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Studies of dense carbon materials formed by bolide impacts or produced by laboratory compression provide key information on the high-pressure behavior of carbon and for identifying and designing unique structures for technological applications. However, a major obstacle to studying and designing these materials is an incomplete understanding of their fundamental structures. Here, we report the remarkable structural diversity of cubic/hexagonally (

Dicaesium octaiodide is composed of layers of zigzag polyiodide units (I8 2−) intercalated with caesium cations. Each I8 2− unit is built of two triiodides bridged with one diiodine molecules. This system was subjected to compression up to 5.9 GPa under hydrostatic conditions. Pressure alters the supramolecular architecture around I8 2−, leading to bending of the triiodide units away from their energetically preferred geometry (D ∞h). Short I2...I3 − contacts compress significantly, reaching lengths typical for the covalently bonded polyiodides. Unlike in reported structures at ambient conditions, pressure-induced catenation proceeds without symmetrization of the polyiodides, pointing to a different electron-transfer mechanism. The structure is shown to be half as compressible [B0 = 12.9 (4) GPa] than the similar CsI3 structure. The high bulk modulus is associated with higher I—I connectivity and a more compact cationic net, than in CsI3. The small discontinuity in the compressibility trend around 3 GPa suggests formation of more covalent I—I bonds. The potential sources of this discontinuity and its implication on the electronic properties of Cs2I8 are discussed.

The direct chemical reactivity between phosphorus and nitrogen was induced under high-pressure and high-temperature conditions (9.1 GPa and 2000-2500 K), generated by a laser-heated diamond anvil cell and studied by synchrotron X-ray diffraction, Raman spectroscopy, and DFT calculations. α-P

High pressure reactivity of phosphorus and hydrogen is relevant to fundamental chemistry, energy conversion and storage, and materials science. Here we report the synthesis of (PH3)2H2, a crystalline van der Waals (vdW) compound (I4cm) made of PH3 and H2 molecules, in a Diamond Anvil Cell by direct catalyst-free high pressure (1.2 GPa) and high temperature (T ≲ 1000 K) chemical reaction of black phosphorus and liquid hydrogen, followed by room T compression above 3.5 GPa. Group 15 elements were previously not known to form H2-containing vdW compounds of their molecular hydrides. The observation of (PH3)2H2, identified by synchrotron X-ray diffraction and vibrational spectroscopy (FTIR, Raman), therefore represents the discovery of a previously missing tile, specifically corresponding to P for pnictogens, in the ability of non-metallic elements to form such compounds. Significant chemical implications encompass reactivity of the elements under extreme conditions, with the observation of the P analogue of the Haber-Bosch reaction for N, fundamental bond theory, and predicted high pressure superconductivity in P-H systems., Chemical elements at high pressure may behave more consistently with their periodic properties than they do at ambient conditions. The authors report the synthesis of PH3 from black phosphorous and hydrogen, and the crystallization of the van der Waals compound (PH3)2H2 which fills a gap in the chemistry of adjacent elements in the periodic table.

International audience; Melting properties of the deep mantle remain controversial due to experimental difficulties; e.g., reports of solidus temperatures of mantle-relevant compositions span over ∼700 K at 2000 km depth. This situation limits our understanding of the thermochemical state of the Earth's interior. Using the laser heated diamond anvil cell (LH-DAC), we performed new experimental determination of the solidus profile of ultra-dry pyrolite and the solidus of two compositions of (Mg,Fe)(Si,Al)O3 bridgmanite (Bg). Melting was detected (i) from -the correlation between laser power and sample temperature, -changes of sample texture and -the level of visible light absorption, for all samples, (ii) using X-ray diffraction, for the MgSiO3 composition and (iii) after scanning electron microscope observations, for selected Fe-bearing samples. Special care was given to using ultra-dry experimental chambers and to determination of sample temperature. In particular, we discuss the wavelength-dependent thermal emission of silicate samples, which lowers the solidus by 100 to 300 K, compared to the grey-body assumption.The solidus of MgSiO3-Bg is in good agreement with previous reports using ab initio calculations and shock wave experiments. We observe a net decrease in the solid-liquid Clapeyron slope at 60(3) GPa and 4400(200) K, which can be related to rapid pressure-induced coordination change of Si in the melt. (Mg0.955,Fe0.045)(Si0.993,Al0.007)O3 Bg melts 600–800 K lower than MgSiO3-Bg. Its solidus evolves smoothly with pressure, suggesting progressive Si coordination change in the melt. In the pressure range investigated (24–135 GPa) Clapeyron slopes suggest rapid decrease of the volume of fusion, from 14 to 2% for MgSiO3 and from 9 to 3% for (Fe,Al)-bearing Bg, assuming congruent melting. By comparing the solidii of various silicates, it appears that the higher the number of cations, the less pronounced is the curvature of the solidus. This observation suggests that the relatively ordered structure of simple liquid compositions with a limited number of distinct network-modifying cations frustrates the coexistence of tetrahedrally and octahedrally coordinated Si polyhedral.The solidus of pyrolite presents a smooth evolution from 2200(100) K to 3950(200) K in the same pressure interval. This is very similar to our previous work on chondritic-type mantle. The new solidus is 200–300 K lower than that of KLB-1 peridotite, which can be related to more incompatible elements in pyrolite. It remains problematic that our solidus plots several 100 K higher than other recent measurements performed on pyrolite; we discuss the possibility of a higher water content in previous samples, compared to our experiments. Assuming a dry lowermost mantle, our results imply a core-mantle boundary temperature lower than 3950(200) K. Modeling the melting diagram at the core-mantle boundary suggests a pseudo-eutectic melt significantly depleted in SiO2, compared to the composition of the mean mantle.

