Crossover from gas-like to liquid-like molecular diffusion in a simple supercritical fluid.

According to textbooks, no physical observable can be discerned allowing to distinguish a liquid from a gas beyond the critical point. Yet, several proposals have been put forward challenging this view and various transition boundaries between a gas-like and a liquid-like behaviour, including the so-called Widom and Frenkel lines, and percolation line, have been suggested to delineate the supercritical state space. Here we report observation of a crossover from gas-like (Gaussian) to liquid-like (Lorentzian) self-dynamic structure factor by incoherent quasi-elastic neutron scattering measurements on supercritical fluid methane as a function of pressure, along the 200 K isotherm.

Synthesis and superconductivity in yttrium-cerium hydrides at high pressures.

Further increasing the critical temperature and/or decreasing the stabilized pressure are the general hopes for the hydride superconductors. Inspired by the low stabilized pressure associated with Ce 4f electrons in superconducting cerium superhydride and the high critical temperature in yttrium superhydride, we carry out seven independent runs to synthesize yttrium-cerium alloy hydrides. The synthetic process is examined by the Raman scattering and X-ray diffraction measurements.

Synthesis of superconducting phase of LaCeHat high pressures.

Clathrate hydrideFm3-m-LaHhas been proven as the most extraordinary superconductor with the critical temperatureabove 250 K upon compression of hundreds of GPa in recent years. A general hope is to reduce the stabilization pressure and maintain the highvalue of the specific phase in LaH. However, strong structural instability distortsFm3-mstructure and leads to a rapid decrease ofat low pressures.

Compression rate of dynamic diamond anvil cells from room temperature to 10 K.

There is an ever increasing interest in studying dynamic-pressure dependent phenomena utilizing dynamic Diamond Anvil Cells (dDACs), devices capable of a highly controlled rate of compression. Here, we characterize and compare the compression rate of dDACs in which the compression is actuated via three different methods: (1) stepper motor (S-dDAC), (2) gas membrane (M-dDAC), and (3) piezoactuator (P-dDAC). The compression rates of these different types of dDAC were determined solely on millisecond time-resolved R-line fluorescence of a ruby sphere located within the sample chamber.

Pressure-Induced Synthesis and Properties of an HS-HSe-H Molecular Alloy.

The chalcogens are known to react with one another to form interchalcogens, which exhibit a diverse range of bonding and conductive behavior due to the difference in electronegativity between the group members. Through a series of high-pressure diamond anvil experiments combined with density functional theory calculations, we report the synthesis of an S-Se hydride. At pressures above 4 GPa we observe the formation of a single solid composed of both HSe and HS molecular units.

Pressure-induced amorphization and existence of molecular and polymeric amorphous forms in dense SO.

We report here the pressure-induced amorphization and reversible structural transformation between two amorphous forms of SO: molecular amorphous and polymeric amorphous, with the transition found at 26 GPa over a broad temperature regime, 77 K to 300 K. The transformation was observed by both Raman spectroscopy and X-ray diffraction in a diamond anvil cell. The results were corroborated by ab initio molecular dynamics simulations, where both forward and reverse transitions were detected, opening a window to detailed analysis of the respective local structures.

Simple-to-Complex Transformation in Liquid Rubidium.

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.

Synthesis and Raman spectroscopy of a layered SiS phase at high pressures.

Dichalcogenides are known to exhibit layered solid phases, at ambient and high pressures, where 2D layers of chemically bonded formula units are held together by van der Waals forces. These materials are of great interest for solid-state sciences and technology, along with other 2D systems such as graphene and phosphorene. SiS is an archetypal model system of the most fundamental interest within this ensemble.

Intermolecular Interactions in Highly Disordered, Confined Dense N.

Molecular nitrogen is a benchmark system for condensed matter and, in particular, for looking at universal properties of strongly confined dense systems. We conducted Raman and X-ray diffraction measurements on a dense and disordered form of molecular nitrogen subnanoconfined in a noncatalytic pure SiO zeolite under pressure, up to 50 GPa. In this form, N-N interactions and, consequently, distances are found to be very close to those of bulk N and intramolecular interactions progressively weaken upon increasing pressure.

