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The density and structure of liquid Fe-C alloys have been measured up to 58 GPa and 3,200 K by in situ X-ray diffraction using a Paris-Edinburgh press and laser-heated diamond anvil cell. Study of the pressure evolution of the local structure inferred by X-ray diffraction measurements is important to understand the compression mechanism of the liquid. Obtained data show that the degree of compression is greater for the first coordination sphere than the second and third coordination spheres. The extrapolation of the measured density suggests that carbon cannot be the only light element alloyed to iron in the Earth's core, as 8–16 at % C (1.8–3.7 wt % C) would be necessary to explain the density deficit of the outer core relative to pure Fe. This concentration is too high to account for outer core velocity. The presence of other light elements (e.g., O, Si, S, and H) is thus required.

A review of some important technical challenges related to in situ diamond anvil cell laser heating experimentation at synchrotron X-ray sources is presented. The problem of potential chemical reactions between the sample and the pressure medium or the carbon from the diamond anvils is illustrated in the case of elemental tantalum. Preliminary results of a comparison between reflective and refractive optics for high temperature measurements in the laser-heated diamond anvil cell are briefly discussed. Finally, the importance of the size and relative alignment of X-ray and laser beams for quantitative X-ray measurements is presented.

Co-auteur étranger; International audience; Noble gases are important geochemical tracers allowing reconstructing global volatile cycles in Earth's reservoirs. To constrain these fundamental processes, precise data on their partitioning behavior at deep Earth conditions are needed. Such data are only available at moderate pressures up to 25 GPa due to experimental challenges. We have investigated the possibility of noble gas storage in the Earth's lower mantle up to 115 GPa. We studied the incorporation of krypton in the second most abundant lower mantle mineral (Mg1-x,Fex)O (ferropericlase) as well as in liquid metal-alloys by performing experiments up to 115 GPa and 3700 K using the laser-heated diamond anvil cell coupled to post-mortem EMPA analysis and X-ray absorption spectroscopy. The results reveal that, at these extreme conditions, up to 3 wt.% of krypton can be stored in (Mg1-x,Fex)O and 3000 ppm in the Fe-rich liquid metal. For both phases the storage capacities increase with pressure (between 40 GPa and 60 GPa) at a constant high temperature of 2300 K. Fpc has never been considered as a NG host, despite being the second most abundant mineral in the Earth's LM. Using recent accurate compressibility data, we demonstrate that a substitution of krypton into the anion site of (Mg1-x,Fex)O in form of neutral oxygen Schottky defects at diluted lower mantle conditions is possible. This noble gas incorporation mechanism is in agreement with a previous study on bridgmanite. We show that (Mg1-x,Fex)O exhibits higher noble gas storage capacities than bridgmanite through the lower mantle using lattice strain modeling and including experimental solubility and thermoelastic data for neon, argon, krypton and xenon. We also demonstrate that both phases exhibit the highest solubilities for argon and krypton. We used the solubility data from lattice strain modeling to predict noble gas abundances stored in the solid lower mantle after magma ocean crystallization. The modeled abundances show apparent similarities with estimates for the deep noble gas reservoir that are based on either 3He abundances in ocean island basalts or radiogenic 40Ar abundances in the bulk Earth. This strongly indicates that the crystalline lower mantle may play an important role as deep noble gas storage reservoir. We propose, based on considerations on noble gas replenishment from the lower mantle to the atmosphere, that the lower mantle can only contribute to a small fraction of the present-day atmospheric noble gases. This suggests that the lower mantle is an un-degassed reservoir.

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.

At ambient pressure the spinel $\mathrm{Li}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$ is a metal with a heavy fermion ground state. However, under applied pressure a transition to an insulating, nonmagnetic state occurs. Powder x-ray diffraction has been used to study structural changes associated with this transition, and reveals that the ambient-pressure $Fd\overline{3}m$ spinel structure distorts to a monoclinic cell of $C2/m,C2$, or $Cm$ symmetry above 11 GPa at low temperatures. The changes of structure and properties provide evidence for orbital molecule formation in the cubic phase at high pressures and low temperatures that leads to long-range orbital molecule order in the monoclinic phase.

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.

We use micro-beam synchrotron x-ray diffraction to study the pressure-induced amorphization of nano-sized and single crystals of Y3Ga5O12 up to pressures exceeding 1 Mbar in static compression. The abrupt pressure-induced amorphization found for both 56 nm and bulk micrometric crystals at around 76 GPa independently of the pressure transmitting medium employed demonstrates its intrinsic nature, previously predicted at 79 GPa by ab initio calculations. The single crystal structural solution at 50 GPa shows that the contraction of the unit-cell, mostly accommodated by the compressible YO8 dodecahedra, gives rise to a regularization and tilting increase of the GaO6 polyhedra with the Y?O-Ga angle changing from 104.84° to 102.34° in 50 GPa. We obtain a bulk modulus of 178(3) GPa for the single crystal and 172(3) GPa for the nanocrystals in excellent agreement with previous calculations. V. M. and J.R–F. thank the Spanish Ministry of Science, Innovation and Universities for the Juan de la Cierva program (FJCI-2016-27921) and for project PGC-2018-097520-A-100, respectively.

Journal of geophysical research / Planets 125(1), 1-19 (2020). doi:10.1029/2019JE006176, Gibbs energy representations for ices II, III, V, and VI are reported. These were constructed using new measurements of volumes at high pressure over a range of low temperatures combined with calculated vibrational energies grounded in statistical physics. The collection of representations are released within the open source SeaFreeze program, together with the Gibbs representation already known for ice Ih and water. This program allows accurate determination of thermodynamics properties (phase boundaries, density, specific heat, bulk modulus, thermal expansivity, chemical potentials) and seismic wave velocities over the entire range of conditions encountered in hydrospheres in our solar system (130���500 K to 2,300 MPa). These comprehensive representations allow exploration of the rich spectrum of thermodynamic behavior in the H$_2$O system. Although these results are broadly applicable in science and engineering, their use is particularly relevant to habitability analysis, interior modeling, and future geophysical sounding of water���rich planetary bodies of our solar system and beyond., Published by Wiley, Hoboken, NJ

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.