Highly brilliant synchrotron source is indispensable to track pressure-induced phenomena in confined crystalline samples in megabar range. In this article, a number of experimental variables affecting the quality high-pressure single-crystal x-ray diffraction data is discussed. An overview of the recent advancements in x-ray diffraction techniques at extreme conditions, in the frame of European Synchrotron Radiation Facility (ESRF)- Extremely Bright Source (EBS), is presented. Particularly, ID15b and ID27 beamlines have profited from the source upgrade, allowing for measurements of a few-micron crystals in megabar range. In case of ID27, a whole new beamline has been devised, including installation of double-multilayer mirrors and double crystal monochromator and construction of custom-made experimental stations. Two case studies from ID27 and ID15b are presented. Hypervalent CsI3 crystals, studied up to 24 GPa, have shown a series of phase transitions: Pnma → P-3c1→ Pm-3 n. First transition leads to formation of orthogonal linear iodine chains made of I3 -. Transformation to the cubic phase at around 21.7 GPa leads to equalization of interatomic I–I distances and formation of homoleptic In m- chains. The second study investigates elastic properties and structure of jadarite, which undergoes isosymmetric phase transition around 16.6 GPa. Despite a few-micron crystal size, twinning and dramatic loss of crystal quality, associated with pressure-induced phase transitions, crystal structures of both compounds have been determined in a straightforward matter, thanks to the recent developments within ESRF-EBS.

Chemical reactivity between As and N2 , leading to the synthesis of crystalline arsenic nitride, is here reported under high pressure and high temperature conditions generated by laser heating in a Diamond Anvil Cell. Single crystal synchrotron X-ray diffraction at different pressures between 30 and 40 GPa provides evidence for the synthesis of a covalent compound of AsN stoichiometry, crystallizing in a cubic P213 space group, in which each of the two elements is single-bonded to three atoms of the other and hosts an electron lone pair, in a tetrahedral anisotropic coordination. The identification of characteristic structural motifs highlights the key role played by the directional repulsive interactions between non-bonding electron lone pairs in the formation of the AsN structure. Additional data indicate the existence of AsN at room temperature from 9.8 up to 50 GPa. Implications concern fundamental aspects of pnictogens chemistry and the synthesis of innovative advanced materials.

International audience; Structural transformation of hot dense water ice is investigated by combining synchrotron x-ray diffraction and a laser-heating diamond anvil cell above 25 GPa. A transition from the body-centered-cubic (bcc) to face-centered-cubic (fcc) oxygen atoms sublattices is observed from 57 GPa and 1500 K to 166 GPa and 2500 K. That is the structural signature of the transition to fcc superionic (fcc SI) ice. The sign of the density discontinuity at the transition is obtained and a phase diagram is disclosed, showing an extended fcc SI stability field. Present data also constrain the stability field of the bcc superionic (bcc SI) ice up to 100 GPa at least. The current understanding of warm dense water ice based on ab initio simulations is discussed in the light of present data.

International audience; We report a new technique for torsional testing of materials under giga-pascal pressures, which uses a shearing module in a large volume Paris-Edinburgh press in combination with highresolution fast radiographic X-ray imaging. The measurement of the relative amplitude and phase lag between the cyclic displacement in the sample and a standard material (Al2O3) provides the effective shear modulus and attenuation factor for the sample. The system can operate in the 0.001 to 0.01 Hz frequency range, and up to 5 GPa and 2000 K; although hightemperature measurements may be affected by grain growth and plastic strain. Preliminary experimental results on San Carlos olivine are in quantitative agreement with previously reported Q-1 factors at lower pressure. This cyclic torsional loading method opens new directions to quantify the viscoelastic properties of minerals/rocks at seismic frequencies and in pressure-temperature conditions relevant to the Earth's mantle, for a better interpretation of seismological data.

