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The phase behaviour of warm dense hydrogen-helium (H-He) mixtures affects our understanding of the evolution of Jupiter and Saturn and their interior structures1,2. For example, precipitation of He from a H-He atmosphere at about 1-10 megabar and a few thousand kelvin has been invoked to explain both the excess luminosity of Saturn1,3, and the depletion of He and neon (Ne) in Jupiter's atmosphere as observed by the Galileo probe4,5. But despite its importance, H-He phase behaviour under relevant planetary conditions remains poorly constrained because it is challenging to determine computationally and because the extremes of temperature and pressure are difficult to reach experimentally. Here we report that appropriate temperatures and pressures can be reached through laser-driven shock compression of H2-He samples that have been pre-compressed in diamond-anvil cells. This allows us to probe the properties of H-He mixtures under Jovian interior conditions, revealing a region of immiscibility along the Hugoniot. A clear discontinuous change in sample reflectivity indicates that this region ends above 150 gigapascals at 10,200 kelvin and that a more subtle reflectivity change occurs above 93 gigapascals at 4,700 kelvin. Considering pressure-temperature profiles for Jupiter, these experimental immiscibility constraints for a near-protosolar mixture suggest that H-He phase separation affects a large fraction-we estimate about 15 per cent of the radius-of Jupiter's interior. This finding provides microphysical support for Jupiter models that invoke a layered interior to explain Juno and Galileo spacecraft observations1,4,6-8.

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; The pressure-induced structural changes in LiBH4 and in NaBH4 have been investigated experimentally up to 290 GPa by coupling Raman spectroscopy, infrared absorption spectroscopy and synchrotron X-ray diffraction. This data set is also analysed in the light of Density Functional Theory calculations performed up to 600 GPa. The [BH4](-) unit appears to be remarkably resistant under pressure. NaBH4 remains stable in the known Pnma gamma-phase up to 200 GPa and calculations predict a transition to a metallic polymeric C2/c phase at about 480 GPa. LiBH4 is confirmed to exhibit a richer polymorphism. A new Pnma orthorhombic phase VI is found to be stable above 60 GPa and there are hints of a possible phase VII above 160 GPa. DFT calculations predict that two other high pressure LiBH4 phases should appear at about 290 and 428 GPa. A very slight solubility of H-2 inside phases II, III and V of LiBH4 is observed. A NaBH4(H-2)(0.5) complex is predicted to be stable above 150 GPa.

International audience; The stable forms of palladium hydrides up to 100 GPa were investigated using the direct reaction of palladium with hydrogen (deuterium) in a laser-heated diamond anvil cell. The structure and volume of PdH(D) x were measured using synchrotron x-ray diffraction. The Pd atoms remain on a fcc lattice. The stoichiometry of the hydride is inferred from the volume expansion due to the hydrogen solubility in the Pd lattice. No evidence for hydrogen to palladium ratio greater than 1 is observed for both isotopes. An inverse isotope effect on the formation enthalpy of the stoichiometric Pd hydride is disclosed by measuring the equilibrium formation pressure of PdH and of PdD, 1:9 GPa and 2:7 GPa, respectively. An isotopic shift between the compression curves of PdH and of PdD is also measured, in good agreement with ab initio calculations quantifying the contribution of the hydrogen zero-point vibrational energy.

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 report the possibility to simultaneously perform wide-field nitrogen-vacancy (NV) diamond magnetic microscopy and synchrotron X-ray diffraction (XRD) measurements at high pressure. NV color centers are created on the culet of a diamond anvil which is integrated in a diamond anvil cell for static compression of the sample. The optically detected spin resonance of the NV centers is used to map the stray magnetic field produced by the sample magnetization. Using this combined scheme, the magnetic and structural behaviors can be simultaneously measured. As a proof-of-principle, we record the correlated {\alpha}-Fe to {\epsilon}-Fe structural and magnetic transitions of iron that occur here between 15 and 20 GPa at 300 K., Comment: 12 pages, 4 figures

A wide variety of Li-N compounds are predicted as stable under pressure and associated with various nitrogen anionic moieties. Accordingly, the LiN5 compound was recently synthesized at 45 GPa by the direct reaction of nitrogen and lithium. In this study, we present an experimental investigation of the Li-N binary phase diagram from ambient pressure up to 73.6 GPa. The samples loaded in the diamond anvil cells were constituted of pure lithium pieces embedded in a much greater quantity of molecular nitrogen and, at incremental pressure steps, were laser-heated to produce the thermodynamically favored solid. The following compounds are observed: Li3N, LiN2, LiN as well as LiN5, and their pressure stability domain is disclosed. Two are synthesized for the first time, namely Cmcm LiN and P63/mmc LiN2. Both are structurally resolved and characterized by X-ray diffraction and Raman spectroscopy measurements. Their high bulk modulus is characteristic of charged N2 dimers.

