Acoustic properties, elasticity, and equation of state of glycerol under pressure.

We employed high-pressure Brillouin scattering to study the pressure dependencies of acoustic modes of glycerol up to 14 GPa at 300 K. We observed longitudinal acoustic velocities and transverse acoustic velocities for the first time from 5 to 14 GPa. The results allow the determination of a complete set of elastic properties and an accurate determination of the pressure-volume (P-V) equation of state (EOS). EOS parameters, K0 = 14.9 ± 1.8 GPa and K'0 = 5.6 ± 0.5, were determined from fits to the data from ambient pressure to 14 GPa. Direct volume measurements of the P-V EOS are consistent with those determined by Brillouin scattering. A deviation from a Cauchy-like relationship for elastic properties was observed, and the pressure dependencies of the photoelastic constants and relaxation times were documented from 5 to 14 GPa. These results have broad implications for glass-forming liquids, viscoelastic theory, and mode coupling theory.

Density dependence of dynamical heterogeneity in fluid methanol.

Brillouin and Raman scattering experiments on methanol through its glass transition under pressure are reported. The Brillouin scattering data were analyzed using viscoelastic theory and a fit to the Vinet equation of state. The variation in the linewidth of the longitudinal acoustic mode with pressure shows a broad maximum centered around 3 GPa. The pressure evolution of the relaxation time in the GHz range is obtained, and the Raman data are analyzed in terms of the Boson peak and its associated relaxation time in the THz range. The pressure evolution of these two relaxation processes extends previous determinations of relaxations at lower frequency based on dielectric measurements in supercooled methanol. The relaxation processes in glass-forming methanol have now been investigated over a wide frequency range and their evolution followed over a large variation of density.

Acoustic properties of Kel F-800 copolymer up to 85 GPa.

Acoustic properties of the fluorinated copolymer Kel F-800 were determined with Brillouin spectroscopy up to pressures of 85 GPa at 300 K. This research addresses outstanding issues in high-pressure polymer behavior, as to date the acoustic properties and equation of state of any polymer have not been determined above 20 GPa. We observed both longitudinal and transverse modes in all pressure domains, allowing us to calculate the C(11) and C(12) moduli, bulk, shear, and Young's moduli, and the density of Kel F-800 as a function of pressure. We found the behavior of the polymer with respect to all parameters to change drastically with pressure. As a result, we find that the data are best understood when split into two pressure regimes. At low pressures (less than ∼5 GPa), analysis of the room temperature isotherm with a semi-empirical equation of state yielded a zero-pressure bulk modulus K(o) and its derivative K(0) (') of 12.8 ± 0.8 GPa and 9.6 ± 0.7, respectively. The same analysis for the higher pressure data yielded values for K(o) and K(0) (') of 34.9 ± 1.7 GPa and 5.1 ± 0.1, respectively. We discuss this significant difference in behavior with reference to the concept of effective free volume collapse.

Brillouin scattering of H2O ice to megabar pressures.

The sound velocity in polycrystalline ice was measured as a function of pressure at room temperature to 100 GPa, through the phase field of ice VII and crossing the ice X transition, by Brillouin scattering in order to examine the elasticity, compression mechanism, and structural transitions in this pressure range. In particular, we focused on previously proposed phase transitions below 60 GPa. Throughout this pressure range, we find no evidence for anomalous changes in compressibility, and the sound velocities and elastic moduli do not exhibit measurable discontinuous shifts with pressure. Subtle changes in the pressure dependence of the bulk modulus at intermediate pressures can be attributed to high shear stresses at these compressions. The C(11) and C(12) moduli are consistent with previously reported results to 40 GPa and increase monotonically at higher pressures.

Inelastic x-ray scattering of dense solid oxygen: evidence for intermolecular bonding.

The detailing of the intermolecular interactions in dense solid oxygen is essential for an understanding of the rich polymorphism and remarkable properties of this element at high pressure. Synchrotron inelastic x-ray scattering measurements of oxygen K-edge excitations to 38 GPa reveal changes in electronic structure and bonding on compression of the molecular solid. The measurements show that O(2) molecules interact predominantly through the half-filled 1pi(g)* orbital <10 GPa. Enhanced intermolecular interactions develop because of increasing overlap of the 1pi(g)* orbital in the low-pressure phases, leading to electron delocalization and ultimately intermolecular bonding between O(2) molecules at the transition to the epsilon-phase. The epsilon-phase, which consists of (O(2))(4) clusters, displays the bonding characteristics of a closed-shell system. Increasing interactions between (O(2))(4) clusters develop upon compression of the epsilon-phase, and provide a potential mechanism for intercluster bonding in still higher-pressure phases.

