Sulfur Melting and Polymorphism under Pressure: Outlines of Fields for 12 Crystalline Phases

The polymorphism of sulfur has been investigated by static and dynamic methods up to 500 degrees C at 35 kilobars and up to 350 degrees C at 100 kilobars. The melting curve of sulfur to 31 kilobars and phase boundaries of the so-called "4.04-angstrom phase" have been determined. Evidence has been obtained for phase fields of nine new high-pressure forms of sulfur.     

Superconductivity of metallic aluminum antimonide

The high-pressure metallic phase of aluminunm antimnonide is super conducting [critical temperature T(c) (P approximately 125 kilobars) = 2.8 degrees +/-0.2 degrees K]. This transition temperature is significantly lower than the transition temperature of metallic germanium under an equivalent high pressure. A similar result had been previously found for superconducting indiumantimonide in comparison to tin.     

A High-Pressure Polymorph of Troilite, FeS

A new polymorph of FeS was produced in a diamond-anvil cell and observed at high pressure both optically and by x-ray diffraction. Fourteen x-ray reflections of the high-pressure FeS were recorded; however, the crystal structure is unknown. This form of FeS is stable at 25 degrees C only at pressures above approximately 55 kilobars. The transition to the lower pressure polymorph, troilite, is rapid and reversible.     

Zeolite Molecular Sieve 4A: Anomalous Compressibility and Volume Discontinuities at High Pressure

Unit cell parameters of synthetic zeolite 4A were measured at several pressures to 40 kilobars with both water and an alcohol mixture as hydrostatic pressure media. Compression in water was normal, with no observed phase transitions. Compression in alcohols was twice as great as in water, and three volume discontinuities were observed. These volume changes in alcohol were rapid with increasing pressure but sluggish in reverse. High-pressure "phases," all of which are dimensionally cubic, are progressively more compressible at high pressure. These unusual high-pressure phenomena, which indicate significant interactions between zeolite 4A and the hydrostatic media, are consistent with differences in zeolite adsorption of water alcohols.     

Polymorphism in benzene, naphthalene, and anthracene at high pressure

Optical observations, in which a microscope was used with the diamond-anvil pressure cell, were carried out on benzene, naphthalene, and anthracene up to temperatures of about 600 degrees C and pressures of approximately 40 kilobars. New high-pressure phases of benzene (benzene III) and anthracene (anthracene II) were observed, and the existence of the high-pressure polymorph, naphthalene II, was verified. All three materials decompose initially to a reddish-orange liquid, and ultimately to amorphous carbon. The decomposition temperatures decrease with increasing molecular size.     

Ultradeep (>300 kilometers) ultramafic xenoliths: petrological evidence from the transition zone

The seismologically delineated transition zone, at depths between 400 and 670 kilometers, is a fundamental discontinuity in the earth that separates the upper mantle from the lower mantle. Xenoliths from within or close to the transition zone are dominated by pyropic garnet and associated pyroxene or mineralogically heterogeneous garnet lherzolite. These xenoliths show evidence for the high-pressure (90 to 120 kilobars) transformation of pyroxene to a solid solution of pyroxene in garnet (majorite) and silicon in octahedral coordination; low-pressure (less than 80 kilobars) exsolution of clinopyroxene or orthopyroxene from the original majorite is preserved. Although mineral modes and rock proportions below the transition zone and the relative amount of eclogite present cannot be accurately assessed from the xenoliths, it is likely that both majorite and beta-spinel help produce the observed seismic gradient of the transition zone.     

Crystal Structures Adopted by Black Phosphorus at High Pressures

Black phosphorus undergoes two reversible structural transitions at high pressures. The first is to a structure of the type arsenic A7. This structure is transformed to a simple cubic structure at higher pressures. The reversibility between the A7 and simple cubic structures at 111 kilobars indicates that the transition obtainable at this pressure provides a good calibration point by which high-pressure x-ray data may be united with volumetric or resistance measurements, or both.     

Compressibility, Phase Transitions, and Oxygen Migration in Zirconium Tungstate, ZrW2O8

In situ neutron diffraction experiments show that at pressures above 2 kilobars, cubic zirconium tungstate (ZrW2O8) undergoes a quenchable phase transition to an orthorhombic phase, the structure of which has been solved from powder diffraction data. This phase transition can be reversed by heating at 393 kelvin and 1 atmosphere and involves the migration of oxygen atoms in the lattice. The high-pressure phase shows negative thermal expansion from 20 to 300 kelvin. The relative thermal expansion and compressibilities of the cubic and orthorhombic forms can be explained in terms of the "cross-bracing" between polyhedra that occurs as a result of the phase transition.     

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.     

Maskelynite: Formation by Explosive Shock

When high pressure (250 to 300 kilobars) was applied suddenly (shock-loading) to gabbro, the plagioclase was transformed to a noncrystalline phase (maskelynite) by a solid-state reaction at a low temperature, while the proxene remained crystalline. The shock-loaded gabbro resembles meteorites of the shergottite class; this suggests that the latter formed as a result of shock. The shock-loading of gabbro at 600 to 800 kilobars raised the temperature above the melting range of the plagioclase.     