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

Melting experiments require rapid data acquisition due to instabilities of the molten sample and optical drifting due to the high required laser power. In this work, the melting curve of zirconium has been determined for the first time up to 80 GPa and 4000 K using in-situ fast x-ray diffraction (XRD) in a laser-heated diamond anvil cell (LH-DAC). The main method used for melt detection was the direct observance of liquid diffuse scattering (LDS) in the XRD patterns and it has been proven to be a reliable melting diagnostic. The effectiveness of other melting criteria such as the appearance temperature plateaus with increasing laser power is also discussed.

The underlying mechanisms of phase transitions and the emergence of complexity are long-standing fundamental subjects for which a complete and unified description is still missing. This is due to the intrinsic nature of condensed matter, which contains a very large number of interacting particles. The partial or complete resolution of these open questions will require a considerable development of the experimental and theoretical means. In this context, the newly developed extremely brilliant x-ray source at the European Synchrotron Radiation Facility with its unprecedented performances will provide the scientific community with a unique tool to tackle such challenging objectives. In this review article, we will discuss, through some selected examples, the potential impact this new instrument could have in the short and long term in this field of research.

Xe–Ni and Xe–Fe systems are studied in a pressure range relevant to the Earth's core (135–210 GPa) using laser-heated diamond anvil cells and synchrotron X-ray diffraction. The stability of several intermetallic compounds, including XeNi3 and XeFe3, has been recently calculated using structural searches and density functional theory (DFT) above 155 and 190 GPa, respectively [Zhu L, Liu H, Pickard CJ, et al. Nat Chem. 2014;6:644–648]. We have synthesized XeNi3 around 150 GPa, confirming the prediction; however, it has a cubic Pm3¯m-Cu3Au structure, different from the predicted one for XeNi3 but identical to the structure predicted for XeFe3. Pm3¯m-XeNi3 is calculated to be metastable with DFT. A disordered Ni1−xXex (x∼0.08) alloy is observed to form prior to this compound. This alloy is interesting in the perspective of a possible storage of xenon in the Earth's core. We have not observed any reaction between Xe and Fe up to 210 GPa.

Transformation microstructures in mantle minerals, such as (Mg,Fe)2SiO4, are critical for predicting the rheological properties of rocks and the interpretation of seismic observations. We present in-situ multi-grain X-ray diffraction experiments on hydrous Mg2SiO4 at the P/T conditions relevant for deep cold subducting slabs (up to 40 GPa and 850 °C) at a low experimental strain rate (~4*10-6s-1). We monitor the orientations of hundreds of grains and grain size variations during the series of α-β-γ (forsterite-wadsleyite-ringwoodite) phase transformations. Microtextural results indicate that the β and an intermediate γ* phase grow incoherently relatively to the host α-phase consistent with a nucleation and growth model. The β and γ-phase exhibit orientation relationships which are in agreement with previous ex-situ observations. The β and intermediate γ* show texturing due to moderate differential stress in the sample. Both the α-β and α-γ transformation induce significant reductions of the mean sample grain size of up to 90% that starts prior to the appearance of the daughter phase. Apart from the γ*, in the newly formed β and γ-phases, the nucleation rate is faster than the growth rate, inhibiting the formation of large grains. These results on grain orientations and grain size reductions in relation to transformation kinetics should allow refining existing slab strength models.

New insight into the superconducting properties of HgBa2CuO4 (Hg-1201) cuprates is provided by combined measurements of electrical resistivity and single crystal X-ray diffraction under pressure. The changes induced by increasing pressure up to 20 GPa in optimally doped single crystals were investigated. The resistivity measurements as a function of temperature show a metallic behavior up to ~10 GPa that gradually passes into an insulating state, typical of charge ordering, which totally suppresses superconductivity above 13 GPa. The changes in resistivity are accompanied by the apparition of sharp Bragg peaks in the X-ray diffraction patterns, indicating that the charge ordering is accompanied by a 3D oxygen ordering. Considering that pressure induces a charge transfer of about 0.02 at 10 GPa, our results are the first observation of charge order competing with superconductivity developed in the overdoped region of the phase diagram of a Hg-based cuprate.

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

This work reports on the pressure dependence of the octahedral tilts and mean Fe-O bond lengths in $R\mathrm{Fe}{\mathrm{O}}_{3}$ ($R=\mathrm{Nd}$, Sm, Eu, Gd, Tb, and Dy), determined through synchrotron x-ray diffraction and Raman scattering, and their role on the pressure-induced phase transition displayed by all of these compounds. For larger rare-earth cations (Nd-Sm), both anti- and in-phase octahedral tilting decrease as pressure increases, whereas the reverse behavior is observed for smaller ones (Gd-Dy). $\mathrm{EuFe}{\mathrm{O}}_{3}$ stands at the borderline, with nearly pressure-independent tilt angles. For the compounds where the tilts increase with pressure, the $\mathrm{Fe}{\mathrm{O}}_{6}$ octahedra are compressed at lower rates than for those ones exhibiting opposite pressure tilt dependence. The crossover between the two opposite pressure behaviors is discussed in relation to the general rules proposed from different theoretical approaches. The similarity of the pressure-induced isostructural insulator-to-metal phase transition, observed in the whole series, points out that the tilts play a minor role in its driving mechanisms. A clear relationship between octahedral compressibility and critical pressure is ascertained.

