

The dynamic diamond anvil cell (dDAC) is a recently developed experimental platform that has shown promise for studying the behavior of materials at strain rates ranging from intermediate to quasi-static and shock compression regimes. Combining dDAC with time-resolved x-ray diffraction (XRD) in the radial geometry (i.e., with incident x-rays perpendicular to the axis of compression) enables the study of material properties such as strength, texture evolution, and deformation mechanisms. This work describes a radial XRD dDAC setup at beamline P02.2 (Extreme Conditions Beamline) at DESY's PETRA III synchrotron. Time-resolved radial XRD data are collected for titanium, zirconium, and zircon samples, demonstrating the ability to study the strength and texture of materials at compression rates above 300 GPa/s. In addition, the simultaneous optical imaging of the DAC sample chamber is demonstrated. The ability to conduct simultaneous radial XRD and optical imaging provides the opportunity to characterize plastic strain and deviatoric strain rates in the DAC at intermediate rates, exploring the strength and deformation mechanisms of materials in this regime., (© 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).)

The inorganic pyrocarbonate salt Na 2 [C 2 O 5 ] crystallizes in the acentric, monoclinic space group P 2 1 with Z = 2. It contains monovalent alkali metal cations together with isolated pyrocarbonate anions. The [C 2 O 5 ] 2- -groups consist of two planar [CO 3 ] 2- -groups which are slightly tilted with respect to each other around the bridging oxygen atom. Na 2 [C 2 O 5 ] was synthesized in a laser-heated diamond anvil cell at 20(2) GPa by heating a mixture of Na 2 [CO 3 ] + CO 2 to ≈800(200) K. Its crystal structure was obtained by single crystal synchrotron X-ray diffraction and confirmed by density functional theory-based calculations in combination with Raman spectroscopy. Second harmonic generation measurements verified the acentric space group symmetry. The crystal structure is characterized by alternating layers of Na + -cations and [C 2 O 5 ] 2- -complex anions.

Giant impact-driven redox processes in the atmosphere and magma ocean played crucial roles in the evolution of Earth. However, because of the absence of rock records from that time, understanding these processes has proven challenging. Here, we present experimental results that simulate the giant impact-driven reactions between iron and volatiles (H 2 O and CO 2 ) using x-ray free electron laser (XFEL) as fast heat pump and structural probe. Under XFEL pump, iron is oxidized to wüstite (FeO), while volatiles are reduced to H 2 and CO. Furthermore, iron oxidation proceeds into formation of hydrides (γ-FeH x ) and siderite (FeCO 3 ), implying redox boundary near 300-km depth. Through quantitative analysis on reaction products, we estimate the volatile and FeO budgets in bulk silicate Earth, supporting the Theia hypothesis. Our findings shed light on the fast and short-lived process that led to reduced atmosphere, required for the emergence of prebiotic organic molecules in the early Earth.

The dynamic diamond anvil cell (dDAC) technique has attracted great interest because it possibly provides a bridge between static and dynamic compression studies with fast, repeatable, and controllable compression rates. The dDAC can be a particularly useful tool to study the pathways and kinetics of phase transitions under dynamic pressurization if simultaneous measurements of physical quantities are possible as a function of time. We here report the development of a real-time event monitoring (RTEM) system with dDAC, which can simultaneously record the volume, pressure, optical image, and structure of materials during dynamic compression runs. In particular, the volume measurement using both Fabry-Pérot interferogram and optical images facilitates the construction of an equation of state (EoS) using the dDAC in a home-laboratory. We also developed an in-line ruby pressure measurement (IRPM) system to be deployed at a synchrotron x-ray facility. This system provides simultaneous measurements of pressure and x-ray diffraction in low and narrow pressure ranges. The EoSs of ice VI obtained from the RTEM and the x-ray diffraction data with the IRPM are consistent with each other. The complementarity of both RTEM and IRPM systems will provide a great opportunity to scrutinize the detailed kinetic pathways of phase transitions using dDAC., (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)

High-pressure and high-temperature experiments using a resistively heated diamond anvil cell have the advantage of heating samples homogeneously with precise temperature control. Here, we present the design and performance of a graphite resistive heated diamond anvil cell (GRHDAC) setup for powder and single-crystal x-ray diffraction experiments developed at the Extreme Conditions Beamline (P02.2) at PETRA III, Hamburg, Germany. In the GRHDAC, temperatures up to 2000 K can be generated at high pressures by placing it in a water-cooled vacuum chamber. Temperature estimates from thermocouple measurements are within +/-35 K at the sample position up to 800 K and within +90 K between 800 and 1400 K when using a standard seat combination of cBN and WC. Isothermal compression at high temperatures can be achieved by employing a remote membrane control system. The advantage of the GRHDAC is demonstrated through the study of geophysical processes in the Earth's crust and upper mantle region., (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)

An experimental platform for dynamic diamond anvil cell (dDAC) research has been developed at the High Energy Density (HED) Instrument at the European X-ray Free Electron Laser (European XFEL). Advantage was taken of the high repetition rate of the European XFEL (up to 4.5 MHz) to collect pulse-resolved MHz X-ray diffraction data from samples as they are dynamically compressed at intermediate strain rates (≤10 3  s -1 ), where up to 352 diffraction images can be collected from a single pulse train. The set-up employs piezo-driven dDACs capable of compressing samples in ≥340 µs, compatible with the maximum length of the pulse train (550 µs). Results from rapid compression experiments on a wide range of sample systems with different X-ray scattering powers are presented. A maximum compression rate of 87 TPa s -1 was observed during the fast compression of Au, while a strain rate of ∼1100 s -1 was achieved during the rapid compression of N 2 at 23 TPa s -1 ., (open access.)