Laser-shocking deuterium into metal The conditions in which hydrogen disassociates and becomes an atomic metal occur in high-energy-density environments, such as the interiors of giant planets and nuclear explosions. Celliers et al. trained 168 lasers on deuterium samples at the National Ignition Facility to measure the pressure and temperature conditions of the hydrogen transition. Careful optical measurements led to the addition of four new points on the phase diagram, consistent with static estimates and theoretical calculations. Science , this issue p. 677

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.

Theory predicts a very rich high pressure chemistry of hydronitrogens with the existence of many NxHy compounds. The stability of these phases under pressure is being investigated by the compression of N2–H2 mixtures of various compositions. A previous study had disclosed a eutectic-type N2–H2 phase diagram with two stoichiometric van der Waals compounds: (N2)6(H2)7 and N2(H2)2. The structure and pressure induced chemistry of the (N2)6(H2)7 compound have already been investigated. Here, we determine the structure of the N2(H2)2 compound and characterize using Raman spectroscopy measurements the chemical changes under a pressure cycle up to 60 GPa and back to ambient conditions. A N2(H2)2 single crystal was grown from a 1 : 2 N2–H2 mixture and its crystalline structure was solved using synchrotron X-ray diffraction. Similar to the (N2)6(H2)7 solid, N2(H2)2 has a remarkable host–guest structure containing N2 molecules orientationally disordered with spherical, ellipsoidal and planar shapes. Above 50 GPa, N2(H2)2 was found to undergo a chemical reaction. The reaction products were determined to be of the azane family, with NH3 as the main constituent, along with molecular nitrogen. Upon pressure decrease, the reaction products are found to react in such a way that below 10 GPa, hydrazine is the sole azane detected. Observed down to the opening of the diamond anvil cell, the formation of metastable hydrazine instead of the energetically favorable ammonia is puzzling and remains to be elucidated. That could change the current view of Jovian planets’ atmospheres in which ammonia is assumed the only stable hydronitrogen molecule.

A new miniature diamond anvil cell was specifically designed to detect superconductivity using a SQUID (Superconducting QUantum Interference Device) magnetometer in dense hydrides directly synthesized by the reaction of hydrogen with a chemical element. The cell, made of a CuTi alloy, is fully symmetric with a very low magnetic background allowing the detection of the superconductivity of a sample as small as 3.4 × 104 µm3 without background subtraction. DC measurements or AC measurements in a Magnetic Property Measurement System 3 SQUID magnetometer from Quantum Design could be performed at temperatures as low as 3 K. This high pressure cell is inserted in a modified conventional membrane diamond anvil cell to be driven for hydrogen gas loading and for fine pressure increase before magnetic measurements are performed. To synthetize and structurally characterize the superconducting sample, a 21° optical and 8.6° X-ray acceptance angle allows one to perform laser heating and X-ray diffraction at th...

Synchrotron nanosecond time-resolved x-ray diffraction has been performed on dynamically compressed bismuth along various compression and release paths, hence exploring the Bi phase diagram up to 8 GPa and 600 K. Marked departures from the static phase diagram is observed. The sequence of structural changes is different upon compression and release. The Bi-III complex host-guest structure is never obtained. Instead the Bi-V phase is observed over a large domain. Melting of Bi-V and crystallization of the fluid into Bi-I are clearly identified on stress release. Supercooling and superheating deviations to the equilibrium melting line are observed. These observations on a prototypical system underline the possible deviations between the phase diagram obtained by dynamical compression and the one under static pressure.

