Melting and High P-T Transitions of Hydrogen up to 300 GPa.

High P-T Raman spectra of hydrogen in the vibron and lattice mode regions were measured up to 300 GPa and 900 K using externally heated diamond anvil cell techniques. A new melting line determined from the disappearance of lattice mode excitations was measured directly for the first time above 140 GPa. The results differ from theoretical predictions and extrapolations from lower pressure melting relations.

New Raman measurements for HO ice VII in the range of 300 cm to 4000 cm at pressures up to 120 GPa.

Raman spectroscopic measurements for HO ice VII have been conducted to 120 GPa at 300 K in the spectroscopic range of 300-4000 cm. Both moissanite and diamond anvils were used for the experiments. This overcomes the problems of overlapping spectra between the diamond anvil and sample, which had prevented the observation of the stretching modes at pressures higher than ∼23 GPa in all previous measurements.

Raman measurements of phase transitions in dense solid hydrogen and deuterium to 325 GPa.

Raman spectroscopy of dense hydrogen and deuterium performed to 325 GPa at 300 K reveals previously unidentified transitions. Detailed analysis of the spectra from multiple experimental runs, together with comparison with previous infrared and Raman measurements, provides information on structural modifications of hydrogen as a function of density through the I-III-IV transition sequence, beginning near 200 GPa at 300 K. The data suggest that the transition sequence at these temperatures proceeds by formation of disordered stacking of molecular and distorted layers.

Carbon precipitation from heavy hydrocarbon fluid in deep planetary interiors.

The phase diagram of the carbon-hydrogen system is of great importance to planetary sciences, as hydrocarbons comprise a significant part of icy giant planets and are involved in reduced carbon-oxygen-hydrogen fluid in the deep Earth. Here we use resistively- and laser-heated diamond anvil cells to measure methane melting and chemical reactivity up to 80 GPa and 2,000 K. We show that methane melts congruently below 40 GPa.

High-pressure measurements of hydrogen phase IV using synchrotron infrared spectroscopy.

Phase IV of dense solid hydrogen has been identified by its infrared spectrum using high-pressure synchrotron radiation techniques. The spectrum exhibits a sharp vibron band at higher frequency and lower intensity than that for phase III, indicating the stability of molecular H(2) with decreased intermolecular interactions and charge transfer between molecules. A low-frequency vibron having a strong negative pressure shift indicative of strongly interacting molecules is also observed.

Synchrotron infrared measurements of dense hydrogen to 360 GPa.

Diamond-anvil-cell techniques have been developed to confine and measure hydrogen samples under static conditions to pressures above 300 GPa from 12 to 300 K using synchrotron infrared and optical absorption techniques. A decreasing absorption threshold in the visible spectrum is observed, but the material remains transparent at photon energies down to 0.1 eV at pressures to 360 GPa over a broad temperature range.

Compressional, temporal, and compositional behavior of H2-O2 compound formed by high pressure x-ray irradiation.

X-ray irradiation was found to convert H(2)O at pressures above 2 GPa into a novel molecular H(2)-O(2) compound. We used optical Raman spectroscopy to explore the behavior of x-ray irradiated H(2)O samples as a function of pressure, time, and composition. The compound was found to be stable over a period of two years, as long as high pressure conditions (>2 GPa) were maintained.

In situ high-pressure x-ray diffraction study of H2O ice VII.

Ice VII was examined over the entire range of its pressure stability by a suite of x-ray diffraction techniques in order to understand a number of unexplained characteristics of its high-pressure behavior. Axial and radial polycrystalline (diamond anvil cell) x-ray diffraction measurements reveal a splitting of diffraction lines accompanied by changes in sample texture and elastic anisotropy. In situ laser heating of polycrystalline samples resulted in the sharpening of diffraction peaks due to release of nonhydrostatic stresses but did not remove the splitting.

Optical study of H2O ice to 120 GPa: dielectric function, molecular polarizability, and equation of state.

The refractive index of H2O ice has been measured to 120 GPa at room temperature using reflectivity methods. The refractive index increases significantly with pressure on initial compression and exhibits small changes with pressure at previously identified phase transitions. Pressure dependencies of the molecular polarizability show changing slopes in different pressure regions.

High pressure studies on silane to 210 GPa at 300 K: optical evidence of an insulator-semiconductor transition.

Silane (SiH(4)) has been studied in a diamond anvil cell from 7-210 GPa by using optical reflection and absorption techniques at 300 K. The reflectivity and transmission measurements showed a dramatic change in the neighbourhood of 100 GPa. On the basis of reflectivity and absorption experimental data, the pressure dependence of the refractive index (n) of solid SiH(4) was derived, which was then used to determine the ratio of the molar refraction (R) to the molar volume (V).