When affected by impact, quartz (SiO 2 ) undergoes an abrupt transformation to glass lamellae, the planar deformation features (PDFs). This shock effect is the most reliable indicator of impacts and is decisive in identifying catastrophic collisions in the Earth´s record such as the Chicxulub impact. Despite the significance of PDFs, there is still no consensus how they form. Here, we present time-resolved in-situ synchroton X-ray diffraction data of single-crystal quartz rapidly compressed in a dynamic diamond anvil cell. These experiments provide evidence for the transformation of quartz at pressures above 15 GPa to lamellae of a metastable rosiaite (PbSb 2 O 6 )-type high-pressure phase with octahedrally coordinated silicon. This phase collapses during decompression to amorphous lamellae, which closely resemble PDFs in naturally shocked quartz. The identification of rosiaite-structured silica provides thus an explanation for lamellar amorphization of quartz. Furthermore, it suggests that the mixed phase region of the Hugoniot curve may be related to the progressive formation of rosiaite-structured silica., (© 2023. The Author(s).)

Both cerium (Ce) and praseodymium (Pr) undergo a volume collapse transition under compression that originate from similar electronic mechanisms. Yet the outcome could not be more different. In the case of Ce with one affected 4f electron the volume collapse leaves the crystal symmetry intact, whereas for Pr with two 4f electrons the crystal symmetry changes from a distorted face centered cubic structure to a lower symmetry orthorhombic structure. In this paper, we present a study of the effect of strain/compression rate spanning nearly 4 orders of magnitude on the volume collapse phase transitions in Ce and Pr. These dynamic compression experiments in a diamond anvil cell also reveal kinetic differences between the phase transformations observed in these two materials. The transition cannot be overdriven in pressure in Ce, which indicates a fast kinetic process, whereas fast compression rates in Pr lead to a shift of the phase boundary to higher pressures, pointing to slower kinetics possibly due to the realization of a new crystal structure., (© 2022. The Author(s).)

A gasket is an important constituent of a diamond anvil cell (DAC) assembly, responsible for the sample chamber stability at extreme conditions for X-ray diffraction studies. In this work, we studied the performance of gaskets made of metallic glass Fe 0.79 Si 0.07 B 0.14 in a number of high-pressure X-ray diffraction (XRD) experiments in DACs equipped with conventional and toroidal-shape diamond anvils. The experiments were conducted in either axial or radial geometry with X-ray beams of micrometre to sub-micrometre size. We report that Fe 0.79 Si 0.07 B 0.14 metallic glass gaskets offer a stable sample environment under compression exceeding 1 Mbar in all XRD experiments described here, even in those involving small-molecule gases (e.g. Ne, H 2 ) used as pressure-transmitting media or in those with laser heating in a DAC. Our results emphasize the material's importance for a great number of delicate experiments conducted under extreme conditions. They indicate that the application of Fe 0.79 Si 0.07 B 0.14 metallic glass gaskets in XRD experiments for both axial and radial geometries substantially improves various aspects of megabar experiments and, in particular, the signal-to-noise ratio in comparison to that with conventional gaskets made of Re, W, steel or other crystalline metals., (open access.)

We have synthesized Pb[C 2 O 5 ], an inorganic pyrocarbonate salt, in a laser-heated diamond anvil cell (LH-DAC) at 30 GPa by heating a Pb[CO 3 ] + CO 2 mixture to ≈2000(200) K. Inorganic pyrocarbonates contain isolated [C 2 O 5 ] 2- groups without functional groups attached. The [C 2 O 5 ] 2- groups consist of two oxygen-sharing [CO 3 ] 3- groups. Pb[C 2 O 5 ] was characterized by synchrotron-based single-crystal structure refinement, Raman spectroscopy, and density functional theory calculations. Pb[C 2 O 5 ] is isostructural to Sr[C 2 O 5 ] and crystallizes in the monoclinic space group P 2 1 / c with Z = 4. The synthesis of Pb[C 2 O 5 ] demonstrates that, just like in other carbonates, cation substitution is possible and that therefore inorganic pyrocarbonates are a novel family of carbonates, in addition to the established sp 2 and sp 3 carbonates.