Hydrogen has been an essential element in the development of atomic, molecular and condensed matter physics1. It is predicted that hydrogen should have a metal state2; however, understanding the properties of dense hydrogen has been more complex than originally thought, because under extreme conditions the electrons and protons are strongly coupled to each other and ultimately must both be treated as quantum particles3,4. Therefore, how and when molecular solid hydrogen may transform into a metal is an open question. Although the quest for metal hydrogen has pushed major developments in modern experimental high-pressure physics, the various claims of its observation remain unconfirmed5–7. Here a discontinuous change of the direct bandgap of hydrogen, from 0.6 electronvolts to below 0.1 electronvolts, is observed near 425 gigapascals. This result is most probably associated with the formation of the metallic state because the nucleus zero-point energy is larger than this lowest bandgap value. Pressures above 400 gigapascals are achieved with the recently developed toroidal diamond anvil cell8, and the structural changes and electronic properties of dense solid hydrogen at 80 kelvin are probed using synchrotron infrared absorption spectroscopy. The continuous downward shifts of the vibron wavenumber and the direct bandgap with increased pressure point to the stability of phase-III hydrogen up to 425 gigapascals. The present data suggest that metallization of hydrogen proceeds within the molecular solid, in good agreement with previous calculations that capture many-body electronic correlations9. The probable transition of hydrogen to its metal state near 425 GPa is observed, with the required high pressures created using a toroidal diamond anvil cell.

In their comment, Desjarlais et al . claim that a small temperature drop occurs after isentropic compression of fluid deuterium through the first-order insulator-metal transition. We show that their calculations do not correspond to the experimental thermodynamic path, and that thermodynamic integrations with parameters from first-principles calculations produce results in agreement with our original estimate of the temperature drop.

The possibility of high pressure formation of lead hydrides has been investigated through the direct reaction of lead with hydrogen in a laser-heated diamond anvil cell. Synchrotron x-ray diffraction was used to detect any structural changes and volume expansion due to hydrogen absorption in the Pb metal. The compression curve of lead embedded in helium, measured under the same experimental conditions, was used as a reference. No volume expansion of the Pb lattice was measured, hence indicating no formation of an interstitial hydride. The observed structural changes match those of pure lead with the sequence of compact structures f c c, h c p, and b c c. The PbH 6 and PbH 4 compounds, found by various crystal structure prediction calculations to be stable below 140 GPa, were not observed. Lead seems very reluctant to absorbing hydrogen, which could be used for plating materials to resist hydrogen embrittlement under extreme conditions.

A wide variety of Li-N compounds are predicted as stable under pressure and associated with various nitrogen anionic moieties. Accordingly, the LiN

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

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.

The ${\mathrm{H}}_{3}\mathrm{S}$ compound was reproducibly synthesized by laser heating hydrogen-embedded solid sulfur samples at various pressures above 75 GPa in a diamond anvil cell. X-ray diffraction studies were conducted up to 160 GPa and the crystal structure has been identified with space group $Cccm$. The stability of this sole orthorhombic ${\mathrm{H}}_{3}\mathrm{S}$ phase up to 160 GPa contradicts ab initio calculations that predict the stability of a sequence of two metallic superconductive structures above 110 GPa, with $R3m$ and $Im\overline{3}m$ symmetries. This work also has strong implications for the current understanding of the $200\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ superconductivity phenomenon in ${\mathrm{H}}_{2}\mathrm{S}$ since it seems to rule out the hypothesis of the decomposition of ${\mathrm{H}}_{2}\mathrm{S}$ into sulfur and superconducting ${\mathrm{H}}_{3}\mathrm{S}$.

Le rayonnement X intense des sources synchrotrons modernes s’est impose comme un outil incontournable pour les etudes in situ sous conditions extremes de pression et de temperature. Deux etudes recentes ont ete selectionnees pour illustrer le potentiel de ce type d’experience.Le premier exemple reside dans la determination precise de la courbe de fusion du tantale par diffraction X. Le second exemple concerne l’etude de la densite du magma lunaire par absorption X; ces mesures permettent d’expliquer simplement l’absence d’activite volcanique a la surface de la Lune.

Understanding Warm Dense Matter (WDM), the state of planetary interiors, is a new frontier in scientific research. There exists very little experimental data probing WDM states at the atomic level to test current models and those performed up to now are limited in quality. Here, we report a proof-of-principle experiment that makes microscopic investigations of materials under dynamic compression easily accessible to users and with data quality close to that achievable at ambient. Using a single 100 ps synchrotron x-ray pulse, we have measured, by K-edge absorption spectroscopy, ns-lived equilibrium states of WDM Fe. Structural and electronic changes in Fe are clearly observed for the first time at such extreme conditions. The amplitude of the EXAFS oscillations persists up to 500 GPa and 17000 K, suggesting an enduring local order. Moreover, a discrepancy exists with respect to theoretical calculations in the value of the energy shift of the absorption onset and so this comparison should help to refine the approximations used in models.

