The effect of deuteration on the optical spectra of compressed methane

The in situ high pressure Raman spectrum of CD_{4} was found to be subtly different from its’ hydrogenous analog, CH_{4}. High quality data were obtained for the first time for pressures between 12 and 20 GPa during both fast and slow compression. Similarly to CH_{4} in phase B, CD_{4} does exhibit peak splitting in the ν_{1} (symmetric stretch) and ν_{3} (antisymmetric stretch) modes, but having the emergent shoulders present on the high-frequency side of the peaks rather than the low-frequency one as in the case of CH_{4}. The general aspect of the Raman spectrum was found to be very different from that of CH_{4}, with modes ν_{1} and ν_{3} having comparable intensities and the latter being sharper and better defined, in stark contrast to how it appears in CH_{4}

When immiscible becomes miscible-Methane in water at high pressures

At low pressures, the solubility of gases in liquids is governed by Henry’s law, which states that the saturated solubility of a gas in a liquid is proportional to the partial pressure of the gas. As the pressure increases, most gases depart from this ideal behavior in a sublinear fashion, leveling off at pressures in the 1- to 5-kbar (0.1 to 0.5 GPa) range with solubilities of less than 1 mole percent (mol %). This contrasts strikingly with the well-known marked increase in solubility of simple gases in water at high temperature associated with the critical point (647 K and 212 bar). The solubility of the smallest hydrocarbon, the simple gas methane, in water under a range of pressure and temperature is of widespread importance, because it is a paradigmatic hydrophobe and occurs widely in terrestrial and extraterrestrial geology. We report measurements up to 3.5 GPa of the pressure dependence of the solubility of methane in water at 100°C—well below the latter’s critical temperature. Our results reveal a marked increase in solubility between 1 and 2 GPa, leading to a state above 2 GPa where the maximum solubility of methane in water exceeds 35 mol %

Krypton and the fundamental flaw of the Lennard-Jones Potential

We have performed a series of neutron scattering experiments on supercritical krypton. Our data and analysis allow us to characterize the Frenkel line crossover in this model monatomic fluid. The data from our measurements was analyzed using Empirical Potential Structure Refinement to determine the short- and medium-range structure of the fluids. We find evidence for several shells of neighbors which form approximately concentric rings of density about each atom. The ratio of second to first shell radius is significantly larger than in any crystal structure. Modeling krypton using a Lennard-Jones potential is shown to give significant errors, notably that the liquid is overstructured. The true potential appears to be longer ranged and with a softer core than the 6–12 powerlaws permit

Transition from gas-like to liquid-like behavior in supercritical N2

We have studied in detail the transition from gas-like to rigid liquidlike behavior in supercritical N2 at 300 K (2.4 TC). Our study combines neutron diffraction and Raman spectroscopy with ab initio molecular dynamics simulations. We observe a narrow transition from gas-like to rigid liquid-like behavior at ca. 150 MPa, which we associate with the Frenkel line. Our findings allow us to reliably characterize the Frenkel line using both diffraction and spectroscopy methods, backed up by simulation, for the same substance. We clearly lay out what parameters change, and what parameters do not change, when the Frenkel line is crossed

Radiation attenuation by single-crystal diamond windows

As artificial diamond becomes more cost effective it is likely to see increasing use as a window for sample environment equipment used in diffraction experiments. Such windows are particularly useful as they exhibit exceptional mechanical properties in addition to being highly transparent to both X-ray and neutron radiation. A key application is in high-pressure studies, where diamond anvil cells (DACs) are used to access extreme sample conditions. However, despite their utility, an important consideration when using single-crystal diamond windows is their interaction with the incident beam. In particular, the Bragg condition will be satisfied for specific angles and wavelengths, leading to the appearance of diamond Bragg spots on the diffraction detectors but also, unavoidably, to loss of transmitted intensity of the beam that interacts with the sample. This effect can be particularly significant for energy-dispersive measurements, for example, in time-of-flight neutron diffraction work using DACs. This article presents a semi-empirical approach that can be used to correct for this effect, which is a prerequisite for the accurate determination of diffraction intensities

X-ray free electron laser heating of water and gold at high static pressure

The study of water at high pressure and temperature is essential for understanding planetary interiors but is hampered by the high reactivity of water at extreme conditions. Here, indirect X-ray laser heating of water in a diamond anvil cell is realized via a gold absorber, showing no evidence of reactivity

Structural markers of the Frenkel line in the proximity of Widom lines

We have performed a neutron scattering experiment on supercritical uid nitrogen at 160 K (1.27 TC) over a wide pressure range (7.8 MPa / 0.260 g/ml to 125 MPa / 0.805 g/ml). This has enabled us to study the process by which nitrogen changes from a fluid that exhibits gas-like behaviour to one that exhibits rigid liquid-like behaviour at a temperature close to, but above, the critical temperature by crossing the Widom lines followed by the Frenkel line on pressure (density) increase. We find that the Frenkel line transition is indicated by a transition to a regime of rigid liquid-like behaviour in which the coordination number remains constant within error, in agreement with our previous work at 300 K. The Frenkel line transition takes place at approximately the same density at 160 K and 300 K. The data do not conclusively show an additional transition at the location of the known Widom lines. We find that behaviour remains gas-like until the Frenkel line is crossed and our data support the hypothesis that the Widom line transitions are density increase-driven