In the last few years, the superconducting transition temperature, T c , of hydrogen-rich compounds has increased dramatically, and is now approaching room temperature. However, the pressures at which these materials are stable exceed one million atmospheres and limit the number of available experimental studies. Superconductivity in hydrides has been primarily explored by electrical transport measurements, whereas magnetic properties, one of the most important characteristic of a superconductor, have not been satisfactory defined. Here, we develop SQUID magnetometry under extreme high-pressure conditions and report characteristic superconducting parameters for Im-3m-H 3 S and Fm-3m-LaH 10 -the representative members of two families of high-temperature superconducting hydrides. We determine a lower critical field H c1 of ∼0.82 T and ∼0.55 T, and a London penetration depth λ L of ∼20 nm and ∼30 nm in H 3 S and LaH 10 , respectively. The small values of λ L indicate a high superfluid density in both hydrides. These compounds have the values of the Ginzburg-Landau parameter κ ∼12-20 and belong to the group of "moderate" type II superconductors, rather than being hard superconductors as would be intuitively expected from their high T c s., (© 2022. The Author(s).)

Scientific tasks aimed at decoding and characterizing complex systems and processes at high pressures set new challenges for modern X-ray diffraction instrumentation in terms of X-ray flux, focal spot size and sample positioning. Presented here are new developments at the Extreme Conditions beamline (P02.2, PETRA III, DESY, Germany) that enable considerable improvements in data collection at very high pressures and small scattering volumes. In particular, the focusing of the X-ray beam to the sub-micrometer level is described, and control of the aberrations of the focusing compound refractive lenses is made possible with the implementation of a correcting phase plate. This device provides a significant enhancement of the signal-to-noise ratio by conditioning the beam shape profile at the focal spot. A new sample alignment system with a small sphere of confusion enables single-crystal data collection from grains of micrometer to sub-micrometer dimensions subjected to pressures as high as 200 GPa. The combination of the technical development of the optical path and the sample alignment system contributes to research and gives benefits on various levels, including rapid and accurate diffraction mapping of samples with sub-micrometer resolution at multimegabar pressures., (open access.)

The ability to visualize a sample undergoing a pressure-induced phase transition allows for the determination of kinetic parameters, such as the nucleation and growth rates of the high-pressure phase. For samples that are opaque to visible light (such as metallic systems), it is necessary to rely on x-ray imaging methods for sample visualization. Here, we present an experimental platform developed at beamline P02.2 at the PETRA III synchrotron radiation source, which is capable of performing simultaneous x-ray imaging and diffraction of samples that are dynamically compressed in piezo-driven diamond anvil cells. This setup utilizes a partially coherent monochromatic x-ray beam to perform lensless phase contrast imaging, which can be carried out using either a parallel- or focused-beam configuration. The capabilities of this platform are illustrated by experiments on dynamically compressed Ga and Ar. Melting and solidification were identified based on the observation of solid/liquid phase boundaries in the x-ray images and corresponding changes in the x-ray diffraction patterns collected during the transition, with significant edge enhancement observed in the x-ray images collected using the focused-beam. These results highlight the suitability of this technique for a variety of purposes, including melt curve determination.

Cubic CaSiO 3 perovskite is a major phase in subducted oceanic crust, where it forms at a depth of about 550 kilometres from majoritic garnet 1,2,28 . However, its rheological properties at temperatures and pressures typical of the lower mantle are poorly known. Here we measured the plastic strength of cubic CaSiO 3 perovskite at pressure and temperature conditions typical for a subducting slab up to a depth of about 1,200 kilometres. In contrast to tetragonal CaSiO 3 , previously investigated at room temperature 3,4 , we find that cubic CaSiO 3 perovskite is a comparably weak phase at the temperatures of the lower mantle. We find that its strength and viscosity are substantially lower than that of bridgmanite and ferropericlase, possibly making cubic CaSiO 3 perovskite the weakest lower-mantle phase. Our findings suggest that cubic CaSiO 3 perovskite governs the dynamics of subducting slabs. Weak CaSiO 3 perovskite further provides a mechanism to separate subducted oceanic crust from the underlying mantle. Depending on the depth of the separation, basaltic crust could accumulate at the boundary between the upper and lower mantle, where cubic CaSiO 3 perovskite may contribute to the seismically observed regions of low shear-wave velocities in the uppermost lower mantle 5,6 , or sink to the core-mantle boundary and explain the seismic anomalies associated with large low-shear-velocity provinces beneath Africa and the Pacific 7-9 ., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)

The success of high-pressure research relies on the inventive design of pressure-generating instruments and materials used for their construction. In this study, the anvils of conical frustum or disk shapes with flat or modified culet profiles (toroidal or beveled) were prepared by milling an Ia-type diamond plate made of a (100)-oriented single crystal using the focused ion beam. Raman spectroscopy and synchrotron x-ray diffraction were applied to evaluate the efficiency of the anvils for pressure multiplication in different modes of operation: as single indenters forced against the primary anvil in diamond anvil cells (DACs) or as pairs of anvils forced together in double-stage DACs (dsDACs). All types of secondary anvils performed well up to about 250 GPa. The pressure multiplication factor of single indenters appeared to be insignificantly dependent on the shape of the anvils and their culets' profiles. The enhanced pressure multiplication factor found for pairs of toroidally shaped secondary anvils makes this design very promising for ultrahigh-pressure experiments in dsDACs.