We present quantitative measurements of the macroscopic non-hydrostatic stress and of the microscopic stress in samples compressed in a diamond anvil cell, using helium as a pressure-transmitting medium. These are based on the analysis of X-ray diffraction line shifts and widths. The macroscopic non-hydrostatic stress reaches 0.3–0.5 GPa at 150 GPa when the sample remains embedded in the pressure medium. The effect of this non-hydrostatic stress on the measured equations of state is estimated in the cases of gold, silver, molybdenum and copper. It is found to be negligible, except in the case of gold. We show that the analysis of the X-ray diffraction line shifts is a more sensitive way of detecting non-hydrostatic compression than the monitoring of the ruby luminescence signal. It can be used to detect when the sample is directly compressed between the diamond anvils. The sample geometry, in particular its thickness, has to be carefully chosen to prevent that situation.

The prediction of novel lithium hydrides with nontraditional stoichiometries at high pressure has been seminal for highlighting a promising line of research on hydrogen-dense materials. Here, we report the evidences of the disproportionation of LiH above 130 GPa to form lithium hydrides containing H2 units. Measurements have been performed using the nonperturbing technique of synchrotron infrared absorption. The observed vibron frequencies match the predictions for LiH2 and LiH6. These polyhydrides remain insulating up to 215 GPa. A disproportionation mechanism based on the diffusion of lithium into the diamond anvil and a stratification of the sample into LiH6/LiH2/LiH layers is proposed. Polyhydrides containing an H2 sublattice do exist and could be ubiquitously stable at high pressure.

International audience; The center-of-mass structure factor, S(Q), of liquid hydrogen and liquid deuterium has been measured upto 5 GPa, mostly along the melting line from 50 to 296 K. Good quality data were achieved thanks to a novelsynchrotron x-ray technique that can isolate the very weak x-ray scattering signal of the micrometric volume ofhydrogen compressed in the diamond anvil cell. S(Q) is dominated by a broad peak and hence, its wave-vectorposition, Qm, is used to appreciate the structural changes in the system. An isotope effect in the position of Qmis observed that can be explained by a density effect. The shift of Qm towards higher Q as density increases isfollowed over a threefold compression. Two simple liquid type evolutions are disclosed with a crossover betweenthem around 37 nm−3/molecule. An interpretation is proposed based on the change in the zero-point motionalrenormalization of the interaction from anharmonic to harmonic

Megabar (1 Mbar = 100 GPa) laser shocks on precompressed samples allow reaching unprecedented high densities and moderately high 10000-100000K temperatures. We describe here a complete analysis framework for the velocimetry (VISAR) and pyrometry (SOP) data produced in these experiments. Since the precompression increases the initial density of both the sample of interest and the quartz reference for pressure-density, reflectivity and temperature measurements, we describe analytical corrections based on available experimental data on warm dense silica and density-functional-theory based molecular dynamics computer simulations. Using our improved analysis framework we report a re-analysis of previously published data on warm dense hydrogen and helium, compare the newly inferred pressure, density and temperature data with most advanced equation of state models and provide updated reflectivity values., Comment: 13 pages, 16 figures, 4 tables