Pressure-induced phase transitions in LiNH2.

In situ high-pressure Raman spectroscopy studies on LiNH2 (lithium amide) have been performed at pressures up to 25 GPa. The pressure-induced changes in the Raman spectra of LiNH2 indicates a phase transition that begins at approximately 12 GPa is complete at approximately 14 GPa from ambient-pressure alpha-LiNH2 (tetragonal, I) to a high-pressure phase denoted here as beta-LiNH2. This phase transition is reversible upon decompression with the recovery of the alpha-LiNH2 phase at approximately 8 GPa. The N-H internal stretching modes (nu([NH2]-)) display an increase in frequency with pressure, and a new stretching mode corresponding to high-pressure beta-LiNH2 phase appears at approximately 12.5 GPa. Beyond approximately 14 GPa, the N-H stretching modes settle into two shouldered peaks at lower frequencies. The lattice modes show rich pressure dependence exhibiting multiple splitting and become well-resolved at pressures above approximately 14 GPa. This is indicative of orientational ordering [NH2]- ions in the lattice of the high-pressure beta-LiNH2 phase.

Pressure-induced phase transformations in LiAlH4.

The pressure-induced phase transformations in pure LiAlH4 have been studied using in situ Raman spectroscopy up to 7 GPa. The analyses of Raman spectra reveal a phase transition at approximately 3 GPa from the ambient pressure monoclinic alpha-LiAlH4 phase (P2(1)/c) to a high pressure phase (beta-LiAlH4, reported recently to be monoclinic with space group I4(1)/b) having a distorted [AlH4]- tetrahedron. The Al-H stretching mode softens and shifts dramatically to lower frequencies beyond the phase transformation pressure. The high pressure beta-LiAlH4 phase was pressure quenchable and can be recovered at lower pressures ( approximately 1.2 GPa). The Al-H stretching mode in the quenched state further shifts to lower frequencies, suggesting a weakening of the Al-H bond.

Pressure-induced bonding and compound formation in xenon-hydrogen solids.

Closed electron shell systems, such as hydrogen, nitrogen or group 18 elements, can form weakly bound stoichiometric compounds at high pressures. An understanding of the stability of these van der Waals compounds is lacking, as is information on the nature of their interatomic interactions. We describe the formation of a stable compound in the Xe-H(2) binary system, revealed by a suite of X-ray diffraction and optical spectroscopy measurements. At 4.8 GPa, a unique hydrogen-rich structure forms that can be viewed as a tripled solid hydrogen lattice modulated by layers of xenon, consisting of xenon dimers. Varying the applied pressure tunes the Xe-Xe distances in the solid over a broad range from that of an expanded xenon lattice to the distances observed in metallic xenon at megabar pressures. Infrared and Raman spectra indicate a weakening of the intramolecular covalent bond as well as persistence of semiconducting behaviour in the compound to at least 255 GPa.

High-pressure Raman spectroscopy of tris(hydroxymethyl)aminomethane.

High-pressure Raman spectroscopy has been used to study tris(hydroxymethyl)aminomethane (C(CH(2)OH)(3)NH(2), Tris). Molecules with globular shapes such as Tris have been studied thoroughly as a function of temperature and are of fundamental interest because of the presence of thermal transitions from orientational order to disorder. In contrast, relatively little is known about their high-pressure behavior. Diamond anvil cell techniques were used to generate pressures in Tris samples up to approximately 10 GPa. A phase transition was observed at a pressure of approximately 2 GPa that exhibited relatively slow kinetics and considerable hysteresis, indicative of a first-order transition. The Raman spectrum becomes significantly more complex in the high-pressure phase, indicating increased correlation splitting and significant enhancement in the intensity of some weak, low-pressure phase Raman-active modes.

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. A general molar refractivity analysis of this change in slope reveals features at 60 GPa due to the onset of the ice VII-X transition. Band gap closure in H2O ice is constrained by the dispersion data using a single oscillator dielectric model. Sample thickness measurements obtained from interference patterns yield pressure-volume relations in excellent agreement with those measured by x-ray diffraction.