A. F. Goncharov

Quantum and Classical Orientational Ordering in Solid Hydrogen

We present a unified view of orientational ordering in phases I, II, and III of solid hydrogen. Phases II and III are orientationally ordered, but the ordering objects in phase II are angular momenta of rotating molecules, whereas in phase III the molecules order themselves. This concept provides a quantitative explanation for the vibron softening, libron and roton spectra, and the increase of the vibron effective charge in phase III, as well as a framework for understanding the topology of the phase diagram and ortho-para state at high pressure. The effective charge and the infrared and Raman vibron frequency shifts are all linear in the order parameter in phase III.

Evidence for ice VI as an inclusion in cuboid diamonds from high P-T near infrared spectroscopy

Abstract Near infrared absorption (NIR) spectra of natural morphologically cubic polycrystalline diamonds (cuboid) were obtained at room temperature, and the stretching plus bending combination band of molecular water was observed. The spectrum consisted of the main band at 5180 cm-1 due to liquid water and a shoulder at 5000 cm-1. The 5000 cm-1 band suggests the presence of a phase with stronger hydrogen bonding in inclusions in the diamond. This shoulder absorption decreased on heating to 120°C. The combination band of H2O at high pressure and temperature was measured using a resistively heated diamond cell and the pressure dependence of the peak position was obtained. Comparison with the present experimental results indicates that the spectral changes induced by heating of the cuboid corresponded to melting of a high-pressure form of ice, and the shoulder absorption at 5000 cm-1 arises from ice VI at 1.9 GPa. On the other hand, the liquid water, a main component of the fluid inclusions in the cuboid, was not under high pressure judging from the frequency of the combination band. This contrast might relate to the texture of the cuboid diamond. The spectral observation enables us to estimate the residual pressure of mantle fluid encapsulated in these diamonds. The diamond-cell data also provide high-P-T NIR fingerprint spectra that could be useful for identifying H2O phases and confining pressures in other samples.

Anomalous optical and electronic properties of dense sodium

Synchrotron infrared spectroscopy on sodium shows a transition from a high reflectivity, nearly free-electron metal to a low-reflectivity, poor metal in an orthorhombic phase at 118 GPa. Optical spectra calculated within density functional theory (DFT) agree with the experimental measurements and predict a gap opening in the orthorhombic phase at compression beyond its stability field, a state that would be experimentally attainable by appropriate choice of pressure-temperature path. We show that a transition to an incommensurate phase at 125 GPa results in a partial recovery of good metallic character up to 180 GPa, demonstrating the strong relationship between structure and electronic properties in sodium.

HIGH-PRESSURE SYNCHROTRON INFRARED SPECTROSCOPY AT THE NATIONAL SYNCHROTRON LIGHT SOURCE

SummaryWe describe a synchrotron infrared facility for high-pressure spectroscopy and microspectroscopy at the National Synchrotron Light Source (NSLS). Located at beamline U2B on the VUV ring of the NSLS, the facility utilizes a commercial FT-IR together with custom-built microscope optics designed for a variety of diamond anvil cell experiments, including low- and high-temperature studies. The system contains an integrated laser optical/grating spectrometer for concurrent optical experiments. The facility has been used to characterize a growing number of materials to ultrahigh pressure and has been instrumental in the identification of new high-pressure phenomena. Experiments on dense hydrogen to > 200 GPa have led to the discovery of numerous unexpected properties of this fundamental system. The theoretically predicted molecular-atomic transition of H2O ice to the symmetric hydrogen-bonded structure has been identified, and new classes of high-density clathrates and molecular compounds have been characterized. Experiments on natural and synthetic mineral samples have been performed to study hydrogen speciation, phase transformations, and microscopic inclusions in multiphase assemblages. Detailed information on the behavior of new materials, including novel high-pressure glasses and ceramics, has also been obtained.

Vibrational dynamics of solid molecular nitrogen to megabar pressures

We report the results of Raman and synchrotron infrared absorption measurements of several molecular phases of solid nitrogen to pressures above 100 GPa (300 K). Low-temperature vibrational spectra to 45 GPa are also presented. The changes in Raman and infrared spectra at 60 GPa and 300 K are interpreted as arising from the e→ζ transition reported at low temperature. The character of splitting of the Raman vibron ν2 observed at 25 GPa and low temperatures differs from that previously reported, a difference that we ascribe to different experimental procedures employed and metastability of the low-temperature phase.

Sound velocities of hexagonal close-packed H2and He under pressure

Bulk, shear, and compressional aggregate sound velocities of hydrogen and helium in the close- packed hexagonal structure are calculated over a wide pressure range using two complementary approaches: semi-empirical lattice dynamics based on the many-body intermolecular potentials and density-functional theory in the generalized gradient approximation. The sound velocities are used to calculate pressure dependence of the Debye temperature. The comparison between experiment and first-principle and semi-empirical calculations provide constraints on the density dependence of intermolecular interactions in the zero-temperature limit.