The melting curve and stability of ammonia (${\mathrm{NH}}_{3}$) is investigated up to 40 GPa and 3500 K by x-ray diffraction and Raman spectroscopy in the laser-heated diamond anvil cell. The ${\mathrm{NH}}_{3}$ samples were directly heated by the $10.6\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$ radiation of a ${\mathrm{CO}}_{2}$ laser to reduce the risks of chemical reactions. Melting was unambiguously detected by the appearance of the liquid diffraction signal upon temperature increase. The melting temperature of ${\mathrm{NH}}_{3}$ is found to steadily increase with pressure up to 40 GPa, and the previously reported turnover is not observed. As a result, the melting line of ${\mathrm{NH}}_{3}$ is expected to cross the isentropes of Neptune and Uranus in the pressure range 55--65 GPa, implying the possible presence of superionic solid ${\mathrm{NH}}_{3}$ in these planets. Our x-ray and Raman measurements confirm the appearance of ${\mathrm{N}}_{2}$ and ${\mathrm{H}}_{2}$ upon heating the liquid phase from 6 to 40 GPa. But while the equilibrium $2{\mathrm{NH}}_{3}\ensuremath{\rightleftharpoons}{\mathrm{N}}_{2}+3{\mathrm{H}}_{2}$ balances towards the dissociated elements at low pressure and high temperature, ammonia is found to the more stable species in the range 20--40 GPa, 300--3000 K.

The crystalline A17 layered structure of Phosphorus, commonly indicated as black Phosphorus, was synthesized for the first time at high pressure by Bridgman back in 1914 [1] and is currently attracting a growing attention from chemists, physicists and materials scientists due to the appealing properties of its monolayer counterpart named Phosphorene [2]. Recently, high pressure studies have reported the observation of a pseudo simple cubic (p-sc) structure in the phase diagram of Phosphorus up to 30 GPa, significantly raising the pressure limit for the layered structures of P and opening new perspectives for their stabilization and functionalization [3-4]. On the other side, the experimental report of superconductivity in compressed PH 3 [5], with no structural characterization so far, has stimulated experimental and theoretical efforts to investigate the high pressure stability of different systems containing Phosphorus and Hydrogen, which can be responsible for such behavior in analogy with H 2 S [6,7]. In this study we investigated the chemical reactivity of black Phosphorus and molecular Hydrogen under high pressure and high temperature conditions, which were generated using a membrane Diamond Anvil Cell (DAC) in combination with laser heating. The sample was probed by means of synchrotron X-ray diffraction at ESRF-ID27 and by FTIR and Raman spectroscopy at LENS. The visual inspection of the sample after laser heating clearly showed the consumption of Phosphorus. Correspondingly, the analysis of the experimental XRD and spectroscopic data indicated the formation of different reaction products containing P-H bonds, particularly PH 3 , H-functionalized Phosphorus fragments and a solid product identified as a van der Waals compound made of PH 3 and H 2 , whose crystal structure was accurately determined from single crystal data. The identification of this compound, representing so far a missing piece, consistently fills a gap in the periodic table for Phosphorus, in agreement with analogous compounds reported in literature formed by the hydrides of Carbon (CH 4 ), Sulphur (H 2 S), Selenium (H 2 Se) and Iodine (HI) in the presence of H 2 [8-11]. Furthermore, the observation of this compound provides new experimental evidence for the formation of unexpected chemical species originating from the high pressure chemistry of Phosphorus and Hydrogen, possibly shedding new light on the high pressure superconductivity of Phosphorus-Hydrogen systems. Acknowledgments: Thanks are expressed to EC through the European Research Council (ERC) for funding the project PHOSFUN "Phosphorene functionalization: a new platform for advanced multifunctional materials" (Grant Agreement No. 670173) through an ERC Advanced Grant. References [1] P. W. Bridgman, J. Am. Chem. Soc. 1914, 36, 1344. [2] M. Peruzzini et al., Eur. J. Inorg. Chem. 2019, 1476. [3] D. Scelta et al., Angew. Chem. Int. Ed. 2017, 56, 14135. [4] D. Scelta et al., Chem. Commun. 2018, 54, 10554. [5] A. P. Drozdov et al., arXiv:1508.06224 (2015). [6] T. Bi et al., Angew. Chem. Int. Ed. 2017, 56, 10192. [7 M. Liu et al. J. Raman Spectrosc. 2018, 49, 721. [8] M. S. Somayazulu et al., Science 1996, 271, 1400. [9] T. A. Strobel et al. Phys. Rev. Lett. 2011, 107, 255503 .[10] E. J. Pace et al., J. Chem. Phys. 2017, 147, 184303. [11] Binns J. et al., Phys. Rev. B 2018, 97, 024111. Keywords: Phosphorus, Hydrogen, high pressure chemistry

Cuprates exhibit antiferromagnetic, charge density wave (CDW), and high-temperature superconducting ground states that can be tuned by means of doping and external magnetic fields. However, disorder generated by these tuning methods complicates the interpretation of such experiments. Here, we report a high-resolution inelastic x-ray scattering study of the high-temperature superconductor YBa₂Cu₃O6.67 under uniaxial stress, and we show that a three-dimensional long-range-ordered CDW state can be induced through pressure along the a axis, in the absence of magnetic fields. A pronounced softening of an optical phonon mode is associated with the CDW transition. The amplitude of the CDW is suppressed below the superconducting transition temperature, indicating competition with superconductivity. The results provide insights into the normal-state properties of cuprates and illustrate the potential of uniaxial-pressure control of competing orders in quantum materials., Facultad de Ciencias Exactas, Instituto de Física de Líquidos y Sistemas Biológicos

With the application of pressure, a material decreases in volume as described in its equation of state, which is governed by energy considerations. At extreme pressures, common materials are thus expected to transform into new dense phases with extremely compact atomic arrangements that may also have unusual physical properties. For aluminium, first principle calculations have consistently predicted a phase transition sequence fcc–hcp–bcc in a pressure range below 0.5 TPa [1–7]. The hcp phase was identified at 217 GPa in an experiment (Akahama et al., 2006), and the bcc phase has been recently confirmed in a dynamic ramp-compression experiment coupled with time-resolved X-ray diffraction (Polsin et al. 2017). Here we confirm this observation with a synchrotron-based X-ray diffraction experiment carried out within a diamond-anvil cell and report indications of the onset of the transition towards a bcc structure at pressures beyond 320 GPa. With this work, we also demonstrate the possibility of routine static high-pressure experiments with conventional bevelled diamond-anvil geometry in the 0.3–0.4 TPa regime.