As well as its importance as a gas, and in a bound form in chemistry, oxygen has interesting properties in its solid form. Solid oxygen comes in six crystallographic phases, one of which, the epsilon phase, is particularly unusual. It is dark red, absorbs strongly in the infrared, has lost the magnetic order seen in other phases and is stable over a large pressure range. The exact structure of epsilon oxygen has remained unknown since its discovery in 1979, until now. Single-crystal X-ray diffraction data reveal epsilon oxygen as O8, a rhombohedral unit made up of four O2 components held in a molecular lattice. Of the simple diatomic molecules, oxygen is the only one to carry a magnetic moment. This makes solid oxygen particularly interesting: it is considered a ‘spin-controlled’ crystal1 that displays unusual magnetic order2. At very high pressures, solid oxygen changes from an insulating to a metallic state3; at very low temperatures, it even transforms to a superconducting state4. Structural investigations of solid oxygen began in the 1920s and at present, six distinct crystallographic phases are established unambiguously1. Of these, the ɛ phase of solid oxygen is particularly intriguing: it exhibits a dark-red colour, very strong infrared absorption, and a magnetic collapse1. It is also stable over a very large pressure domain and has been the subject of numerous X-ray diffraction5,6,7, spectroscopic8,9,10,11 and theoretical studies12,13,14. But although ɛ-oxygen has been shown to have a monoclinic C2/m symmetry5,6,7,15 and its infrared absorption behaviour attributed to the association of oxygen molecules into larger units9,14, its exact structure remains unknown. Here we use single-crystal X-ray diffraction data collected between 13 and 18 GPa to determine the structure of ɛ-oxygen. We find that ɛ-oxygen is characterized by the association of four O2 molecules into a rhombohedral molecular unit, held together by what are probably weak chemical bonds. This structure is consistent with existing spectroscopic data, and further validated by the observation of a newly predicted Raman stretching mode.

We have spectroradiometrically measured a suite of temperatures between 761 ± 128 and 7307 ± 958 K in the laser-heated diamond cell using Br2 sample material. Critical evaluation of each spectrum and the dataset as a whole in comparison with synthetic data confirms the high temperatures and demonstrates the observable effects of temperature gradients and wavelength-dependent emissivity or absorbance on a single spectrum. In addition, we quantitatively demonstrate the use of a sliding two-color pyrometer analysis to produce a consistent and meaningful estimate of uncertainty of spectral temperature measurements.

We demonstrate here a laser-driven shock wave in a hydrogen sample, pre-compressed in a diamond anvil cell. The compression factors of the dynamic and static techniques are multiplied. This approac...

We report quantitatively accurate structure-factor and radial-distribution-function measurements of liquid water in a diamond-anvil cell (DAC) using x-ray diffraction. During the analysis of our diffraction data, we found it possible (and necessary) to also determine the density. Thus, we believe we present the first-ever diffraction-based determination of a liquid structure factor and equation of state in a DAC experiment.

The equation of state and the unit cell parameters of triamino-trinitrobenzene (TATB) have been measured up to 66 GPa by x-ray diffraction on multi-grain samples. A custom indexing of the diffraction pattern was implemented to enable an unambiguous assignment of the diffraction peaks. The structural data reveal a more isotropic response to compression above 10 GPa, indicating that the van der Waals forces are no more the dominant interplanar interactions. The unit cell parameters are compared with density functional theory calculations, including the empirical D2 correction for the van der Waals interactions. An excellent agreement is obtained up to 20 GPa and an increasing deviation above. The present determination of the TATB isothermal equation of state is reliable for detonation physics applications.

The Fe-H system has been investigated by combined x-ray diffraction studies and total energy calculations at pressures up to 136 GPa. The experiments involve laser annealing of hydrogen-embedded iron in a diamond anvil cell. Two new FeHx compounds, with x∼2 and x=3, are discovered at 67 and 86 GPa, respectively. Their crystal structures are identified (unit cell and Fe positional parameters from x-ray diffraction, H positional parameters from ab initio calculations) as tetragonal with space group I4/mmm for FeH(∼2) and as simple cubic with space group Pm3m for FeH3. Large metastability regimes are observed that allowed to measure the P(V) equation of state at room temperature of FeH, FeH(∼2), and FeH3.

The binary phase diagram of ${\mathrm{N}}_{2}\text{\ensuremath{-}}\mathrm{Ne}$ mixtures has been measured at 296 K by visual observation and Raman spectroscopy. The topology of the phase diagram points to the existence of the stoichiometric compound $({\mathrm{N}}_{2}{)}_{6}{\mathrm{Ne}}_{7}$. Its structure has been solved by single-crystal synchrotron x-ray diffraction. The ${\mathrm{N}}_{2}$ molecules form a guest lattice that hosts the Ne atoms. This insertion compound can be viewed as a clathrate with the centers of the ${\mathrm{N}}_{2}$ molecules forming distorted dodecahedron cages, each enclosing 14 Ne atoms. Remarkably, the $({\mathrm{N}}_{2}{)}_{6}{\mathrm{Ne}}_{7}$ compound is somehow the first clathrate organized by the quadrupolar interaction.