The synthesis of a novel type of carbonate, namely of the inorganic pyrocarbonate salt Sr[C 2 O 5 ], which contains isolated [C 2 O 5 ] 2- -groups, significantly extends the crystal chemistry of inorganic carbonates beyond the established sp 2 - and sp 3 -carbonates. We synthesized Sr[C 2 O 5 ] in a laser-heated diamond anvil cell by reacting Sr[CO 3 ] with CO 2 . By single crystal synchrotron diffraction, Raman spectroscopy, and density functional theory (DFT) calculations, we show that it is a pyrocarbonate salt. Sr[C 2 O 5 ] is the first member of a novel family of inorganic carbonates. We predict, based on DFT calculations, that further inorganic pyrocarbonates can be obtained and that these will be relevant to geoscience and may provide a better understanding of reactions converting CO 2 into useful inorganic compounds.

Magnetite, Fe 3 O 4 , is the oldest known magnetic mineral and archetypal mixed-valence oxide. Despite its recognized role in deep Earth processes, the behavior of magnetite at extreme high-pressure high-temperature (HPHT) conditions remains insufficiently studied. Here, we report on single-crystal synchrotron X-ray diffraction experiments up to ∼80 GPa and 5000 K in diamond anvil cells, which reveal two previously unknown Fe 3 O 4 polymorphs, γ-Fe 3 O 4 with the orthorhombic Yb 3 S 4 -type structure and δ-Fe 3 O 4 with the modified Th 3 P 4 -type structure. The latter has never been predicted for iron compounds. The decomposition of Fe 3 O 4 at HPHT conditions was found to result in the formation of exotic phases, Fe 5 O 7 and Fe 25 O 32 , with complex structures. Crystal-chemical analysis of iron oxides suggests the high-spin to low-spin crossover in octahedrally coordinated Fe 3+ in the pressure interval between 43 and 51 GPa. Our experiments demonstrate that HPHT conditions promote the formation of ferric-rich Fe-O compounds, thus arguing for the possible involvement of magnetite in the deep oxygen cycle.

A laser heating system for samples confined in diamond anvil cells paired with in situ X-ray diffraction measurements at the Extreme Conditions Beamline of PETRA III is presented. The system features two independent laser configurations (on-axis and off-axis of the X-ray path) allowing for a broad range of experiments using different designs of diamond anvil cells. The power of the continuous laser source can be modulated for use in various pulsed laser heating or flash heating applications. An example of such an application is illustrated here on the melting curve of iron at megabar pressures. The optical path of the spectroradiometry measurements is simulated with ray-tracing methods in order to assess the level of present aberrations in the system and the results are compared with other systems, that are using simpler lens optics. Based on the ray-tracing the choice of the first achromatic lens and other aspects for accurate temperature measurements are evaluated., (open access.)

We have synthesized the orthocarbonate Sr 3 [CO 4 ]O in a laser-heated diamond anvil cell at 20 and 30 GPa by heating to ≈3000 (300) K. Afterward, we recovered the orthocarbonate with [CO 4 ] 4- groups at ambient conditions. Single-crystal diffraction shows the presence of [CO 4 ] 4- groups, i.e., sp 3 -hybridized carbon tetrahedrally coordinated by covalently bound oxygen atoms. The [CO 4 ] 4- tetrahedra are located in a cage formed by corner-sharing OSr 6 octahedra, i.e., octahedra with oxygen as a central ion, forming an antiperovskite-type structure. At high pressures, the octahedra are nearly ideal and slightly rotated. The high-pressure phase is tetragonal ( I 4/ mcm ). Upon pressure release, there is a phase transition with a symmetry lowering to an orthorhombic phase ( Pnma ), where the octahedra tilt and deform slightly.

The European XFEL delivers up to 27000 intense (>10 12 photons) pulses per second, of ultrashort (≤50 fs) and transversely coherent X-ray radiation, at a maximum repetition rate of 4.5 MHz. Its unique X-ray beam parameters enable groundbreaking experiments in matter at extreme conditions at the High Energy Density (HED) scientific instrument. The performance of the HED instrument during its first two years of operation, its scientific remit, as well as ongoing installations towards full operation are presented. Scientific goals of HED include the investigation of extreme states of matter created by intense laser pulses, diamond anvil cells, or pulsed magnets, and ultrafast X-ray methods that allow their diagnosis using self-amplified spontaneous emission between 5 and 25 keV, coupled with X-ray monochromators and optional seeded beam operation. The HED instrument provides two target chambers, X-ray spectrometers for emission and scattering, X-ray detectors, and a timing tool to correct for residual timing jitter between laser and X-ray pulses., (open access.)

It is qualitatively well known that kinetics related to nucleation and growth can shift apparent phase boundaries from their equilibrium value. In this work, we have measured this effect in Bi using time-resolved X-ray diffraction with unprecedented 0.25 ms time resolution, accurately determining phase transition pressures at compression rates spanning five orders of magnitude (10 -2 -10 3  GPa/s) using the dynamic diamond anvil cell. An over-pressurization of the Bi-III/Bi-V phase boundary is observed at fast compression rates for different sample types and stress states, and the largest over-pressurization that is observed is ΔP = 2.5 GPa. The work presented here paves the way for future studies of transition kinetics at previously inaccessible compression rates., (© 2021. The Author(s).)

