Metastable water at several compression rates and its freezing kinetics into ice VII.

Water can be dynamically over-compressed well into the stability field of ice VII. Whether water then transforms into ice VII, vitreous ice or a metastable novel crystalline phase remained uncertain. We report here the freezing of over-compressed water to ice VII by time-resolved X-ray diffraction.

Ethane under pressure revisited using x-ray diffraction, Raman spectroscopy, infrared absorption, and ab initio calculations up to 150 GPa.

Ethane (C2H6) is anticipated to be the most stable compound within the carbon-hydrogen system under the 100 GPa pressure range. Nevertheless, the properties of ethane under pressure are still poorly documented. Here, we present a comprehensive study of the structural and vibrational properties of C2H6 in a diamond anvil cell at pressures up to 150 GPa.

Phonon Dispersion and Proton Disorder of Ice VII and VIII.

The phonon dispersion of ice VII and that of its proton-ordered analog ice VIII are investigated through a combination of inelastic x-ray scattering (IXS) measurements and first-principles calculations of the oxygen sublattice dynamic structure factor. Particular attention is devoted to hydrogen-disorder in ice VII, addressed theoretically through a statistical ensemble of fictitious ordered supercell configurations. Similar phonon densities of states are found in both phases but are significantly less structured in the case of ice VII.

Compression of D_{2} to 460 GPa and Isotopic Effects in the Path to Metal Hydrogen.

How are nuclear quantum fluctuations affecting the properties of dense hydrogen approaching metallization? We report here Raman spectroscopy and synchrotron infrared absorption measurements on deuterium up to 460 GPa at 80 K. By comparing to a previous similar study on hydrogen, isotopic effects on the electronic and vibrational properties in phase III are disclosed. Also, evidence of a probable transition to metal deuterium is observed, shifted by about 35 GPa compared to that in hydrogen.

High pressure polymorphism of LiBH and of NaBH.

The pressure-induced structural changes in LiBH and in NaBH 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 [BH] unit appears to be remarkably resistant under pressure.

Evidence of hydrogen-helium immiscibility at Jupiter-interior conditions.

The phase behaviour of warm dense hydrogen-helium (H-He) mixtures affects our understanding of the evolution of Jupiter and Saturn and their interior structures. 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 Saturn, and the depletion of He and neon (Ne) in Jupiter's atmosphere as observed by the Galileo probe. 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.

Sound velocity and refractive index of pure N fluid and of equimolar N-CO fluid mixture up to 15 GPa.

The sound velocity and refractive index of pure N and of the equimolar N-CO mixture are measured up to 15 GPa and 700 K in a resistive heating diamond anvil cell. The refractive index vs pressure is obtained by an interferometric method. The adiabatic sound velocity is then determined from the measurement of the Brillouin frequency shift in the backscattering geometry and the refractive index data.

Synchrotron infrared spectroscopic evidence of the probable transition to metal hydrogen.

Hydrogen has been an essential element in the development of atomic, molecular and condensed matter physics. It is predicted that hydrogen should have a metal state; 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 particles. Therefore, how and when molecular solid hydrogen may transform into a metal is an open question.

Magnetic measurements on micrometer-sized samples under high pressure using designed NV centers.

Pressure can be used to tune the interplay among structural, electronic, and magnetic interactions in materials. High pressures are usually applied in the diamond anvil cell, making it difficult to study the magnetic properties of a micrometer-sized sample. We report a method for spatially resolved optical magnetometry based on imaging a layer of nitrogen-vacancy (NV) centers created at the surface of a diamond anvil.

Response to Comment on "Insulator-metal transition in dense fluid deuterium".

In their comment, Desjarlais 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.

Hexagonal Layered Polymeric Nitrogen Phase Synthesized near 250 GPa.

The nitrogen triple bond dissociates in the 100 GPa pressure range and a rich variety of single-bonded polymeric nitrogen structures unique to this element have been predicted up to the terapascal pressure range. The nonmolecular cubic-gauche (cg-N) structure was first observed above 110 GPa, coupled to high temperature (>2000  K) to overcome the kinetic barrier. A mixture of cg-N with a layered phase was afterwards reported between 120 and 180 GPa.

Direct Reaction of Nitrogen and Lithium up to 75 GPa: Synthesis of the LiN, LiN, LiN, and LiN Compounds.

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

Insulator-metal transition in dense fluid deuterium.

Dense fluid metallic hydrogen occupies the interiors of Jupiter, Saturn, and many extrasolar planets, where pressures reach millions of atmospheres. Planetary structure models must describe accurately the transition from the outer molecular envelopes to the interior metallic regions. We report optical measurements of dynamically compressed fluid deuterium to 600 gigapascals (GPa) that reveal an increasing refractive index, the onset of absorption of visible light near 150 GPa, and a transition to metal-like reflectivity (exceeding 30%) near 200 GPa, all at temperatures below 2000 kelvin.

Toroidal diamond anvil cell for detailed measurements under extreme static pressures.

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.

High-Pressure Synthesized Lithium Pentazolate Compound Metastable under Ambient Conditions.

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 around 45 GPa along with its recovery under ambient conditions.

Pressure-induced chemical reactions in the N(H) compound: from the N and H species to ammonia and back down into hydrazine.

Theory predicts a very rich high pressure chemistry of hydronitrogens with the existence of many NH compounds. The stability of these phases under pressure is being investigated by the compression of N-H mixtures of various compositions. A previous study had disclosed a eutectic-type N-H phase diagram with two stoichiometric van der Waals compounds: (N)(H) and N(H).

Melting Curve and Liquid Structure of Nitrogen Probed by X-ray Diffraction to 120 GPa.

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.

Synthesis of FeH: A layered structure with atomic hydrogen slabs.

High pressure promotes the formation of polyhydrides with unusually high hydrogen-to-metal ratios. These polyhydrides have complex hydrogenic sublattices. We synthesized iron pentahydride (FeH) by a direct reaction between iron and H above 130 gigapascals in a laser-heated diamond anvil cell.

Probing local and electronic structure in Warm Dense Matter: single pulse synchrotron x-ray absorption spectroscopy on shocked Fe.

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.

Synthesis of lithium polyhydrides above 130 GPa at 300 K.

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.

New iron hydrides under high pressure.

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.

Pressure-induced chemistry in a nitrogen-hydrogen host-guest structure.

New topochemistry in simple molecular systems can be explored at high pressure. Here we examine the binary nitrogen/hydrogen system using Raman spectroscopy, synchrotron X-ray diffraction, synchrotron infrared microspectroscopy and visual observation. We find a eutectic-type binary phase diagram with two stable high-pressure van der Waals compounds, which we identify as (N2)6(H2)7 and N2(H2)2.

(N(2))(6)Ne(7): A high pressure van der Waals insertion compound.

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.