Structure and density of silicon carbide to 1.5 TPa and implications for extrasolar planets.

There has been considerable recent interest in the high-pressure behavior of silicon carbide, a potential major constituent of carbon-rich exoplanets. In this work, the atomic-level structure of SiC was determined through in situ X-ray diffraction under laser-driven ramp compression up to 1.5 TPa; stresses more than seven times greater than previous static and shock data. Here we show that the B1-type structure persists over this stress range and we have constrained its equation of state (EOS). Using this data we have determined the first experimentally based mass-radius curves for a hypothetical pure SiC planet. Interior structure models are constructed for planets consisting of a SiC-rich mantle and iron-rich core. Carbide planets are found to be ~10% less dense than corresponding terrestrial planets.

High-pressure phase transition in Y3Fe5O12.

Yttrium iron garnet (YIG, Y3Fe5O12) was examined up to 74 GPa and 1800 K using synchrotron x-ray diffraction in a diamond anvil cell. At room temperature, YIG remained in the garnet phase until abrupt amorphization occurred at 51 GPa, consistent with earlier studies. Upon laser heating up to 1800 K, the material transformed to a single-phase orthorhombic GdFeO3-type perovskite of composition (Y(0.75)Fe(0.25))FeO3. No evidence of decomposition of the sample was observed. Both the room-temperature amorphization and high-temperature transformation to the perovskite structure are consistent with the behaviour of other rare earth oxide garnets. The perovskite sample was compressed between 28-74 GPa with annealing to 1450-1650 K every 3-5 GPa. Between 46 and 50 GPa, a 6.8% volume discontinuity was observed without any accompanying change in the number or intensity of diffraction peaks. This is indicative of a high-spin to low-spin electronic transition in Fe(3+), likely in the octahedrally coordinated B-site of the perovskite. The volume change of the inferred spin transition is consistent with those observed in other rare earth ferric iron perovskites at high pressures.

Ramp compression of diamond to five terapascals.

The recent discovery of more than a thousand planets outside our Solar System, together with the significant push to achieve inertially confined fusion in the laboratory, has prompted a renewed interest in how dense matter behaves at millions to billions of atmospheres of pressure. The theoretical description of such electron-degenerate matter has matured since the early quantum statistical model of Thomas and Fermi, and now suggests that new complexities can emerge at pressures where core electrons (not only valence electrons) influence the structure and bonding of matter. Recent developments in shock-free dynamic (ramp) compression now allow laboratory access to this dense matter regime. Here we describe ramp-compression measurements for diamond, achieving 3.7-fold compression at a peak pressure of 5 terapascals (equivalent to 50 million atmospheres). These equation-of-state data can now be compared to first-principles density functional calculations and theories long used to describe matter present in the interiors of giant planets, in stars, and in inertial-confinement fusion experiments. Our data also provide new constraints on mass-radius relationships for carbon-rich planets.

Sound velocity and elasticity of tetragonal lysozyme crystals by Brillouin spectroscopy.

Quasilongitudinal sound velocities and the second-order elastic moduli of tetragonal hen egg-white lysozyme crystals were determined as a function of relative humidity (RH) by Brillouin scattering. In hydrated crystals the measured sound velocities in the [110] plane vary between 2.12 +/- 0.03 km/s along the [001] direction and 2.31 +/- 0.08 km/s along the [110] direction. Dehydration from 98% to 67% RH increases the sound velocities and decreases the velocity anisotropy in (110) from 8.2% to 2.0%. A discontinuity in velocity and an inversion of the anisotropy is observed with increasing dehydration providing support for the existence of a structural transition below 88% RH. Brillouin linewidths can be described by a mechanical model in which the phonon is coupled to a relaxation mode of hydration water with a single relaxation time of 55 +/- 5 ps. At equilibrium hydration (98% RH) the longitudinal moduli C(11) + C(12) + 2C(66) = 12.81 +/- 0.08 GPa, C(11) = 5.49 +/- 0.03 GPa, and C(33) = 5.48 +/- 0.05 GPa were directly determined. Inversion of the measured sound velocities in the [110] plane constrains the combination C(44) + (1/2)C(13) to 2.99 +/- 0.05 GPa. Further constraints on the elastic tensor are obtained by combining the Brillouin quasilongitudinal results with axial compressibilities determined from high-pressure x-ray diffraction. We constrain the adiabatic bulk modulus to the range 2.7-5.3 GPa.