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., (open access.)

The ultrafast synthesis of ε-Fe 3 N 1+ x in a diamond-anvil cell (DAC) from Fe and N 2 under pressure was observed using serial exposures of an X-ray free electron laser (XFEL). When the sample at 5 GPa was irradiated by a pulse train separated by 443 ns, the estimated sample temperature at the delay time was above 1400 K, confirmed by in situ transformation of α- to γ-iron. Ultimately, the Fe and N 2 reacted uniformly throughout the beam path to form Fe 3 N 1.33 , as deduced from its established equation of state (EOS). We thus demonstrate that the activation energy provided by intense X-ray exposures in an XFEL can be coupled with the source time structure to enable exploration of the time-dependence of reactions under high-pressure conditions.

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 blueschist to eclogite transition is one of the major geochemical-metamorphic processes typifying the subduction zone, which releases fluids triggering earthquakes and arc volcanism. Although glaucophane is an index hydrous mineral for the blueschist facies, its stability at mantle depths in diverse subduction regimes of contemporary and early Earth has not been experimentally determined. Here, we show that the maximum depth of glaucophane stability increases with decreasing thermal gradients of the subduction system. Along cold subduction geotherm, glaucophane remains stable down ca. 240 km depth, whereas it dehydrates and breaks down at as shallow as ca. 40 km depth under warm subduction geotherm or the Proterozoic tectonic setting. Our results imply that secular cooling of the Earth has extended the stability of glaucophane and consequently enabled the transportation of water into deeper interior of the Earth, suppressing arc magmatism, volcanism, and seismic activities along subduction zones.

A resistively-heated dynamic diamond anvil cell (RHdDAC) setup is presented. The setup enables the dynamic compression of samples at high temperatures by employing a piezoelectric actuator for pressure control and internal heaters for high temperature. The RHdDAC facilitates the precise control of compression rates and was tested in compression experiments at temperatures up to 1400 K and pressures of ∼130 GPa. The mechanical stability of metallic glass gaskets composed of a FeSiB alloy was examined under simultaneous high-pressure/high-temperature conditions. High-temperature dynamic compression experiments on H 2 O ice and (Mg, Fe)O ferropericlase were performed in combination with time-resolved x-ray diffraction measurements to characterize crystal structures and compression behaviors. The employment of high brilliance synchrotron radiation combined with two fast GaAs LAMBDA detectors available at the Extreme Conditions Beamline (P02.2) at PETRA III (DESY) facilitates the collection of data with excellent pressure resolution. The pressure-temperature conditions achievable with the RHdDAC combined with its ability to cover a wide range of compression rates and perform tailored compression paths offers perspectives for a variety of future experiments under extreme conditions.

Polynitrides are intrinsically thermodynamically unstable at ambient conditions and require peculiar synthetic approaches. Now, a one-step synthesis of metal-inorganic frameworks Hf 4 N 20 ⋅N 2 , WN 8 ⋅N 2 , and Os 5 N 28 ⋅3 N 2 via direct reactions between elements in a diamond anvil cell at pressures exceeding 100 GPa is reported. The porous frameworks (Hf 4 N 20 , WN 8 , and Os 5 N 28 ) are built from transition-metal atoms linked either by polymeric polydiazenediyl (polyacetylene-like) nitrogen chains or through dinitrogen units. Triply bound dinitrogen molecules occupy channels of these frameworks. Owing to conjugated polydiazenediyl chains, these compounds exhibit metallic properties. The high-pressure reaction between Hf and N 2 also leads to a non-centrosymmetric polynitride Hf 2 N 11 that features double-helix catena-poly[tetraz-1-ene-1,4-diyl] nitrogen chains [-N-N-N=N-] ∞ ., (© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

We present an improved setup for the experimental study of deformation of solids at simultaneous high pressures and temperatures by radial x-ray diffraction. This technique employs a graphite resistive heated Mao-Bell type diamond anvil cell for radial x-ray diffraction in combination with a water-cooled vacuum chamber. The new chamber has been developed by the sample environment group at PETRA III and implemented at the Extreme Conditions Beamline P02.2 at PETRA III, DESY (Hamburg, Germany). We discuss applications of the new setup to study deformation of a variety of materials, including ferropericlase, calcium perovskite, bridgmanite, and tantalum carbide, at high-pressure/temperature.

Superconductivity near room temperature in the sulfur-hydrogen system arises from a sequence of reactions at high pressures, with X-ray diffraction experiments playing a central role in understanding these chemical-structural transformations and the corresponding S:H stoichiometry. Here we document X-ray irradiation acting as both a probe and as a driver of chemical reaction in this dense hydride system. We observe a reaction between molecular hydrogen (H 2 ) and elemental sulfur (S 8 ) under high pressure, induced directly by X-ray illumination, at photon energies of 12 keV using a free electron laser. The rapid synthesis of hydrogen sulfide (H 2 S) at 0.3 GPa was confirmed by optical observations, spectroscopic measurements, and microstructural changes detected by X-ray diffraction. These results document X-ray induced chemical synthesis of superconductor-forming dense hydrides, revealing an alternative production strategy and confirming the disruptive nature of X-ray exposure in studies on high-pressure hydrogen chalcogenides, from water to high-temperature superconductors.

