Etude des propriétés de l'oxygène sous haute pression et haute température

PARIS-BIUSJ-Thèses (751052125) / SudocCentre Technique Livre Ens. Sup. (774682301) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

X-ray crystallography of simple molecular solids up to megabar pressures: application to solid oxygen and carbon dioxide

International audienceA review of recently developed methods to study the structural properties of simple molecular solids under pressures up to the megabar range using synchrotron X-ray diffraction techniques is presented. This includes the growth of single-crystals at high pressure and temperature conditions and the use of He as pressuretransmitting medium. The application of these methods to solid O2 and CO2 is then presented as illustrative examples, showing how they enabled to solve long-standing debates on the structure of the high-pressure crystalline phases of these compounds

Strategies for in situ laser heating in the diamond anvil cell at an X-ray diffraction beamline

An overview of several innovations regarding in situ laser-heating techniques in the diamond anvil cell at the high-pressure beamline ID27 of the European Synchrotron Radiation Facility is presented. Pyrometry measurements have been adapted to allow simultaneous double-sided temperature measurements with the installation of two additional online laser systems: a CO2 and a pulsed Nd:YAG laser system. This reiteration of laser-heating advancements at ID27 is designed to pave the way for a new generation of state-of-the-art experiments that demand the need for synchrotron diffraction techniques. Experimental examples are provided for each major development. The capabilities of the double pyrometer have been tested with the Nd:YAG continuous-wave lasers but also in a time-resolved configuration using the nanosecond-pulsed Nd:YAG laser on a Fe sample up to 180 GPa and 2900 K. The combination of time-resolved X-ray diffraction with in situ CO2 laser heating is shown with the crystallization of a high-pressure phase of the naturally found pyrite mineral MnS2 (11 GPa, 1100–1650 K)

Liquid–liquid transition and critical point in sulfur

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

Liquid hydrogen structure factor to 5 GPa and evidence of a crossover between two density evolutions

International audienceThe 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 harmoni

Structure of liquid ammonia at high pressures and temperatures

International audienceThe structure of liquid ammonia (NH3) 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 mol (Q), pair distribution function (PDF) g mol (r), and the density of NH3. There is an excellent agreement between present S mol (Q) and g mol (r) at our lowest density and those reported in reference neutron experiments. Our densities agree better with the equation of state of Tillner-Roth et al. [1] than with more recent equation of state (EoS) models. The experimental structure factor and PDF are well reproduced by AIMD simulations using either the BLYP or the PBE exchange-correlation functional. The shapes of S mol (Q) and g 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 ∼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

Evidence and stability field of fcc superionic water ice using static compression

International audienceStructural 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

Evidence and stability field of fcc superionic water ice using static compression

International audienceStructural 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

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

International audienceNew 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 synthesi