Silicon coordination and speciation changes in a silicate liquid at high pressures

Coordination and local geometry around Si cations in silicate liquids are of primary importance in controlling the chemical and physical properties of magmas. Pressure-induced changes from fourfold to sixfold coordination of Si in silicate glass samples quenched from liquids has been detected with (29)Si magic-angle spinning nuclear magnetic resonance spectrometry. Samples of Na(2)Si(2)O(5) glass quenched from 8 gigapascals and 1500 degrees C contained about 1.5 percent octahedral Si, which was demonstrably part of a homogeneous, amorphous phase. The dominant tetrahedral Si speciation in these glasses became disproportionated to a more random distribution of bridging and nonbridging oxygens with increasing pressure.     

Stress gradient in solid methane at high pressures

The magnitude of the axial pressure gradient at 77 degrees K (11 percent per centimeter) in solid methane compressed along one axis for applied pressures up to 10 kilobars was determined by comparing the electrical resistance of a pair of doped tellurium pressure gages with that of a set of single-crystal gages made of high-purity bismuth. The existence of the pressure gradient revealed the causes of deformation in metal tensile specimens embedded in solid methane and cycled to high pressure.     

Densities of liquid silicates at high pressures

Densities of molten silicates at high pressures (up to approximately 230 kilobars) have been measured for the first time with shock-wave techniques. For a model basaltic composition (36 mole percent anorthite and 64 mole percent diopside), a bulk modulus K(s), of approximately 230 kilobars and a pressure derivative (dK(s)/dP) of approximately 4 were derived. Some implications of these results are as follows: (i) basic to ultrabasic melts become denser than olivine-and pyroxene-rich host mantle at pressures of 60 to 100 kilobars; (ii) there is a maximum depth from which basaltic melt can rise within terrestrial planetary interiors; (iii) the slopes of silicate solidi [(dT(m)/dP), where T(m) is the temperature] may become less steep at high pressures; and (iv) enriched mantle reservoirs may have developed by downward segregation of melt early in Earth history.     

Melting of tin teliuride at high pressures

The melting curve of tintelluride (Sn(0.496)Te(0.504) was determined by differential thermal analysis at pressures between 5 and 40 kilobars. Near 844 degrees +/-4 degrees C and 12.0+/-1.0 kb, the liquid and two solid polymorphscoexist.     

Phase transformations in carbon fullerenes at high shock pressures

C(60) powders were shock-compressed quasi-isentropically and quenched from pressures in the range 10 to 110 GPa (0.1 to 1.1 Mbar). Recovered specimens were analyzed by Raman spectroscopy and optical microscopy. C(60) fullerenes are stable into the 13- to 17-GPa pressure range. The onset of a fast ( approximately 0.5 micros) reconstructive transformation to graphite occurs near 17 GPa. The graphite recovered from 27 GPa and about 600 degrees C is relatively well ordered with crystal planar domain size of about 100 A. Above 50 GPa a continuous transformation to an amorphous state is observed in recovered specimens. The fast transformation to graphite is proposed to occur by pi-electron rehybridization which initiates breakup of the ball structure and formation of the graphite structure at high density.     

Animals at very high pressures of helium and neon

At pressures up to 125 atmospheres, helium failed to anesthetize mice; at slightly higher pressures (135 to 145 atmospheres) it proved lethal. With Italian newts (Triturus italicus), whose sensitivity to anesthesia by nitrogen is similar to that of mice, responsiveness was lost at pressures between 165 and 245 atmospheres, whether the pressure was achieved with helium or neon, or hydrostatically. It was concluded that the anesthetic pressures of helium and neon, for mice and newts, are higher than the tolerable mechanical pressures.     

Rare-Earth elements and high pressures

Praseodymium, under very high pressures, shows a magnetic behavior similar to that of cerium at normal pressure.     