The binary phase diagram of N(2)-Ne mixtures has been measured at 296 K by visual observation and Raman spectroscopy. The topology of the phase diagram points to the existence of the stoichiometric compound N(2))(6)Ne(7). Its structure has been solved by single-crystal synchrotron x-ray diffraction. The N(2) molecules form a guest lattice that hosts the Ne atoms. This insertion compound can be viewed as a clathrate with the centers of the N(2) molecules forming distorted dodecahedron cages, each enclosing 14 Ne atoms. Remarkably, the N(2))(6)Ne(7) compound is somehow the first clathrate organized by the quadrupolar interaction.

International audience; New topochemistry in simple molecular systems can be explored at high pressure. Here weexamine the binary nitrogen/hydrogen system using Raman spectroscopy, synchrotron X-raydiffraction, synchrotron infrared microspectroscopy and visual observation. We find aeutectic-type binary phase diagram with two stable high-pressure van derWaals compounds,which we identify as (N2)6(H2)7 and N2(H2)2. The former represents a new type of van derWaals host–guest compound in which hydrogen molecules are contained within channels in anitrogen lattice. This compound shows evidence for a gradual, pressure-induced change inbonding from van der Waals to ionic interactions near 50 GPa, forming an amorphousdinitrogen network containing ionized ammonia in a room-temperature analogue of theHaber–Bosch process. Hydrazine is recovered on decompression. The nitrogen–hydrogensystem demonstrates the potential for new pressure-driven chemistry in high-pressurestructures and the promise of tailoring molecular interactions for materials synthesis

Fourier-transform infrared and Raman studies are performed on $\mathrm{Ar}({\mathrm{H}}_{2}{)}_{2}$ compound single crystals over a large pressure (5--30 GPa) and temperature (30--300 K) range in a diamond-anvil cell. The effect of polarization on the infrared absorption is investigated. On the basis of the spectroscopic features and comparing to the cases of pure solid ${\mathrm{H}}_{2}$ and of matrix isolated ${\mathrm{H}}_{2}$ in argon crystal, we assign the quite complex infrared spectra to the pure vibron and vibron plus phonons and vibron plus rotons combination bands. The difference of the infrared and Raman vibron frequency, as a function of pressure, is interpreted quantitatively with reference to vibrational coupling between hydrogen molecules, as in the hydrogen crystal case. Information on the structural properties can thus be derived. Finally, this study highlights the similarity in the molecular problems for the $\mathrm{Ar}({\mathrm{H}}_{2}{)}_{2}$ compound and for solid hydrogen.

The phase transformations under pressure at 296 K in solid H${}_{2}$ and D${}_{2}$ are carefully investigated here up to 290 GPa by combining infrared (IR) and Raman spectroscopy measurements. Two phase transitions, previously detected by Raman studies, are clearly measured at the same pressures for both isotopes. The transition I-III is observed at 215 GPa characterized by a vibron Raman frequency discontinuity and the appearance of a strong IR vibron. At 237 GPa, the III-IV transition is observed associated to a large vibron Raman frequency discontinuity and the appearance of two IR and two Raman vibron modes. There is no sign of free charge carriers in the long-wavelength infrared absorption region, even up to 290 GPa. Hydrogen is thus not entering a metal nor a semimetal state in phase IV. The Raman and IR vibron spectra in phase IV seems to support the alternating layers structural models. However, the comparison for the IR and Raman vibron frequencies between present data and calculations indicates that a graphenelike sheet order is overestimating the coupling between H${}_{2}$ molecules.

We present an experimental determination of the ambient temperature equation of state, P(ρ/ρ0,293 K), up to 45 GPa, of the glow discharge polymer (GDP) used as a confining capsule for the fusible deuterium-tritium mixture in inertial confinement fusion experiments. An original method has been implemented to measure both the compression factor and the refractive index versus pressure. The data are obtained in a diamond anvil cell with two sample chambers of equal thickness containing, respectively, the GDP and a NaCl reference. This experimental equation of state is compared to numerical first principles simulations. Deviations are ascribed to the difficulty to simulate the detailed atomic structure of the polymer under moderate pressure.