We investigated the atomic structure of liquid Rb along an isothermal path at 573 K, up to 23 GPa, by X-ray diffraction measurements. By raising the pressure, we observed a liquid–liquid transformation from a simple metallic liquid to a complex one. The transition occurs at 7.5 ± 1 GPa which is slightly above the first maximum of the T–P melting line. This transformation is traced back to the density-induced hybridization of highest electronic orbitals leading to the accumulation of valence electrons between Rb atoms and to the formation of interstitial atomic shells, a behavior that Rb shares with Cs and is likely to be common to all alkali metals.

We present the pressure-temperature phase diagram of the antiferromagnet CeAg2Si2 established via resistivity and calorimetry measurements under quasi-hydrostatic conditions up to 22.5 GPa. With increasing pressure, the Neel temperature [ T N ( p = 0 ) = 8.6 K] slowly increases up to T N = 13.4 K at 9.4 GPa and then vanishes abruptly at the magnetic critical pressure p c ∼ 13 GPa. For the first time, heavy fermion superconductivity is observed in CeAg2Si2. Superconductivity emerges at ∼ 11 GPa and persists over roughly 10 GPa. Partial- and bulk-transition temperatures are highest at p = 16 GPa, with a maximal T c bulk = 1.25 K. In the pressure region of superconductivity, Kondo and crystal-field splitting energies become comparable and resistivity exhibits clear signatures of a Ce-ion valence crossover. The crossover line is located at a rapid collapse in resistivity as function of pressure and extrapolates to a valence transition critical endpoint at critical pressure and temperature of p cr ∼ 17 GPa and T cr ∼ − 13 K , respectively. Both critical spin and valence fluctuations may build up superconductivity in CeAg2Si2.

Polynitrogen compounds have been actively pursued driven by their potential as ultra-high-performing propellants or explosives. Despite remarkable breakthroughs over the past two decades, the two figures of merit for a compelling material, namely a large fraction of nitrogen by weight and a bulk stability under ambient conditions, have not yet been achieved. We report the synthesis of a lithium pentazolate solid by compressing and laser-heating lithium embedded in molecular N

Solid krypton (Kr) undergoes a pressure-induced martensitic phase transition from a face-centered cubic (fcc) to a hexagonal close-packed (hcp) structure. These two phases coexist in a very wide pressure domain inducing important modifications of the bulk properties of the resulting mixed phase system. Here, we report a detailed in situ x-ray diffraction and absorption study of the influence of the fcc-hcp phase transition on the compression behavior of solid krypton in an extended pressure domain up to 140 GPa. The onset of the hcp-fcc transformation was observed in this study at around 2.7 GPa and the coexistence of these two phases up to 140 GPa, themaximum investigated pressure. The appearance of the hcp phase is also evidenced by the pressure-induced broadening and splitting of the first peak in the XANES spectra. We demonstrate that the transition is driven by a continuous nucleation and intergrowth of nanometric hcp stacking faults that evolve in the fcc phase. These hcp stacking faults are unaffected by high-temperature annealing, suggesting that plastic deformation is not at their origin. The apparent small Gibbs free-energy differences between the two structures that decrease upon compression may explain the nucleation of hcp stacking faults and the large coexistence domain of fcc and hcp krypton. We observe a clear anomaly in the equation of state of the fcc solid at ∼20 GPa when the proportion of the hcp form reaches ∼20%.We demonstrate that this anomaly is related to the difference in stiffness between the fcc and hcp phases and propose two distinct equation of states for the low and high-pressure regimes.

We report on the effects of hydrostatic pressure (HP) on the charge density wave observed in underdoped cuprates. We studied ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.6}\phantom{\rule{4pt}{0ex}}({T}_{c}=61$ K) using high-resolution inelastic x-ray scattering (IXS), and reveal an extreme sensitivity of the phonon anomalies related to the charge density wave (CDW) order to HP. The amplitudes of the normal state broadening and superconductivity-induced phonon softening at ${\mathbf{Q}}_{\text{CDW}}$ rapidly decrease as HP is applied, resulting in the complete suppression of signatures of the CDW below $\ensuremath{\sim}10$ kbar. Additional IXS measurements on ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.75}$ demonstrate that this very rapid effect cannot be explained by pressure-induced modification of the doping level and highlight the different role of external pressure and doping in tuning the phase diagram of the cuprates. Our results provide insights into the mechanisms underlying the CDW formation and its interplay with superconductivity.