The formation of chondrules at high gas pressures in the solar nebula

High-precision magnesium isotope measurements of whole chondrules from the Allende carbonaceous chondrite meteorite show that some aluminum-rich Allende chondrules formed at or near the time of formation of calcium-aluminum-rich inclusions and that some others formed later and incorporated precursors previously enriched in magnesium-26. Chondrule magnesium-25/magnesium-24 correlates with [magnesium]/[aluminum] and size, the aluminum-rich, smaller chondrules being the most enriched in the heavy isotopes of magnesium. These relations imply that high gas pressures prevailed during chondrule formation in the solar nebula.     

Elastic wave velocities of lunar samples at high pressures and their geophysical implications

Ultrasonic measurement of P and S velocities of Apollo 11 lunar samples 10020, 10057, and 10065 to 5 kilobars pressure at room temperature shows a pronounced increase of velocity (as much as twofold) for the first 2 kilobars. The travel times predicted from the velocity-depth curve of sample 10057 are consistent with the results of the Apollo 12 seismic experiments. At pressures below 200 bars, the samples are highly attenuating; for both P and S waves, the value of Q is about 10.     

Quartzlike carbon dioxide: An optically nonlinear extended solid at high pressures and temperatures

An extended-solid phase, carbon dioxide phase V (CO2-V), was synthesized in a diamond anvil cell by laser heating the molecular orthorhombic phase, carbon dioxide phase III, above 40 gigapascals and 1800 kelvin. This new material can be quenched to ambient temperature above 1 gigapascal. The vibration spectrum of CO2-V is similar to that of the quartz polymorph of silicon dioxide, indicating that it is an extended covalent solid with carbon-oxygen single bonds. This material is also optically nonlinear, generating the second harmonic of a neodymium-yttrium-lithium-fluoride laser at a wavelength of 527 nanometers with a conversion efficiency that is near 0.1 percent.     

Study by synchrotron radiation of the structure of a working catalyst at high temperatures and pressures

A relation among activity, composition, and structure was determined for a working catalyst by means of a stainless-steel reactor cell of novel design that permitted operation at temperatures and pressures similar to those in industrial reactors. Molybdenum K-edge x-ray absorption spectra were used to probe the structural environment of molybdenum in CoMoS/[unknown]-alumina catalysts while hydro-desulfurization of benzothiophene was proceeding at high temperature and pressure. For catalyst samples with different contents of cobalt, radial structure functions obtained from extended x-ray absorption fine structure data presented the same features as those obtained from the spectra of MoS(2)/[unknown]-alumina reference samples. Moreover, Mo-S and Mo-Mo coordination numbers were maximum for the sample with an atomic ratio of Co to (Co + Mo) of 0.33; this sample was also the most active catalyst tested.     

Sound velocities in olivine at Earth mantle pressures

The independent elastic constants of an upper mantle mineral, San Carlos olivine [(Mg(1.8)Fe(0.2))SiO(4)], were measured from 0 to 12.5 gigapascals. Evidence is offered in support of the proposition that the explicit temperature dependence of the bulk modulus is small over the range of temperatures and pressures thought to prevail above the 400-kilometer discontinuity, and thus the data can be extrapolated to estimate the properties of olivine under mantle conditions at a depth of 400 kilometers. In the absence of high-temperature data at high pressures, estimates are made of the properties of olivine under mantle conditions to a depth of 400 kilometers. In contrast with low-pressure laboratory data, the predicted covariance of shear and compressional velocities as a function of temperature nearly matches the seismically estimated value for the lower mantle.     

Noble gas partitioning between metal and silicate under high pressures

Measurements of noble gas (helium, neon, argon, krypton, and xenon) partitioning between silicate melt and iron melt under pressures up to 100 kilobars indicate that the partition coefficients are much less than unity and that they decrease systematically with increasing pressure. The results suggest that the Earth's core contains only negligible amounts of noble gases if core separation took place under equilibrium conditions.     