Dynamical Crossover in Hot Dense Water: The Hydrogen Bond Role.

We investigate the terahertz dynamics of liquid H2O as a function of pressure along the 450 K isotherm, by coupled quasielastic neutron scattering and inelastic X-ray scattering experiments. The pressure dependence of the single-molecule dynamics is anomalous in terms of both microscopic translation and rotation. In particular, the Stokes-Einstein-Debye equations are shown to be violated in hot water compressed to the GPa regime.

Carbon enters silica forming a cristobalite-type CO2-SiO2 solid solution.

Extreme conditions permit unique materials to be synthesized and can significantly update our view of the periodic table. In the case of group IV elements, carbon was always considered to be distinct with respect to its heavier homologues in forming oxides. Here we report the synthesis of a crystalline CO2-SiO2 solid solution by reacting carbon dioxide and silica in a laser-heated diamond anvil cell (P = 16-22 GPa, T>4,000 K), showing that carbon enters silica.

Dynamical crossover at the liquid-liquid transformation of a compressed molten alkali metal.

Density-driven phase transformations are a known phenomenon in liquids. Pressure-driven transitions from an open low-density to a higher-density close-packed structure were observed for a number of systems. Here, we show a less intuitive, inverse behavior.

High-pressure synthesis of a polyethylene/zeolite nano-composite material.

Meso/micro-porous solids, such as zeolites, are complex materials used in an impressive range of applications. Here we photo-polymerized ethylene using non-catalytic high-pressure techniques at 0.5-1.

Partially collapsed cristobalite structure in the non molecular phase V in CO2.

Non molecular CO(2) has been an important subject of study in high pressure physics and chemistry for the past decade opening up a unique area of carbon chemistry. The phase diagram of CO(2) includes several non molecular phases above 30 GPa. Among these, the first discovered was CO(2)-V which appeared silica-like.

Equation of state and anharmonicity of carbon dioxide phase I up to 12 GPa and 800 K.

We present an extended investigation of phase I of carbon dioxide by x-ray diffraction and spectroscopic techniques at simultaneous high pressure and high temperature, up to 12 GPa and 800 K. Based on the present and literature data, we show that a Mie-Grüneisen-Debye model reproduces within experimental uncertainties the equation of state of CO(2) over the entire range of stability of phase I. Using infrared and Raman spectroscopy, we have determined the frequencies of the zone-center lattice modes as a function of pressure and temperature.

Triggering dynamics of the high-pressure benzene amorphization.

Success in designing and tailoring solid-state reactions depends on the knowledge of the mechanisms regulating the reactivity at the microscopic level. In spite of several attempts to rationalize the reactivity of crystals, the question of the existence of a critical distance for a reaction to occur remains unsolved. In this framework, the role of lattice phonons, which continuously tune the relative distance and orientation of the molecules, is still not fully understood.

Amorphous silica-like carbon dioxide.

Among the group IV elements, only carbon forms stable double bonds with oxygen at ambient conditions. At variance with silica and germania, the non-molecular single-bonded crystalline form of carbon dioxide, phase V, only exists at high pressure. The amorphous forms of silica (a-SiO2) and germania (a-GeO2) are well known at ambient conditions; however, the amorphous, non-molecular form of CO2 has so far been described only as a result of first-principles simulations.

Linear carbon dioxide in the high-pressure high-temperature crystalline phase IV.

High-temperature IR absorption spectra of solid CO2 in phases II and IV were measured in a resistive heated diamond anvil cell up to 30 GPa. The spectral structures of the bending mode, observed in high quality thin crystalline samples, and of the IR lattice phonons, measured for the first time between 80 and 640 K, are discussed using group theory arguments. According to this analysis the claimed bent molecular geometry of CO2 in phase IV can be unambiguously ruled out.