The recent synthesis of Phosphorene [1,2] and the theoretical predictions of a variety of related 2D allotropes [3] have raised a growing interest from the scientific community about the layered structures of Phosphorus. Indeed, black Phosphorus (bP, A17), formed by the ordered stacking of Phosphorene layers in a similar way as graphene is related to graphite, is actually the starting material for the synthesis of Phosphorene by exfoliation techniques. However, while the synthesis and stabilization of Phosphorene are still challenging tasks, its chemistry and functionalization represent frontier research topics. Within this perspective, pressure has been shown to be a very effective tool. Besides bP, another layered structure of P, rhombohedral A7, made by the stacking of blue Phosphorene layers, is indeed experimentally accessible by pressure. Remarkably, pressure has recently allowed to gain fundamental insight about the mechanism ruling the formation of chemical bonds between P layers, unveiling the existence of an intermediate p-sc structure between the layered rhombohedral A7 and the non-layered simple-cubic phases of P, significantly raising the pressure limit where the layers of P can be observed up to at least 30 GPa [4]. In this study we report the high pressure and high temperature chemistry of Phosphorus in the presence of N2 by means of state-of-the-art synchrotron X-ray diffraction (at ESRF-ID27), FTIR and Raman spectroscopy, using membrane Diamond Anvil Cell for the generation of static high pressure and laser heating for in situ generation of high temperature. Besides the N- functionalization of Phosphorene layers, this study is relevant to the substantially unexplored chemistry of the lowest Z pnictogens and to the synthesis of new PN compounds. Crystalline black Phosphorus was laser heated under different pressure and temperature conditions, corresponding to different crystalline structures of Phosphorus (A17, A7 and sc), above and below the melting line of N2. The results indicate a pressure dependent chemical reactivity, leading to different reaction products, according to the applied pressure. The low pressure data demonstrated the first direct synthesis of gamma-P3N5 from the elements, avoiding any precursor or byproduct [5]. Raman spectra and XRD patterns were acquired at high and ambient pressure, tracing the equation of state of the material, and providing new experimental input about the existence of the predicted delta- P3N5 phase [6-7]. References [1] Liu, H.; Neal, A. T.; Zhu, Z.; Luo, Z.; Xu, X.; Tománek, D.; Ye, P. D. Phosphorene: An Unexplored 2D Semiconductor with a High Hole Mobility, ACS Nano 2014, 8, 4033-4041. [2] Li, L.; Yu, Y.; Ye, G. J.; Ge, Q.; Ou, X.; Wu, H.; Feng, D.; Chen, X. H.; Zhang, Y. Black phosphorus field-effect transistors, Nat. Nanotechnol. 2014, 9, 372-377. [3] Woo Hyun Han, W.H. Sunghyun Kim, S.; Lee, I-H.; Chang, L.J. Prediction of Green Phosphorus with Tunable Direct Band Gap and High Mobility, J. Phys. Chem. Lett. 2017, 8, 4627-4632. [4] Scelta, D.; Baldassarre, A.; Serrano-Ruiz, M.; Dziubek, K.; Cairns, A. B.; Peruzzini, M.; Bini, R.; Ceppatelli, M. Interlayer Bond Formation in Black Phosphorus at High Pres- sure, Angew. Chem. Int. Ed. 2017, 56, 14135-14140. [5] Landskron, K.; Huppertz, H.; Senker, J. and Schnick, W. High-Pressure Synthesis of gamma- P3N5 at 11 GPa and 1500 °C in a Multianvil Assembly: A Binary Phosphorus (V) Nitride with a Three-Dimensional Network Structure from PN4 Tetrahedra and Tetragonal Pyramids Angew. Chem. Int. Ed. 2001, 40, 2643-2645. [6] Dong, J.; Kinkhabwala, A. A.; McMillan, P. F. , High-pressure polymorphism in phosphorus nitrides Physica Status Solidi B, 2004, 210, 2319-2325. [7] Kroll, P.; Schnick, W. A Density Functional Study of Phosphorus Nitride P3N5 : Refined Geometries, Properties, and Relative Stability of alpha-P3N5 and gamma-P3N5 and a Further Possible High-Pressure Phase delta-P3N5 with Kyanite-Type Structure Chem. Eur. J. 2002, 8, 3530- 3537.

International audience; As the main constituent of planetary cores, pure iron phase diagram under high pressure and temperature is of fundamental importance in geophysics and planetary science. However, previously reported iron‐melting curves show large discrepancies (up to 1000 K at the Earth's core–mantle boundary, 136 GPa), resulting in persisting high uncertainties on the solid‐liquid phase boundary. Here we unambiguously show that the observed differences commonly attributed to the nature of the used melting diagnostic are due to a carbon contamination of the sample as well as pressure overestimation at high temperature. The high melting temperature of pure iron under core‐mantle boundary (4250 ± 250 K), here determined by X‐ray absorption experiments at the Fe K‐edge, indicates that volatile light elements such as sulfur, carbon, or hydrogen are required to lower the crystallization temperature of the Earth's liquid outer core in order to prevent extended melting of the surrounding silicate mantle.

International audience; The insertion of neon and argon in the 1-D pore system of the zeolite TON was studied at high pressure by X-ray diffraction and by Monte Carlo (MC) molecular modeling. Rietveld refinements of the crystal structure of TON and the MC results indicate that 12 Ne atoms enter the unit cell of TON, completely filling the pores. This is much greater than the degree of filling observed for argon, which due to size considerations lies in a vertical plane in the pores. A phase transition from the Cmc21 to a Pbn21 structure occurs at 0.6 GPa with cell doubling. The compressibility and structural distortions, such as pore ellipticity, are considerably reduced as compared to the argon-filled or the empty-pore material. In addition, the crystalline form persists to pressures of the order of 20 GPa, and the Pbn21 phase is recovered after decompression. The results show the very strong and different effects of pore filling by noble gases on the structural stability and mechanical properties of this prototypical 1-D zeolite-type material.