Introduction In light of the low oxygen fugacity inferred for Mercury, it is expected that Si is the dominant light constituent of Mercury’s core, potentially combined with smaller amounts of C and S. Depending on specific assumptions on Mercury’s oxidation state and bulk composition (for example, assuming a CB or EH chondrite-like bulk composition), geochemical estimates for the amount of Si, C, and S in Mercury’s core differ [1,2]. The concentration of light elements in Mercury’s core can be assessed independently from geochemical inferences using geophysical measurements. Geodetic constraints on the interior mass distribution of Mercury are derived from Mercury’s (close proximity to a) Cassini state 1 [3]. The compositional range of Mercury’s core for which the mass distribution of the planet is in agreement with geodetic measurements can be examined by planetary interior structure modelling [4-6]. Such interior structure modelling requires a composition-dependent parametrisation of the thermodynamic properties of metallic core alloys. Experimental measurements of density and of ternary Fe-Si-C liquids at high pressures have been lacking, although they are crucial to constrain the compositional range of Mercury’s core, and of bulk Mercury, using geodetic information of the planet. We experimentally measured the density and P-wave velocity (VP) of Fe-Si-C liquid metallic mixtures at high pressure, and investigated the feasible range of Mercury’s core composition in light of present geodetic constraints by interior structure modelling. Methods Density measurements of Fe-Si-C liquid metals are performed at high-pressure and high-temperature by X-ray absorption techniques with a Paris-Edinburgh (PE) press at beamline ID-27 of the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Binary Fe-Si sample powder is loaded into a cylindrical diamond (C) sample container, which saturates the sample in C. X-ray absorption profiles of the C-saturated Fe-Si-C metallic liquid sample are obtained in-situ, and of the quenched sample after the experiment ended and brought to ambient conditions. The density of the recovered samples is measured by hydrostatic weighing at the IPGP in Paris, France. The density of the liquid sample at experimental conditions is derived by combining the density of the quenched sample and the post-situ X-ray absorption profile with the in-situ X-ray absorption profile, using the Beer-Lambert law for X-ray absorption. Ultrasonic VP measurements of Fe-Si liquid metals are performed at high-pressure and high-temperature with a PE press at beamline 16-BMB of the Advanced Photon Source (APS) synchrotron facility, in Argonne, Illinois. Ultrasonic signals of 20 MHz, 25 MHz, and 30 MHz are generated and received in-situ by a lithium niobite transducer located at the top of the setup. Vertical wave reflections at the top and at the bottom of the sample are recorded and used to determine the vertical two-way travel time of the ultrasonic wave through the sample. The sample length is determined from radiographic images that are collected in situ. The ultrasonic sound speed (VP) through the sample is calculated using the sample length and the two-way travel time of sound waves. The samples that are successfully recovered from both the density experiments and the VP experiments are analysed by electron microprobe at the Institute for Mineralogy in Münster, Germany. A ternary ideal mixing model (FESIC) is fitted to the Fe-Si-C liquid metal density and data obtained by us and to data available in literature. This mixing model is implemented in spherically symmetric interior structure models of Mercury that are constrained by the normalized moment of inertia of Mercury I=0.346±0.014 MR2 and that of its outer solid shell Im=0.1475±0.006 MR2 [3]. The thermal profile is taken linear in the upper part (assumingly conductive) of the core, and adiabatic lower in the core. An Fe-Si-C solid inner core is considered where temperature is below the melting temperature of the liquid alloy. The mantle and an assumed 40 km thick curst are parametrized as single density spherical shells with fixed density ratio. Results Figure 1 compares the obtained density and VP measurements with the FESIC mixing model. The results confirm previous studies that VP of Fe-rich liquid metal increases and the density decreases with increasing concentration of Si. For the C-saturated density measurements, the decrease of C with increasing Si concentration confirms that the solubility of C in Fe-Si metals decreases with increasing concentration of Si. In the VP experiments, the sample was not loaded in a C-saturated environment. Nonetheless, we did detect a small amount of C in the sample with ~10wt%Si. The bulk-core compositions of interior structure models, constrained by the I and Im mentioned above, are compared to estimated core composition from by geochemical methods [1] from assuming a bulk composition of Mercury similar to an EH chondrite or CB chondrites (figure 2). The Si and C concentrations of Mercury’s core that are estimated from assuming a CB-like bulk composition of Mercury are consistent with the core composition of interior structure models with the central value of I (0.3458·MR2). A EH chondrite-like bulk composition of Mercury is thought to have a large core concentration of Si, and can only be met with an obliquity of Mercury at the upper end of the uncertainty limit, the planet is cold, and the inner core is large. The latter is inconsistent with the deep-dynamo explanation for Mercury’s broad-scale low-intensity magnetic field [7]. This study is published and openly accessible [8]. Acknowledgements We acknowledge the Marie Sklodowska-Curie grant No 845354 awarded to JSK and the BRAIN-be program grant (BR/143/A2/COME-IN) to TVH. The X-ray absorption measurements were performed at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. The APS is operated by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. [1] Steenstra E. S. and W. van Westrenen (2020) Icarus, 340, 113621 [2] Vander Kaadden et al. (2020) JGR:Planets, 125(5), e2019JE006239 [3] Margot et al. (2012) JGR:Planets, 117(E12), E00L09 [4] Hauck et al. (2013), JGR:planets, 118(6), 1204-1220 [5] [Rivoldini A. and T. Van Hoolst (2013) Earth Planet. Sci. Lett., 377-378, 62-72 [6] Knibbe J. S. and W. van Westrenen (2015) JGR:Planets, 120(11), 1904-1923. [7] Christensen, U. (2006) nature, 444(7122), 1056-1058. [8] Knibbe et al. (2021), JGR:Planets, 126(1), e2020JE006651.