High-pressure synthesis in diamond anvil cells can yield unique compounds with advanced properties, but often they are either unrecoverable at ambient conditions or produced in quantity insufficient for properties characterization. Here we report the synthesis of metallic, ultraincompressible (K 0  = 428(10) GPa), and very hard (nanoindentation hardness 36.7(8) GPa) rhenium nitride pernitride Re 2 (N 2 )(N) 2 . Unlike known transition metals pernitrides Re 2 (N 2 )(N) 2 contains both pernitride N 2 4- and discrete N 3- anions, which explains its exceptional properties. Re 2 (N 2 )(N) 2 can be obtained via a reaction between rhenium and nitrogen in a diamond anvil cell at pressures from 40 to 90 GPa and is recoverable at ambient conditions. We develop a route to scale up its synthesis through a reaction between rhenium and ammonium azide, NH 4 N 3 , in a large-volume press at 33 GPa. Although metallic bonding is typically seen incompatible with intrinsic hardness, Re 2 (N 2 )(N) 2 turned to be at a threshold for superhard materials.

Combination of in situ laser heating with single-crystal X-ray diffraction (scXRD) in diamond anvil cells (DACs) provides a tool to study crystal structures and/or chemistry of materials at simultaneous high pressures and high temperatures. Here, we describe the first dedicated single-crystal X-ray diffractometer coupled with double-sided laser heating (dsLH) system. The scXRD/dsLH setup was developed for the P02.2 Extreme Conditions Beamline at PETRA III and became available for general users in 2017. It enables the collection of reliable scXRD data at simultaneous high pressure and high temperature. We demonstrate the performance of the setup on example of studies of iron and chromium nitrides.

Fast compression experiments performed using dynamic diamond anvil cells (dDACs) employing piezoactuators offer the opportunity to study compression-rate dependent phenomena. In this paper, we describe an experimental setup which allows us to perform time-resolved x-ray diffraction during the fast compression of materials using improved dDACs. The combination of the high flux available using a 25.6 keV x-ray beam focused with a linear array of compound refractive lenses and the two fast GaAs LAMBDA detectors available at the Extreme Conditions Beamline (P02.2) at PETRA III enables the collection of x-ray diffraction patterns at an effective repetition rate of up to 4 kHz. Compression rates of up to 160 TPa/s have been achieved during the compression of gold in a 2.5 ms fast compression using improved dDAC configurations with more powerful piezoactuators. The application of this setup to low-Z compounds at lower compression rates is described, and the high temporal resolution of the setup is demonstrated. The possibility of applying finely tuned pressure profiles opens opportunities for future research, such as using oscillations of the piezoactuator to mimic propagation of seismic waves in the Earth.

Modelling of processes involving deep Earth liquids requires information on their structures and compression mechanisms. However, knowledge of the local structures of silicates and silica (SiO 2 ) melts at deep mantle conditions and of their densification mechanisms is still limited. Here we report the synthesis and characterization of metastable high-pressure silica phases, coesite-IV and coesite-V, using in situ single-crystal X-ray diffraction and ab initio simulations. Their crystal structures are drastically different from any previously considered models, but explain well features of pair-distribution functions of highly densified silica glass and molten basalt at high pressure. Built of four, five-, and six-coordinated silicon, coesite-IV and coesite-V contain SiO 6 octahedra, which, at odds with 3 rd Pauling's rule, are connected through common faces. Our results suggest that possible silicate liquids in Earth's lower mantle may have complex structures making them more compressible than previously supposed.

A nitrogen-rich compound, ReN 8 ⋅x N 2 , was synthesized by a direct reaction between rhenium and nitrogen at high pressure and high temperature in a laser-heated diamond anvil cell. Single-crystal X-ray diffraction revealed that the crystal structure, which is based on the ReN 8 framework, has rectangular-shaped channels that accommodate nitrogen molecules. Thus, despite a very high synthesis pressure, exceeding 100 GPa, ReN 8 ⋅x N 2 is an inclusion compound. The amount of trapped nitrogen (x) depends on the synthesis conditions. The polydiazenediyl chains [-N=N-] ∞ that constitute the framework have not been previously observed in any compound. Ab initio calculations on ReN 8 ⋅x N 2 provide strong support for the experimental results and conclusions., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Poly-nitrogen compounds have been considered as potential high energy density materials for a long time due to the large number of energetic N-N or N=N bonds. In most cases high nitrogen content and stability at ambient conditions are mutually exclusive, thereby making the synthesis of such materials challenging. One way to stabilize such compounds is the application of high pressure. Here, through a direct reaction between Fe and N 2 in a laser-heated diamond anvil cell, we synthesize three ironnitrogen compounds Fe 3 N 2 , FeN 2 and FeN 4 . Their crystal structures are revealed by single-crystal synchrotron X-ray diffraction. Fe 3 N 2 , synthesized at 50 GPa, is isostructural to chromium carbide Cr 3 C 2 . FeN 2 has a marcasite structure type and features covalently bonded dinitrogen units in its crystal structure. FeN 4 , synthesized at 106 GPa, features polymeric nitrogen chains of [N 4 2- ] n units. Based on results of structural studies and theoretical analysis, [N 4 2- ] n units in this compound reveal catena-poly[tetraz-1-ene-1,4-diyl] anions.