We carried out sound velocity and density measurements on solid hcp-Fe and an hcp-Fe–Si alloy with 9 wt.% Si at 300 K up to ∼170 and ∼140 GPa, respectively. The results allow us to assess the density (ρ) dependence of the compressional sound velocity ( V P ) and of the shear sound velocity ( V S ) for pure Fe and the Fe–Si alloy. The established V P –ρ and V S –ρ relations are used to address the effect of Si on the velocities in the Fe–FeSi system in the range of Si concentrations 0 to 9 wt.% applicable to the Earth's core. Assuming an ideal linear mixing model, velocities vary with respect to those of pure Fe by ∼ + 80 m / s for V P and ∼ − 80 m / s for V S for each wt.% of Si at the inner core density of 13 000 kg/m3. The possible presence of Si in the inner core and the quantification of its amount strongly depend on anharmonic effects at high temperature and on actual core temperature.

International audience; Synchrotron x-ray diffraction measurements of nitrogen are performed up to 120 GPa to determine the melting curve and the structural changes of the solid and liquid phases along it. The melting temperature exhibits a monotonic increase up to the triple point where the epsilon molecular solid, the cubic gauche covalent solid, and the fluid meet at 116 GPa, 2080 K. Above, the stability of the cubic gauche phase induces a sharp increase of the melting curve. The structural data on liquid nitrogen show that the latter remains molecular over the whole probed domain, which contradicts the prediction of a liquid-liquid transition at 88 GPa, 2000 K. These findings thus largely revisit the phase diagram of hot dense nitrogen and challenge the current understanding of this model system.

Eutectic melting temperatures in the Fe–FeO and Fe–Fe3C systems have been determined up to 150 GPa. Melting criteria include observation of a diffuse scattering signal by in situ X-Ray diffraction, and textural characterisation of recovered samples. In addition, compositions of eutectic liquids have been established by combining in situ Rietveld analyses with ex situ chemical analyses. Gathering these new results together with previous reports on Fe–S and Fe–Si systems allow us to discuss the specific effect of each light element (Si, S, O, C) on the melting properties of the outer core. Crystallization temperatures of Si-rich core compositional models are too high to be compatible with the absence of extensive mantle melting at the core–mantle boundary (CMB) and significant amounts of volatile elements such as S and/or C (>5 at%, corresponding to >2 wt%), or a large amount of O (>15 at% corresponding to ∼5 wt%) are required to reduce the crystallisation temperature of the core material below that of a peridotitic lower mantle.

Magnetic, electronic, and structural properties of $M\mathrm{F}{\mathrm{e}}_{2}{\mathrm{O}}_{4} (M=\mathrm{Mg},\mathrm{Zn},\mathrm{Fe})$ ferric spinels have been studied by $^{57}\mathrm{Fe}$ M\"ossbauer spectroscopy, electrical conductivity, and powder and single-crystal x-ray diffraction (XRD) to a pressure of 120 GPa and in the 2.4--300 K temperature range. These studies reveal for all materials, at the pressure range 25--40 GPa, an irreversible first-order structural transition to the postspinel $\mathrm{CaT}{\mathrm{i}}_{2}{\mathrm{O}}_{4}\ensuremath{-}$ type structure (Bbmm) in which the HS $\mathrm{F}{\mathrm{e}}^{3+}$ occupies two different crystallographic sites characterized by six- and eightfold coordination polyhedra, respectively. Above 40 GPa, an onset of a sluggish second-order high-to-low spin (HS-LS) transition is observed on the octahedral $\mathrm{F}{\mathrm{e}}^{3+}$ sites while $\mathrm{F}{\mathrm{e}}^{3+}$ occupying bicapped trigonal prism sites remain in the HS state. Despite an appreciable resistance decrease, corroborating with the transition to the LS state, $\mathrm{MgF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}$ and $\mathrm{ZnF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}$ remain semiconductors at this pressure range. However, in the case of $\mathrm{F}{\mathrm{e}}_{3}{\mathrm{O}}_{4}$, the second-order HS-LS transition on the $\mathrm{F}{\mathrm{e}}^{3+}$ octahedral sites corroborates with a clear trend to a gap closure and formation of a semimetal state above 50 GPa. Above 65 GPa, another structural phase transition is observed in $\mathrm{F}{\mathrm{e}}_{3}{\mathrm{O}}_{4}$ to a new Pmma structure. This transition coincides with the onset of nonmagnetic $\mathrm{F}{\mathrm{e}}^{2+}$, signifying further propagation of the gradual collapse of magnetism corroborating with a sluggish metallization process. With this, half of $\mathrm{F}{\mathrm{e}}^{3+}$ sites remain in the HS state. Thus, this paper demonstrates that, in a material with a complex crystal structure containing transition metal cation(s) in different environments, a HS-LS transition and delocalization/metallization of the $3d$ electrons does not necessarily occur simultaneously and may propagate through different crystallographic sites at different degrees of compression.

Knowledge of the elastic properties and equations of state of iron and iron alloys are of fundamental interest in Earth and planetary sciences as they are the main constituents of telluric planetary cores. Here, we present results of X-ray diffraction measurements on a ternary Fe&ndash, Ni&ndash, Si alloy with 5 wt% Ni and 5 wt% Si, quasi-hydrostatically compressed at ambient temperature up to 56 GPa, and under simultaneous high pressure and high temperature conditions, up to 74 GPa and 1750 K. The established pressure dependence of the c/a axial ratio at ambient temperature and the pressure&ndash, volume&ndash, temperature (P&ndash, V&ndash, T) equation of state are compared with previous work and literature studies. Our results show that Ni addition does not affect the compressibility and axial compressibility of Fe&ndash, Si alloys at ambient temperature, but we suggest that ternary Fe&ndash, Si alloys might have a reduced thermal expansion in respect to pure Fe and binary Fe&ndash, Si alloys. In particular, once the thermal equations of state are considered together with velocity measurements, we conclude that elements other than Si and Ni have to be present in the Earth&rsquo, s inner core to account for both density and seismic velocities.