Stability and structure of MgSiO3 perovskite to 2300-kilometer depth in Earth's mantle.

Unexplained features have been observed seismically near the middle (approximately 1700-kilometer depth) and bottom of the Earth's lower mantle, and these could have important implications for the dynamics and evolution of the planet. (Mg,Fe)SiO3 perovskite is expected to be the dominant mineral in the deep mantle, but experimental results are discrepant regarding its stability and structure. Here we report in situ x-ray diffraction observations of (Mg,Fe)SiO3 perovskite at conditions (50 to 106 gigapascals, 1600 to 2400 kelvin) close to a mantle geotherm from three different starting materials, (Mg0.9Fe0.1)SiO enstatite, MgSiO3 glass, and an MgO+SiO2 mixture. Our results confirm the stability of (Mg,Fe)SiO3 perovskite to at least 2300-kilometer depth in the mantle. However, diffraction patterns above 83 gigapascals and 1700 kelvin (1900-kilometer depth) cannot presently rule out a possible transformation from Pbnm perovskite to one of three other possible perovskite structures with space group P2(1)/m, Pmmn, or P4(2)/nmc.

The post-spinel transformation in Mg2SiO4 and its relation to the 660-km seismic discontinuity.

The 660-km seismic discontinuity in the Earth's mantle has long been identified with the transformation of (Mg,Fe)2SiO4 from gamma-spinel (ringwoodite) to (Mg,Fe)SiO3-perovskite and (Mg,Fe)O-magnesiowüstite. This has been based on experimental studies of materials quenched from high pressure and temperature, which have shown that the transformation is consistent with the seismically observed sharpness and the depth of the discontinuity at expected mantle temperatures. But the first in situ examination of this phase transformation in Mg2SiO4 using a multi-anvil press indicated that the transformation occurs at a pressure about 2 GPa lower than previously thought (equivalent to approximately 600 km depth) and hence that it may not be associated with the 660-km discontinuity. Here we report the results of an in situ study of Mg2SiO4 at pressures of 20-36 GPa using a combination of double-sided laser-heating and synchrotron X-ray diffraction in a diamond-anvil cell. The phase transformation from gamma-Mg2SiO4 to MgSiO3-perovskite and MgO (periclase) is readily observed in both the forward and reverse directions. In contrast to the in situ multi-anvil-press study, we find that the pressure and temperature of the post-spinel transformation in Mg2SiO4 is consistent with seismic observations for the 660-km discontinuity.

No end in sight: the nurse's role in ensuring a holistic approach in the care of a patient undergoing a drainage implant for end-stage glaucoma.

This article addresses issues that relate to the surgical care of patients in imminent danger of permanently losing their eyesight as a result of advanced glaucoma. An overview of the stages of glaucoma and concurrent treatment is reviewed. Close attention is given to the psychosocial and physiologic ramifications of this end-stage surgical procedure. The nurse's role in assisting patients through this life crisis is essential to the patient's well being, lifestyle changes, and quality of life.

Sound velocities in dense hydrogen and the interior of jupiter.

Sound velocities in fluid and crystalline hydrogen were measured under pressure to 24 gigapascals by Brillouin spectroscopy in the diamond anvil cell. The results provide constraints on the intermolecular interactions of dense hydrogen and are used to construct an intermolecular potential consistent with all available data. Fluid perturbation theory calculations with the potential indicate that sound velocities in hydrogen at conditions of the molecular layer of the Jovian planets are lower than previously believed. Jovian models consistent with the present results remain discrepant with recent free oscillation spectra of the planet by 15 percent. The effect of changing interior temperatures, the metallic phase transition depth, and the fraction of high atomic number material on Jovian oscillation frequencies is also investigated with the Brillouin equation of state. The present data place strong constraints on sound velocities in the Jovian molecular layer and provide an improved basis for interpreting possible Jovian oscillations.