Niobium (Nb) is one of the key trace elements used to understand Earth's formation and differentiation, and is remarkable for its deficiency relative to tantalum in terrestrial rocks compared to the building chondritic blocks. In this context, the local environment of Nb in silica-rich melts and glasses is studied by in situ x-ray absorption spectroscopy (XAS) at high pressure (P) up to 9.3 GPa and 1350 K using resistive-heating diamond-anvil cells. Nb is slightly less oxidized in the melt (intermediate valence between  +4 and  +5) than in the glass (+5), an effect evidenced from the shift of the Nb-edge towards lower energies. Changes in the pre-edge features are also observed between melt and glass states, consistently with the observed changes in oxidation state although likely enhanced by temperature (T) effects. The oxidation state of Nb is not affected by pressure neither in the molten nor glassy states, and remains constant in the investigated P-range. The Nb-O coordination number is constant and equal to [Formula: see text] below 5 GPa, and only progressively increases up to [Formula: see text] at 9.3 GPa, the maximum P investigated. If these findings were to similarly apply to basaltic melts, that would rule out the hypothesis of Nb/Ta fractionation during early silicate Earth's differentiation, thus reinforcing the alternative hypothesis of fractionation during core formation on reduced pre-planetary bodies.

Due to their high technological and geological relevance, silicates are one of the most studied classes of inorganic compounds. Under ambient conditions, the silicon in silicates is almost exclusively coordinated by four oxygen atoms, while high-pressure treatment normally results in an increase in the coordination from four- to sixfold. Reported here is a high-pressure single-crystal X-ray diffraction study of danburite, CaB 2 Si 2 O 8 , the first compound showing a step-wise transition of Si coordination from tetrahedral to octahedral through a trigonal bipyramid. Along the compression, the Si 2 O 7 groups of danburite first transform into chains of vertice-sharing SiO 5 trigonal bipyramids (danburite-II) and later into chains of edge-sharing SiO 6 octahedra (danburite-III). It is suggested that the unusual formation of an SiO 5 configuration is a consequence of filling up the pentacoordinated voids in the distorted hexagonal close packing of danburite-II.

In various shocked meteorites, low-pressure silica polymorph α-cristobalite is commonly found in close spatial relation with the densest known SiO 2 polymorph seifertite, which is stable above ∼80 GPa. We demonstrate that under hydrostatic pressure α-cristobalite remains untransformed up to at least 15 GPa. In quasi-hydrostatic experiments, above 11 GPa cristobalite X-I forms-a monoclinic polymorph built out of silicon octahedra; the phase is not quenchable and back-transforms to α-cristobalite on decompression. There are no other known silica polymorphs, which transform to an octahedra-based structure at such low pressures upon compression at room temperature. Further compression in non-hydrostatic conditions of cristobalite X-I eventually leads to the formation of quenchable seifertite-like phase. Our results demonstrate that the presence of α-cristobalite in shocked meteorites or rocks does not exclude that materials experienced high pressure, nor is the presence of seifertite necessarily indicative of extremely high peak shock pressures.

Synchrotron x-ray diffraction reveals a pressure induced crystallization at about 3.4 GPa and a polymorphic transition near 10.3 GPa when compressed a liquid GaIn eutectic alloy up to ~13 GPa at room temperature in a diamond anvil cell. Upon decompression, the high pressure crystalline phase remains almost unchanged until it transforms to the liquid state at around 2.3 GPa. The ab initio molecular dynamics calculations can reproduce the low pressure crystallization and give some hints on the understanding of the transition between the liquid and the crystalline phase on the atomic level. The calculated pair correlation function g(r) shows a non-uniform contraction reflected by the different compressibility between the short (1st shell) and the intermediate (2nd to 4th shells). It is concluded that the pressure-induced liquid-crystalline phase transformation likely arises from the changes in local atomic packing of the nearest neighbors as well as electronic structures at the transition pressure.