High-pressure room-temperature angle-dispersive powder X-ray diffraction measurements on Cu2S chalcocite were performed up to 30 GPa using a diamond-anvil cell, He as pressure transmitting medium and synchrotron radiation. Two first-order phase transitions were found at 3.2 and 7.4 GPa. The indexation of the powder diffraction patterns suggests three different monoclinic cells for the low-pressure chalcocite and the two high-pressure phases. Subtle changes in the X-ray diffraction patterns suggest a third pressure-induced transition above 26 GPa. Structural parameters and compressibility are discussed and compared to those reported in a previous study on Cu2S nanowires.

We have obtained three-component systems: the complexes comprising of two different benzenoid hydrocarbons together with one molecule of 7,7,8,8-tetracyanoquinodimethane (TCNQ). The X-ray single-crystal structures of naphthalene–perylene–TCNQ and pyrene–perylene–TCNQ revealed that they form face-to-face stacking between perylene and TCNQ molecules containing another hydrocarbon as a guest in the structure. We also present a pyrene–TCNQ complex with two pyrene moieties acting in similar way. In this system, charge transfer is far more efficient than in any other complexes of TCNQ with benzenoid hydrocarbons. Additionally, we have also obtained as references pyrene–TCNQ, chrysene–TCNQ, phenanthrene–TCNQ and naphthalene–TCNQ with 1 : 1 molecular ratios. These systems were also used to estimate the degrees of charge transfer.

[1] An understanding of the physical properties of the hydrous magnesium silicate phase D is important for the interpretation of the seismic anomalies observed in subducted slabs and to evaluate the effect of hydration on slab dynamics. Here we report the equation of state of phase D (Mg1.1Si1.8H2.5O6) up to 65 GPa obtained from high-precision single-crystal X-ray diffraction. A single-crystal of phase D was loaded in a diamond anvil cell using helium as pressure transmitting medium to ensure quasi-hydrostatic conditions during the entire data collection. The volume of phase D decreases smoothly over the entire pressure range, without the anomalies in the compressibility reported at 40 GPa in previous powder diffraction studies. If existing in phase D, a hydrogen bond symmetrization transition as predicted by first-principles calculation is therefore not associated with anomalies in the volume compression behavior. The isothermal bulk modulus KT and its pressure derivative K′T obtained from the fitting of the unit cell volumes using a third-order Birch-Murnaghan equation of state are KT = 151.4 ± 1.2 GPa and K′T = 4.89 ±0.08, respectively. This bulk modulus is in good agreement with a recent single-crystal Brillouin scattering experiment. The presence of 16 vol. % of phase D in hydrous peridotite lithologies reduces the density by up to 2.6% at upper most lower mantle pressure-temperature conditions (1273 K and 30 GPa), phase D is thus a potential candidate to influence the buoyancy of hydrated stagnant slabs below the transition zone.

The siliceous zeolite TON with a 1-D pore system was studied at high pressure by X-ray diffraction, infrared spectroscopy, and DFT calculations. The behavior of this material was investigated using nonpenetrating pressure transmitting media. Under these conditions, a phase transition from the Cmc2(1) to a Pbn2(1) structure occurs at close to 0.6 GPa with doubling of the primitive unit cell based on Rietveld refinements. The pores begin to collapse with a strong increase in their ellipticity. Upon decreasing the pressure below this value the initial structure was not recovered. DFT calculations indicate that the initial empty pore Cmc2(1) phase is dynamically unstable. Irreversible, progressive pressure-induced amorphization occurs upon further increases in pressure up to 21 GPa. These changes are confirmed in the mid-and far-infrared spectra by peak splitting at the Cmc2(1) to Pbn2(1) phase transition and strong peak broadening at high pressure due to amorphization.

Co-auteur étranger; International audience; Knowledge of melting properties is critical to predict the nature and the fate of melts produced in the deep mantle. Early in the Earth’s history, melting properties controlled the magma ocean crystallization, which potentially induced chemical segregation in distinct reservoirs. Today, partial melting most probably occurs in the lowermost mantle as well as at mid upper-mantle depths, which control important aspects of mantle dynamics, including some types of volcanism. Unfortunately, despite major experimental and theoretical efforts, major controversies remain about several aspects of mantle melting. For example, the liquidus of the mantle was reported (for peridotitic or chondritic-type composition) with a temperature difference of ∼1000 K at high mantle depths. Also, the Fe partitioning coefficient (DFeBg/melt) between bridgmanite (Bg, the major lower mantle mineral) and a melt was reported between ∼0.1 and ∼0.5, for a mantle depth of ∼2000 km. Until now, these uncertainties had prevented the construction of a coherent picture of the melting behavior of the deep mantle.In this article, we perform a critical review of previous works and develop a coherent, semi-quantitative, model. We first address the melting curve of Bg with the help of original experimental measurements, which yields a constraint on the volume change upon melting (ΔVm). Secondly, we apply a basic thermodynamical approach to discuss the melting behavior of mineralogical assemblages made of fractions of Bg, CaSiO3-perovskite and (Mg,Fe)O-ferropericlase. Our analysis yields quantitative constraints on the SiO2-content in the pseudo-eutectic melt and the degree of partial melting (F) as a function of pressure, temperature and mantle composition; For examples, we find that F could be more than 40% at the solidus temperature, except if the presence of volatile elements induces incipient melting. We then discuss the melt buoyancy in a partial molten lower mantle as a function of pressure, F and DFeBg/melt. In the lower mantle, density inversions (i.e. sinking melts) appear to be restricted to low F values and highest mantle pressures.The coherent melting model has direct geophysical implications: (i) in the early Earth, the magma ocean crystallization could not occur for a core temperature higher than ∼5400 K at the core-mantle boundary (CMB). This temperature corresponds to the melting of pure Bg at 135 GPa. For a mantle composition more realistic than pure Bg, the right CMB temperature for magma ocean crystallization could have been as low as ∼4400 K. (ii) There are converging arguments for the formation of a relatively homogeneous mantle after magma ocean crystallization. In particular, we predict the bulk crystallization of a relatively large mantle fraction, when the temperature becomes lower than the pseudo-eutectic temperature. Some chemical segregation could still be possible as a result of some Bg segregation in the lowermost mantle during the first stage of the magma ocean crystallization, and due to a much later descent of very low F, Fe-enriched, melts toward the CMB. (iii) The descent of such melts could still take place today. There formation should to be related to incipient mantle melting due to the presence of volatile elements. Even though, these melts can only be denser than the mantle (at high mantle depths) if the controversial value of DFeBg/melt is indeed as low as suggested by some experimental studies. This type of melts could contribute to produce ultra-low seismic velocity anomalies in the lowermost mantle.