Earth’s deep carbon cycle affects atmospheric CO2, climate, and habitability. Owing to the extreme solubility of CaCO3, aqueous fluids released from the subducting slab could extract all carbon from the slab. However, recycling efficiency is estimated at only around 40%. Data from carbonate inclusions, petrology, and Mg isotope systematics indicate Ca2+ in carbonates is replaced by Mg2+ and other cations during subduction. Here we determined the solubility of dolomite [CaMg(CO3)2] and rhodochrosite (MnCO3), and put an upper limit on that of magnesite (MgCO3) under subduction zone conditions. Solubility decreases at least two orders of magnitude as carbonates become Mg-rich. This decreased solubility, coupled with heterogeneity of carbon and water subduction, may explain discrepancies in carbon recycling estimates. Over a range of slab settings, we find aqueous dissolution responsible for mobilizing 10 to 92% of slab carbon. Globally, aqueous fluids mobilise \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${35}_{-17}^{+20}$$\end{document}35−17+20% (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${27}_{-13}^{+16}$$\end{document}27−13+16 Mt/yr) of subducted carbon from subducting slabs., Carbonate mineral aqueous solubility decreases as carbonates become more Mg-rich during subduction. Coupled with regional variations in amounts of carbon and water subducted, this explains discrepancies in estimates of carbon recycling, suggesting that only around a third returns to the surface.

The phase transition between a face-centered cubic (fcc) and hexagonal close-packed (hcp) structures in Fe-4wt% Si alloy was examined in an internally resistive heated diamond-anvil cell (DAC) under high-pressure (P) and high-temperature (T) conditions to 71 GPa and 2000 K by in situ synchrotron X-ray diffraction. Complementary laser-heated DAC experiments were performed in Fe-6.5wt% Si. The fcc-hcp phase transition boundaries in the Fe-Si alloys are located at higher temperatures than that in pure Fe, indicating that the addition of Si expands the hcp stability field. The dP/dT slope of the boundary of the entrant fcc phase in Fe-4wt% Si is similar to that of pure Fe, but the two-phases region is observed over a temperature range increasing with pressure, going from 50 K at 15 GPa to 150 K at 40 GPa. The triple point, where the fcc, hcp, and liquid phases coexist in Fe-4wt% Si, is placed at 90–105 GPa and 3300–3600 K with the melting curve same as in Fe is assumed. This supports the idea that the hcp phase is stable at Earth's inner core conditions. The stable structures of the inner cores of the other terrestrial planets are also discussed based on their P-T conditions relative to the triple point. In view of the reduced P-T conditions of the core of Mercury (well below the triple point), an Fe-Si alloy with a Si content up to 6.5 wt% would likely crystallize an inner core with an fcc structure. Both cores from Venus and Mars are currently believed to be totally molten. Upon secular cooling, Venus is expected to crystallize an inner core with an hcp structure, as the pressures are similar to those of the Earth's core (far beyond the triple point). Martian inner core will take an hcp or fcc structure depending on the actual Si content and temperature.

Single crystals of solid chlorine (Cl2) were synthesized at room temperature and high pressure (HP, P=1.45 GPa) in a diamond anvil cell (DAC). At these conditions Cl2 adapts the same structure as its corresponding low-temperature (LT) ambient pressure modification (TCmce spacegroup) with Cl2 molecules forming monolayers parallel to the bc plane and this structure is preserved up to at least 64 GPa. The pressure of 1.45 GPa is to be considered as a solidification point of liquid Cl2 at room temperature.