The high pressure-temperature behavior of titanium has been studied with x-ray diffraction in resistively heated and laser-heated diamond anvil cells up to 200 GPa and $\ensuremath{\sim}3500$ K. The stability fields of $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Ti},\ensuremath{\omega}\text{\ensuremath{-}}\mathrm{Ti},\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Ti},\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Ti}$, and $\ensuremath{\delta}\text{\ensuremath{-}}\mathrm{Ti}$ have been determined in this range. $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Ti}$ and $\ensuremath{\delta}\text{\ensuremath{-}}\mathrm{Ti}$, which had been evidenced earlier under nonhydrostatic compression, are also observed in helium pressure transmitting medium. Equation-of-state parameters are proposed for $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Ti}$ and $\ensuremath{\omega}\text{\ensuremath{-}}\mathrm{Ti}$ at 300 K, and $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Ti}$ at high temperature. The stability fields of the $\ensuremath{\alpha},\ensuremath{\omega},\ensuremath{\gamma}$, and $\ensuremath{\delta}$ phases are also studied using the projector-augmented wave method based on density-functional theory. Using the relevant core radius to avoid overlapping between atomic spheres, and relaxing cells and atomic positions, we show that all those phases have a stability domain at 0 K. We explain why $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Ti}$ and $\ensuremath{\delta}\text{\ensuremath{-}}\mathrm{Ti}$ were calculated to be unstable in earlier works. In addition, a new phase, called ${\ensuremath{\delta}}^{\ensuremath{'}}$-Ti, which is a distortion of $\ensuremath{\delta}\text{\ensuremath{-}}\mathrm{Ti}$, is predicted to form between 80 and 120 GPa and below $\ensuremath{\simeq}200$ K.

Xenon-oxygen mixtures, with low oxygen contents below 3 mol.$%$ O${}_{2}$, have been studied under pressure by x-ray diffraction, Raman, and infrared absorption spectroscopies. The presence of O${}_{2}$ at substitutional sites affects only marginally xenon equation of state---measured here up to 259 GPa---but impedes the face-centered cubic to hexagonal close packed martensitic phase transformation. On the other hand, a small amount of O${}_{2}$ impurities drastically reduces the Xe metallization pressure: from 130 GPa for pure xenon to 49 GPa for Xe-0.6 mol.$%$ O${}_{2}$. A significant softening of the O${}_{2}$ intramolecular vibration frequency is observed in the same pressure range, which suggests a redistribution of the electronic density between O${}_{2}$ and xenon.

Quantum nuclear zero-point motions in solid H$_2$ and D$_2$ under pressure are investigated at 80 K up to 160 GPa by first-principles path-integral molecular dynamics calculations. Molecular orientations are well-defined in phase II of D$_2$, while solid H$_2$ exhibits large and very asymmetric angular quantum fluctuations in this phase, with possible rotation in the (bc) plane, making it difficult to associate a well-identified single classical structure. The mechanism for the transition to phase III is also described. Existing structural data support this microscopic interpretation., 5 pages, 3 figures

The structural and vibrational properties of the van der Waals compound (N${}_{2}$)${}_{11}$He have been measured up to 175 GPa. The structure, as determined at 14.5 GPa using single-crystal x-ray diffraction, is hexagonal $(P{6}_{3}/m)$ with seven nonequivalent atoms: 6 N and 1 He. The ${\mathrm{N}}_{2}$ molecule centered at (0,0,0) is orientationally disordered. A close relationship with the $\ensuremath{\varepsilon}$ structure of pure solid ${\mathrm{N}}_{2}$ is observed. A phase transition to a hexagonal superstructure occurs at 28 GPa, which is attributed to the ordering of these disordered ${\mathrm{N}}_{2}$ molecules, driven by quadrupole-quadrupole interaction. A transition to an amorphous state then takes place at 135 GPa. Strong similarities in the structural evolution under pressure of the (N${}_{2}$)${}_{11}$He compound and of $\ensuremath{\varepsilon}$-N${}_{2}$ are discussed.

Measurements of the undercooling temperature of crystallization of 3 He, 4 He, and Ne fluids show that there exists a temperature T a along the melting curve of each system below which no undercooling is observed and above which the undercooling temperature is a linear function of the melting temperature at the same pressure. When the melting temperature is normalized by T a , the measurements in the three systems scale perfectly. T a is seen to be a linear function of 1/m, m being the mass number, and the observed undercooling can be explained through standard nucleation theory