We report the synthesis and characterization of the novel silicide LaFeSiH displaying superconductivity with onset at 11 K. We find that this pnictogen-free compound is isostructural to LaFeAsO, with a similar low-temperature tetragonal to orthorhombic distortion. Using density functional theory we show that this system is also a multiband metal in which the orthorhombic distortion is likely related to single-stripe antiferromagnetic order. Electrical resistivity and magnetic susceptibility measurements reveal that these features occur side-by-side with superconductivity, which is suppressed by external pressure., Comment: 5 + epsilon pages, 7 figures, Supplemental Material on demand; v2 includes additional magnetization and resistivity data; v3 includes additional resistivity data from LaFeSiH single crystal

There is interest in identifying novel materials for use in radioactive waste applications and studying their behavior under high pressure conditions. The mineral zirconolite (CaZrTi(2)O(7)) exists naturally in trace amounts in diamond-bearing deep-seated metamorphic/igneous environments, and it is also identified as a potential ceramic phase for radionuclide sequestration. However, it has been shown to undergo radiation-induced metamictization resulting in amorphous forms. In this study we probed the high pressure structural properties of this pyrochlore-like structure to study its phase transformations and possible amorphization behavior. Combined synchrotron X-ray diffraction and Raman spectroscopy studies reveal a series of high pressure phase transformations. Starting from the ambient pressure monoclinic structure, an intermediate phase with P2(1)/m symmetry is produced above 15.6 GPa via a first order transformation resulting in a wide coexistence range. Upon compression to above 56 GPa a disordered metastable phase III with a cotunnite-related structure appears that is recoverable to ambient conditions. We examine the similarity between the zirconolite behavior and the structural evolution of analogous pyrochlore systems under pressure.

Synchrotron X-ray and neutron diffraction experiments at various temperatures, down to 3 K, along with ab initio calculations, are carried out to elucidate the magnetic order of layered β-cobalt-hydroxide. This combination of techniques allows for the unambiguous assignment of the magnetic structure of this material. Our results confirm that below the Néel temperature high-spin cobalt centers are ferromagnetically coupled within a layer, and antiferromagnetically coupled across layers (magnetic propagation vector k = (0,0,½)), in agreement with the indirect interpretation based on magnetic susceptibility measurements. A paramagnetic/antiferromagnetic transition is observed at around 15 K. Moreover, the thermal expansion behavior along the c-lattice direction, perpendicular to the layers, shows an inflection slightly above this temperature, at around 30 K. The neutron diffraction patterns and the non-collinear DFT+U calculations indicate that the magnetization forms an angle of about 35° with the cobalt planes. In particular, for an isolated ferromagnetic layer, the electronic structure calculations reveal sharp cusps on the potential energy surface when the spins point parallel or perpendicular to the planes, suggesting that the ferromagnetic superexchange mechanism is strongly sensitive to the orientation of the magnetic moment., JARV acknowledges CSIC for a JAEdoc contract. This study has been supported by grants of ANPCYT/PICT 2012-2292 and UBACYT 20020120100333BA.

The melting point of Sn was determined between 20-105 GPa using laser-heated diamond anvil cell experiments, coupled with in situ synchrotron X-ray diffraction studies. In agreement with previous LH-DAC speckle experiments, we observe a flattening of the melting slope between P = 40-60 GPa. However, we also observe that this plateau is followed by a further increase in the melting slope above P ∼ 70 GPa, leading to a remarkably high melting point of T m = 5500 K by P = 105 GPa.

International audience; We have developed an experimental setup at the ESRF beamline ID12 dedicated to X-ray absorption and magnetic circular dichroism measurements at high pressure adapted for the tender X-ray energy range and compatible with low temperatures and with high magnetic field. The focused incoming X-ray beam passes through a thin diamond disk attached to a fully perforated diamond anvil and X-ray fluorescence photons from the sample are collected in back-scattering geometry through the same diamond disk. The pressure in the cell is measured using the ruby luminescence through a full diamond anvil. The highest pressure attainable with this diamond anvil cell (DAC) depends on the thickness of the diamond disk and it is above 16 GPa for a 80-μm thick plate and exceeds 4.5 GPa in the case of 30-μm diamond disk. Excellent performances of this setup in the tender X-ray range are illustrated with X-ray absorption near-edge structure studies of the phase transitions in KCl at the potassium and chlorine K-edges (3.61 and 2.82 keV, respectively) as well as in CdS at the sulfur K-edge (2.47 keV). This DAC together with a dedicated total fluorescence yield (TFY) detector could be mounted in the main heat exchanger of a cryostat and inserted in a bore of a superconducting magnet, this makes possible to perform X-ray magnetic circular dichroism (XMCD) experiments at low temperature, high magnetic field and high pressure. Feasibility of this approach is shown with the XMCD results obtained at the U M4,5-edges in ferromagnetic superconductor UGe2.