Journal of the American Chemical Society 143(21), 7920 - 7924 (2021). doi:10.1021/jacs.1c03765, Ge and Sn are unreactive at ambient conditions. Their significant promise for optoelectronic applications is thus largely confined to thin film investigations. We sought to remove barriers to reactivity here by accessing a unique pressure, 10 GPa, where the two elements can adopt the same crystal structure (tetragonal, $I4_1$/$amd$) and exhibit compatible atomic radii. The route to GeSn solid solution, however, even under these directed conditions, is different. Reaction upon heating at 10 GPa occurs between unlike crystal structures (Ge, $Fd3m$ and Sn, $I$4/$mmm$), which also have highly incompatible atomic radii. They should not react, but they do. A reconstructive transformation of $I$4/$mmm$ into the $I$4$_1$/$amd$ solid solution then follows. The new tetragonal GeSn solid solution ($I$4$_1$/$amda$ = 5.280(1) Å, c = 2.915(1) Å, Z = 4 at 9.9 GPa and 298 K) also constitutes the structural and electronic bridge between 4-fold and newly prepared 8-fold coordinated alloy cubic symmetries. Furthermore, using this high-pressure route, bulk cubic diamond-structured GeSn alloys can now be obtained at ambient pressure. The findings here remove confining conventional criteria on routes to synthesis. This opens innovative avenues to advanced materials development., Published by American Chemical Society, Washington, DC

Experimental measurements of density by X-ray absorption and of P-wave velocity by ultrasonic techniques of liquid Fe-(

International audience; The phase diagram and melting behavior of the equimolar water-ammonia mixture have been investigated by Raman spectroscopy, x-ray diffraction, and visual observations from 295 K to 675 K and up to 9 GPa. Our results show non-congruent melting behavior of ammonia monohydrate (AMH) solid below 324 K and congruent melting at higher temperatures. The congruent melting is associated with the stability of a previously unobserved solid phase of AMH, which we named AMH-VII. Another, presumably water-rich, hydrate has also been detected in the range 4 GPa-7 GPa at 295 K on decompression of the high pressure disordered ionico-molecular alloy (DIMA) phase. Comparing our melting data to the literature suggests that non-congruent melting extends from 220 K to 324 K and that the solid phase that borders the fluid between 220 K and 270 K, called AMH-III, is not a proper phase of AMH but a solid solution of ammonia hemihydrate and ice. These results allow us to propose a revised and extended experimental phase diagram of AMH. Published under license by AIP Publishing. https://doi.org/10.1063/5.0021207 ., s

International audience; The liquid-liquid transition (LLT), in which a single-component liquid transforms into another one via a first-order phase transition, is an intriguing phenomenon that has changed our perception of the liquid state. LLTs have been predicted from computer simulations of water 1,2 , silicon 3 , carbon dioxide 4 , carbon 5 , hydrogen 6 and nitrogen 7. Experimental evidence has been found mostly in supercooled (that is, metastable) liquids such as Y 2 O 3-Al 2 O 3 mixtures 8 , water 9 and other molecular liquids 10-12. However, the LLT in supercooled liquids often occurs simultaneously with crystallization, making it difficult to separate the two phenomena 13. A liquid-liquid critical point (LLCP), similar to the gas-liquid critical point, has been predicted at the end of the LLT line that separates the low-and high-density liquids in some cases, but has not yet been experimentally observed for any materials. This putative LLCP has been invoked to explain the thermodynamic anomalies of water 1. Here we report combined in situ density, X-ray diffraction and Raman scattering measurements that provide direct evidence for a first-order LLT and an LLCP in sulfur. The transformation manifests itself as a sharp density jump between the low-and high-density liquids and by distinct features in the pair distribution function. We observe a non-monotonic variation of the density jump with increasing temperature: it first increases and then decreases when moving away from the critical point. This behaviour is linked to the competing effects of density and entropy in driving the transition. The existence of a first-order LLT and a critical point in sulfur could provide insight into the anomalous behaviour of important liquids such as water.

We present a study on the phase stability of dense carbon dioxide (${\mathrm{CO}}_{2}$) at extreme pressure-temperature conditions, up to 6200 K within the pressure range $37\ifmmode\pm\else\textpm\fi{}9$ to $106\ifmmode\pm\else\textpm\fi{}17\text{ }\text{ }\mathrm{GPa}$. The investigations of high-pressure high-temperature in situ x-ray diffraction patterns recorded from laser-heated ${\mathrm{CO}}_{2}$, as densified in diamond-anvil cells, consistently reproduced the exclusive formation of polymeric tetragonal ${\mathrm{CO}}_{2}$-V at any condition achieved in repetitive laser-heating cycles. Using well-considered experimental arrangements, which prevent reactions with metal components of the pressure cells, annealing through laser heating was extended individually up to approximately 40 min per cycle in order to keep track of upcoming instabilities and changes with time. The results clearly exclude any decomposition of ${\mathrm{CO}}_{2}$-V into the elements as previously suggested. Alterations of the Bragg peak distribution on Debye-Scherrer rings indicate grain coarsening at temperatures $g4000\text{ }\text{ }\mathrm{K}$, giving a glimpse of the possible extension of the stability of the polymeric solid phase.