The doping effect of Sr(OH) 2 on the Mg(NH 2 ) 2 -2LiH system is investigated considering different amounts of added Sr(OH) 2 in the range of 0.05 to 0.2 mol. Experimental results show that both the thermodynamic and the kinetic properties of Mg(NH 2 ) 2 -2LiH are influenced by the presence of Sr(OH) 2 . The addition of 0.1 mol Sr(OH) 2 leads to a decrease in both the dehydrogenation onset and peak temperatures of ca. 70 and 13 °C, respectively, and an acceleration in the de/re-hydrogenation rates of one time at 150 °C compared to Mg(NH 2 ) 2 -2LiH alone. Based on differential scanning calorimetry (DSC) analysis, the overall reaction enthalpy of the 0.1 Sr(OH) 2 -doped sample is calculated to be 44 kJ per mol-H 2 and there are two absorption events occurring in the doped sample instead of one in the pristine sample. For the applied experimental conditions, according to the in situ synchrotron radiation powder X-ray diffraction (SR-PXD) and Fourier Transform Infrared spectroscopy (FT-IR) analysis, the reaction mechanism has been finally defined: Sr(OH) 2 , Mg(NH 2 ) 2 and LiH react with each other to form SrO, MgO and LiNH 2 during ball milling. After heating, SrO interacts with Mg(NH 2 ) 2 producing MgO and Sr(NH 2 ) 2 . Then Mg(NH 2 ) 2 , LiNH 2 and Sr(NH 2 ) 2 react with LiH to produce Li 2 NH, SrNH, Li 2 Mg(NH) 2 and Li 2 Mg 2 (NH) 3 in traces. After re-hydrogenation, LiSrH 3 , LiH and LiNH 2 are formed along with amorphous Mg(NH 2 ) 2 . The reasons for the improved kinetics are: (a) during dehydrogenation, the in situ formation of SrNH appears to increase the interfacial contacts between Mg(NH 2 ) 2 and LiH and also weakens the N-H bond of Mg(NH 2 ) 2 ; (b) during absorption, the formation of LiSrH 3 at around 150 °C could be the key factor for improving the hydrogenation properties.

Titanium(III) phosphate, TiPO 4  , is a typical example of an oxyphosphorus compound containing covalent P-O bonds. Single-crystal X-ray diffraction studies of TiPO 4 reveal complex and unexpected structural and chemical behavior as a function of pressure at room temperature. A series of phase transitions lead to the high-pressure phase V, which is stable above 46 GPa and features an unusual oxygen coordination of the phosphorus atoms. TiPO 4 -V is the first inorganic phosphorus-containing compound that exhibits fivefold coordination with oxygen. Up to the highest studied pressure of 56 GPa, TiPO 4 -V coexists with TiPO 4 -IV, which is less dense and might be kinetically stabilized. Above a pressure of about 6 GPa, TiPO 4 -II is found to be an incommensurately modulated phase whereas a lock-in transition at about 7 GPa leads to TiPO 4 -III with a fourfold superstructure compared to the structure of TiPO 4 -I at ambient conditions. TiPO 4 -II and TiPO 4 -III are similar to the corresponding low-temperature incommensurate and commensurate magnetic phases and reflect the strong pressure dependence of the spin-Peierls interactions., (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The physical and chemical properties of Earth's mantle, as well as its dynamics and evolution, heavily depend on the phase composition of the region. On the basis of experiments in laser-heated diamond anvil cells, we demonstrate that Fe,Al-bearing bridgmanite (magnesium silicate perovskite) is stable to pressures over 120 GPa and temperatures above 3000 K. Ferric iron stabilizes Fe-rich bridgmanite such that we were able to synthesize pure iron bridgmanite at pressures between ~45 and 110 GPa. The compressibility of ferric iron-bearing bridgmanite is significantly different from any known bridgmanite, which has direct implications for the interpretation of seismic tomography data.

Although chemically very simple, Fe2O3 is known to undergo a series of enigmatic structural, electronic and magnetic transformations at high pressures and high temperatures. So far, these transformations have neither been correctly described nor understood because of the lack of structural data. Here we report a systematic investigation of the behaviour of Fe2O3 at pressures over 100 GPa and temperatures above 2,500 K employing single crystal X-ray diffraction and synchrotron Mössbauer source spectroscopy. Crystal chemical analysis of structures presented here and known Fe(II, III) oxides shows their fundamental relationships and that they can be described by the homologous series nFeO·mFe2O3. Decomposition of Fe2O3 and Fe3O4 observed at pressures above 60 GPa and temperatures of 2,000 K leads to crystallization of unusual Fe5O7 and Fe25O32 phases with release of oxygen. Our findings suggest that mixed-valence iron oxides may play a significant role in oxygen cycling between earth reservoirs.

Metallic osmium (Os) is one of the most exceptional elemental materials, having, at ambient pressure, the highest known density and one of the highest cohesive energies and melting temperatures. It is also very incompressible, but its high-pressure behaviour is not well understood because it has been studied so far only at pressures below 75 gigapascals. Here we report powder X-ray diffraction measurements on Os at multi-megabar pressures using both conventional and double-stage diamond anvil cells, with accurate pressure determination ensured by first obtaining self-consistent equations of state of gold, platinum, and tungsten in static experiments up to 500 gigapascals. These measurements allow us to show that Os retains its hexagonal close-packed structure upon compression to over 770 gigapascals. But although its molar volume monotonically decreases with pressure, the unit cell parameter ratio of Os exhibits anomalies at approximately 150 gigapascals and 440 gigapascals. Dynamical mean-field theory calculations suggest that the former anomaly is a signature of the topological change of the Fermi surface for valence electrons. However, the anomaly at 440 gigapascals might be related to an electronic transition associated with pressure-induced interactions between core electrons. The ability to affect the core electrons under static high-pressure experimental conditions, even for incompressible metals such as Os, opens up opportunities to search for new states of matter under extreme compression.