Orbital reconstruction and covalent bonding at an oxide interface

Orbital reconstructions and covalent bonding must be considered as important factors in the rational design of oxide heterostructures with engineered physical properties. We have investigated the interface between high-temperature superconducting (Y,Ca)Ba(2)Cu3O7 and metallic La(0.67)Ca(0.33)MnO3 by resonant x-ray spectroscopy. A charge of about -0.2 electron is transferred from Mn to Cu ions across the interface and induces a major reconstruction of the orbital occupation and orbital symmetry in the interfacial CuO2 layers. In particular, the Cu d(3z(2)-r(2)) orbital, which is fully occupied and electronically inactive in the bulk, is partially occupied at the interface. Supported by exact-diagonalization calculations, these data indicate the formation of a strong chemical bond between Cu and Mn atoms across the interface. Orbital reconstructions and associated covalent bonding are thus important factors in determining the physical properties of oxide heterostructures.     

Melting of normal hydrogen under high pressures between 20 and 300 kelvins

The melting curve of normal hydrogen has been determined up to 52 kilobars between 20 and 300 Kelvins. The results are in excellent agreement with the modified Simon equation proposed for hydrogen below 19 kilobars, but not with the existing theoretical predictions. The results also provide an independent check on the validity of the ruby high-pressure scale at low temperature.     

Magnetic transitions observed in sulfide minerals at elevated pressures and their geophysical significance

The magnetic behavior of iron in chalcopyrite (CuFeS(2)) and pyrrhotite (Fe(7)S(8)) in the pressure range from 1 atmosphere to 20 kilobars has been studied by M?ssbauer spectroscopy. Both chalcopyrite and pyrrhotite exhibit transitions from magnetically ordered to disordered states over the range from 5 to 16 kilobars. Both transitions, particularly the loss of ferrimagnetism in pyrrhotite, have geophysical consequences.     

Hydrogen-helium mixtures at megabar pressures: implications for jupiter and saturn

Models of Jupiter and Saturn postulate a central rock core surrounded by a fluid mixture of hydrogen and helium. These models suggest that the mixture is undergoing phase separation in Saturn but not Jupiter. State-of-the-art total energy calculations of the enthalpy of mixing for ordered alloys of hydrogen and helium confirm that at least partial phase separation has occurred in Saturn and predict that this process has also begun in Jupiter.     

Hyperbaric oxygen: toxicity to fish at pressures present in their swimbladders

When juvenile Pacific rock-fish, Sebastodes miniatus, are exposed to oxygen tensions equal to those in their swimbladders, they exhibit symptoms characteristic of oxygen poisoning in mammals and ultimately die. Thus their central nervous system appears to be as sensitive to elevated oxygen pressure as that of higher vertebrates, whereas the cells of the gas gland tissue inside the swimbladder must be insensitive to the partial pressure of oxygen which they help to produce.     

Supersaturation of gases in water: absence of cavitation on decompression from high pressures

Very high supersaturations ranging from 140 atmospheres for oxygen to more than 270 atmospheres for helium, were produced without Cavitation of the water at atmospheric pressure. Thus, the lower limits for de novo bubble formation are increased substantially over those previously obtained.     

Electron- and photon-stimulated desorption: probes of structure and bonding at surfaces

Techniques for analyzing the structure and composition of solid surfaces with electron and photon beams often cause radiation damage in samples. Damage-producing processes compete with information-producing events during measurements, and beam damage can be a serious perturbation in quantitative surface analysis. There are, however, substantial benefits of electron- and photonstimulated damage processes for studying molecules adsorbed on surfaces. Direct information about the geometric structure of surface molecules can be obtained from measurements of the angular distributions of ions released by electron- or photon-stimulated desorption. The directions of ion emission are determined by the orientation of the surface bonds that are ruptured by beam irradiation. Moreover, photon-stimulated desorption studies that make use of synchrotron radiation reveal the fundamental electronic excitations that lead to bondbreaking processes at surfaces. These measurements provide new insights into radiation-damage processes in areas as diverse as x-ray optics and semiconductor electronics.