The phase diagram and melting curve of water ice is investigated up to 45 GPa and 1600 K by synchrotron x-ray diffraction in the resistively and laser heated diamond anvil cell. Our melting data evidence a triple point at 14.6 GPa, 850 K. The latter is shown to be related to a first-order solid transition from the dynamically disordered form of ice VII, denoted ice ${\mathrm{VII}}^{\ensuremath{'}}$, toward a high-temperature phase with the same bcc oxygen lattice but larger volume and higher entropy. Our experiments are compared to ab initio molecular dynamics simulations, enabling us to identify the high-temperature bcc phase with the predicted superionic ice ${\mathrm{VII}}^{\ensuremath{'}\ensuremath{'}}$ phase [J.-A. Hernandez and R. Caracas, Phys. Rev. Lett. 117, 135503 (2016).].

International audience; Properties of liquid silicates under high pressure and high temperature conditions are critical for modeling the dynamics and solidification mechanisms of the magma ocean in the early Earth, as well as for constraining entrainment of melts in the mantle and in the present-day core-mantle boundary. Here, we present in situ structural measurements by X-ray diffraction of selected amorphous silicates compressed statically in diamond anvil cells (up to 157 GPa at room temperature) or dynamically by laser-generated shock compression (up to 130 GPa and 6000 K along the MgSiO3 glass Hugoniot). The X-ray diffraction patterns of silicate glasses and liquids reveal similar characteristics over a wide pressure and temperature range. Beyond the increase in Si coordination observed at 20 GPa, we find no evidence for major structural changes occurring in the silicate melts studied up to pressures and temperatures exceeding Earth's core mantle boundary conditions. This result is supported by molecular dynamics calculations. Our findings reinforce the widely-used assumption that the silicate glasses studies are appropriate structural analogs for understanding the atomic arrangement of silicate liquids at these high pressures.

International audience; Black phosphorus (bP) is a crystalline material which can be seen as an ordered stacking of two-dimensional layers, referred to as Phosphorene. The knowledge of the linear thermal expansion coefficients (LTEC) of bP is of great interest in the field of 2D materials for a better understanding of the anistropic thermal properties and exfoliation mechanism of this material. Despite several theoretical and experimental studies important uncertainties remain in the determination of the LTEC of bP. Here, we report accurate thermal expansion measurements along the three crystallographic axes using in-situ high temperature x-ray diffraction. From the progressive reduction of the diffracted intensities with temperature we monitored the loss of the crystal structure of bP across the investigated temperature range, evidencing two thermal expansion regimes at temperature below and above 706 K. Below 706 K, we observe a strong out-of-plane anisotropy, while at temperatures above 706 K a larger thermal expansion occurs along the a crystallographic direction. From our data and by taking advantage of ab-initio optimization, we propose a detailed anisotropic thermal expansion mechanism of bP, which leads to an inter- and intra-layer destabilization. An interpretation of it, based on the high T perturbation of the stabilizing sp orbital mixing effect, is provided, consistently with high pressure data.

Co-auteur étranger; International audience; The structure of liquid silicates is commonly described as a statistical mixture of various atomic entities with relative abundances that can vary with pressure, temperature and composition. Unfortunately, this view remains largely theoretical due to scarce experimental reports on the silicate melt structure, in particular under pressure. We performed X-ray diffraction of the SiO2 end-member to probe the melting curve up to ~120 GPa and 7000 K, and the melt structure up to ~80 GPa. We confirm the steep increase of the melting curve above ~14 GPa when stishovite becomes stable over coesite in subsolidus conditions, with a slope of about 80 K/GPa. Then, around 45 GPa and 5400 K, the melting curve flattens significantly, an effect most likely reflecting the densification of the SiO2 melt structure. The signal of diffuse X-ray scattering is compatible with a change of the Si coordination number from 4 to 6 along the melting curve, in agreement with previous works reporting a similar evolution during the cold compression of SiO2-glass. Because of the limited pressure range (within 10 to 20 GPa) in which the melting curve changes its slope, we speculate a difficult coexistence of tetrahedral SiO4 and octahedral SiO6 units in SiO2 melt at high pressures.

International audience; The melting curve of gold has been measured up to 110 GPa using laser-heated diamond anvil cells and synchrotron x-ray diffraction techniques. Accurate pyrometry temperature measurements and a homogeneous heating of the gold sample were achieved by implementing a sample assembly consisting of two boron-doped diamond cupped disks sandwiching the gold sample. In the investigated pressure range, the fcc solid gold remains stable up to melting. A clear structural signature of bulk melting is observed. Ab initio molecular dynamics simulations within the two-phase approach give a melting curve in good agreement with the experimental one. We discuss the validity of calculations based on the Lindemann criteria of melting which have been up to now used to obtain the melting line of Au in the 100 GPa range.

Compared to other body-centered cubic (bcc) transition metals, Nb has been the subject of fewer compression studies and there are still aspects of its phase diagram which are unclear. Here, we report a combined theoretical and experimental study of Nb under high pressure and temperature. We present the results of static laser-heated diamond anvil cell experiments up to 120 GPa using synchrotron-based fast x-ray diffraction combined with ab initio quantum molecular dynamics simulations. The melting curve of Nb is determined and evidence for a solid-solid phase transformation in Nb with increasing temperature is found. The high-temperature phase of Nb is orthorhombic Pnma. The bcc-Pnma transition is clearly seen in the experimental data on the Nb principal Hugoniot. The bcc-Pnma coexistence observed in our experiments is explained. Agreement between the measured and calculated melting curves is very good except at 40-60 GPa where three experimental points lie below the theoretical melting curve by 250 K (or 7%); a possible explanation is given. The study of materials under extreme conditions can reveal interesting physics in diverse areas such as condensed matter and geophysics. Here, the authors investigate experimentally and theoretically the high pressure-high temperature phase diagram of niobium revealing a previously unobserved phase transition from body-centered cubic to orthorhombic phase. Funding Agencies|Spanish Ministerio de Ciencia, Innovacion y Universidades [MAT2016-75586-C4-1-P, PGC2018-097520-A-100, RED2018-102612-T]; Generalitat ValencianaGeneralitat Valenciana [Prometeo/2018/123 EFIMAT]; Spanish governmentSpanish Government [RyC-2014-15643]; US Department of EnergyUnited States Department of Energy (DOE) [DE-AC52-07NA27344]

Cubic Pm-3n Nb3Sn0.92 superconductor (T c ∼ 16 K) was found to exhibit tetragonal instabilities at the superconducting state (T = 10 K). These instabilities are manifested through the appearance of reflections which are forbidden in the Pm-3n symmetry but are compatible with the P42/mmc structure which is observed in the Nb3Sn1−x system for higher Sn content at temperatures lower than ∼43 K. Nevertheless, the low-temperature structure of Nb3Sn0.92 remains metrically fully cubic, as concluded from single crystal synchrotron radiation diffraction experiments. Subsequent application of external pressure amplifies the observed instabilities with a resulting pseudo-cubic–tetragonal transformation at P = 3 GPa at 10 K and this transition is energy driven, as concluded from ab initio calculations. The electronic structures of the corresponding phases are virtually identical and, therefore, the pseudo-cubic–tetragonal transformation does not influence significantly the underlying electronic interactions. Consequently, no anomalies in the behavior of the critical temperature, T c, are expected at this pressure. However, anomalies in the upper critical field are anticipated during this transition, in analogy to the corresponding behavior observed during the cubic–tetragonal transformation in Nb3Sn1−x induced by increase in Sn content. Therefore targeted changes in composition could be used to enhance upper critical field of Nb3Sn1−x for specific extreme conditions of temperature and pressure.

The structure of liquid ammonia (${\mathrm{NH}}_{3}$) is investigated from 1 to 6.3 GPa and up to 800 K by means of synchrotron x-ray diffraction (XRD) and ab initio molecular dynamics (AIMD) simulations. The XRD data are used to extract the molecular structure factor ${S}_{\mathrm{mol}}(Q)$, pair distribution function (PDF) ${g}_{\mathrm{mol}}(r)$, and the density of ${\mathrm{NH}}_{3}$. There is an excellent agreement between present ${S}_{\mathrm{mol}}(Q)$ and ${g}_{\mathrm{mol}}(r)$ at our lowest density and those reported in reference neutron experiments. Our densities agree better with the equation of state (EoS) of Tillner-Roth et al. [DKV-Tagungsbericht (Hannover, New York, 1993), Vol. 20, p. 167] than with more recent EoS models. The experimental structure factor and PDF are well reproduced by AIMD simulations using either the Becke-Lee-Yang-Parr or the Perdew-Burke-Ernzerhof exchange-correlation functional. The shapes of ${S}_{\mathrm{mol}}(Q)$ and ${g}_{\mathrm{mol}}(r)$ vary little over the investigated pressure range and suggest a compact liquid with weak orientational correlations between molecules, which is corroborated by the coordination number varying from 12.7 to $\ensuremath{\sim}14$. The simulations are used to study the evolution of the site-site pair distribution functions, which reveals that the number of H bonds per molecule (between 1.5 and 2) do not evolve with density and that the distribution of H atoms around N atoms becomes more and more anisotropic with pressure.

A face-centered cubic (fcc) polymerized C60 phase has been synthesized at 9.5 GPa and 550 °C. The observed short lattice constant, 13.19 Å, indicates that a three-dimensional (3D) polymer with adjacent C60 molecules covalently bonded was obtained. Possible polymerized structures, with each molecule adopting one of the two standard orientations, have been investigated through density functional theory (DFT). It was found that intermolecular bonds, 56/56 2 + 2 cycloaddition, are formed between molecules with different orientations but no bonds are formed between molecules with the same orientation. The computed bulk moduli for these structures gave values ranging from 88 to 132 GPa. Although the distances between neighboring molecules and the volume per molecule from the models are close to the experimental values, the relaxed structures displayed symmetries lower than cubic, suggesting that the experimental fcc structure is likely disordered or frustrated. Keywords: Fullerene polymers, X-ray diffraction, DFT calculations, Ordered binary-alloy structures

Over the past 60 years, the diamond anvil cell (DAC) has been developed into a widespread high static pressure device. The adaptation of laboratory and synchrotron analytical techniques to DAC enables a detailed exploration in the 100 GPa range. The strain of the anvils under high load explains the 400 GPa limit of the conventional DAC. Here we show a toroidal shape for a diamond anvil tip that enables to extend the DAC use toward the terapascal pressure range. The toroidal-DAC keeps the assets for a complete, reproducible, and accurate characterization of materials, from solids to gases. Raman signal from the diamond anvil or X-ray signal from the rhenium gasket allow measurement of pressure. Here, the equations of state of gold, aluminum, and argon are measured with X-ray diffraction. The data are compared with recent measurements under similar conditions by two other approaches, the double-stage DAC and the dynamic ramp compression., Extreme static pressures exceeding a million atmospheres exist in a variety of natural environments, but obtaining such pressures in a laboratory is still a challenge. Here, the authors develop a toroidal diamond anvil design that allows for the generation of 600 GPa (6 million atmospheres) in routinely used diamond anvil cells.

In this article we present a direct comparison between reflective (Schwarzschild mirrors) and refractive (achromatic doublets) optics commonly used in spectroradiometric temperature measurements in...

Nanodiamonds (ND) with 1-5 nm dimensions found in meteorites or produced by chemical vapour deposition (CVD) and detonation synthesis are typically described in terms of an sp3-bonded carbon network. However, ultra-high-resolution transmission electron microscopy (uHRTEM) combined with density functional theory (DFT) modelling leads to a different structural interpretation. uHRTEM imaging and nanodiffraction studies of many NDs show six-fold symmetry features whose identity has long been controversial. We also observe diffraction patterns with twelve equally-spaced and symmetrically but unequally arranged reflections, indicating structures with crystallographically-forbidden ideal and distorted twelve-fold symmetry. Structural models based on our DFT calculations lead to an interpretation of these unusual features found throughout the meteoritic and CVD samples in terms of sp3 domains arranged around and coherently bonded to graphitic domains embedded within the diamond matrix. The bonding at the sp2-sp3 interface can explain the unusual features observed in electron energy-loss spectra (EELS) below the onset of the main diamond C1s core-loss edge leading to predictions of low-dimensional conductivity behaviour. The presence of sp2- as well as sp3-bonded regions allows us to interpret previously unexplained features of the Raman spectra and EELS data of ND materials.

JETP letters 106(11), 733 - 738 (2017). doi:10.1134/S0021364017230011, The crystal structure of elemental Sn was investigated by synchrotron X-ray diffraction at ultra high pressures up to ∼230 GPa creating in diamond anvil cells. Above 70 GPa, a pure bcc structure of Sn was observed, which is stable up to 160GPa, until an occurrence of the hcp phase was revealed. At the onset of the bcc-hcp transition at pressure of about 160GPa, the drop of the unit cell volume is about 1%. A mixture of the bcc-hcp states was observed at least up to 230GPa, and it seems that this state could exist even up to higher pressures. The fractions of the bcc and hcp phases were evaluated in the pressure range of the phase coexistence 160–230 GPa. The difference between static and dynamic compression and its effect on the V–P phase diagram of Sn are discussed., Published by Springer, Heidelberg [u.a.]

Synchrotron X-ray diffraction studies of the structure of SnTe have been performed at room temperature and high pressures under the conditions of quasihydrostatic compression up to 193.5 GPa created in diamond anvil cells. Two structural phase transitions have been detected at P ≈ 3 and 23 GPa. The first phase transition is accompanied by a stepwise decrease in the volume of the unit cell by 4% because of the orthorhombic distortion of the initial SnTe-B1 cubic structure of the NaCl type. It has been found that two intermediate rhombic phases of SnTe with the space groups Cmcm and Pnma coexist in the pressure range of 3–23 GPa. The second phase transition at 23 GPa occurs from the intermediate rhombic modification to the SnTe-B2 cubic phase with the CsCl structure type. This phase transition is accompanied by an abrupt decrease in the volume of the unit cell by 8%. The pressure dependence of the volumes per formula unit at room temperature has been determined.