Mantle-derived peridotites in southwestern Oregon: relation to plate tectonics

A group of peridotites in southwestern Oregon contains high-pressure mineral assemblages reflecting recrystallization at high temperatures (1100 degrees to 1200 degrees C) over a range of pressure decreasing from 19 to 5 kilobars. It is proposed that the peridotites represent upper-mantle material brought from depth along the ancestral Gorda-Juan de Fuca ridge system, transported eastward by the spreading Gorda lithosphere plate, and then emplaced by thrust-faulting in the western margin of the Cordillera during late Mesozoic time.     

Melting of diamond

Radiation from a Q-switched YAG laser, focused on the (100) face of a single crystal diamond anvil in a high-pressure diamond cell, caused a portion of the diamond anvil face to melt. Potassium bromide mixed with graphite was under pressure between the anvils when melting occurred. The diamond surface melted at pressures greater than approximately 120 kilobars and graphitized at lower pressures. Evidence for the melting and graphitization of the diamond was obtained by optical and scanning electron microscopy.     

Crystal Structure of Benzene II at 25 Kilobars

Crystals of a high-pressure form of benzene (benzene 11) were grown in the diamond-anvil pressure cell at elevated temperature and pressure from the transition of solid I to solid II. X-ray precession data were obtained from a single-crystal in the high-pressure cell. At 21 degrees C and about 25 kilobars, benzene II crystallizes in the monoclinic system with a = 5.417 +/- 0.005 angstroms (S.D.), b = 5.376 +/- 0.019 angstroms, c = 7.532 +/- 0.007 angstroms, beta = 110.00 degrees +/- 0.08 degrees , space group P2(1)/ c, Pc= 1.26 grams per cubic centimeter. The crystal structure was solved by generating all possible molecular packing configurations and calculating structure factors, reliability factors, and packing energies for each configuration. This procedure produced a unique solution for the molecular packing of benzene II.     

High-Pressure Polymorph of Thulium: An X-ray Diffraction Study

An x-ray diffractiotn study of thulium at room temperature and high pressure by means of a diamond-anvil press has shown that thulium transforms from a hexagonal close-packed structure to the samarium type, as other rareearth elements (gadolinium, terbium, dysprosium, and holmium) do. Unlike the other rare-earth elements, thulium (hexagonal close-packed) has an axial ratio (c/a) that is independent of pressure within experimental error and the transition is reversible. The transition occurs with increasing pressure in the range of 60 to 116 kilobars. The lattice paralieters of the samarium-type phase of thulium at about 116 kilobars are a = 3.327 +/- 0.005 angstroms and c = 23.48 +/- 0.04 angstroms, and the volume change at the transition is estimated to be - 0.5 percent of the volume of the hexagonal close-packed phase at the transition.     

Enthalpy of transformation of a high-pressure polymorph of titanium dioxide to the rutile modification

The enthalpy of transformation of a high-pressure form of titanium dioxide, which has the alpha lead dioxide structure, to the rutile modification was measured by the method of "transposed temperature-drop calorimetry." For the reaction, titanium dioxide (alpha lead dioxide form) transforming to rutile, the change in the heat content is -0.76 +/- 0.17 kilocalorie per mole. From this value and the volume change (+ 2.8 percent) associated with the transformation, we estimate the equilibrium pressure at 294 degrees K to be 60 +/- 20 kilobars.     

Pressure measurement made by the utilization of ruby sharp-line luminescence

A rapid, convenient technique for precision pressure measurement in the diamond-anvil high-pressure cell, which makes use of the sharp-line (R-line) luminescence of ruby, has been developed. The observed shift is -0.77 +/-0.03 reciprocal centimeters per kilobar for R(1) and -0.84+/- 0.03 reciprocal centimeters per kilobar for R(2) to lower energy and is approximately linear in the range studied (to 22 kilobars). Line-broadening has been observed in some instances and has been tentatively identified with nonhydrostatic conditions surrounding the ruby sample.     

Quenchable high-pressure polymorph of zinc selenate

A new quenchable high-pressure form of zinc selenate (ZnSeO(4)) was produced by subjecting the low-pressure modification to 40 kilobars at 400 degrees C for 30 minutes. The new form is orthorhombic, space group D(2h),(17)-Cmcm. The cell constants at 29 degrees C are: a, 5.511 angstroms; b, 8.110 angstroms; and c, 6.585 angstroms. The calculated density is 4.70 grams per cubic centimeter in comparison with 4.61 grams per cubic centimeter for the low-pressure modification. This implies a volume change of 2 percent at the transition.     

Polymorphism of shock loaded fe-mn and fe-ni alloys

Addition of nickel or manganese to iron lowers the pressure of the "130-k" dynamic polymorphic transition to about 55 kilobars at the limits of the body-centered cubic alloy phase.     

Garnet-like structures of high-pressure cadmium germanate and calcium germanate

Crystals of CdGeO(3) grown at a pressure of 65 kilobars are tetragonal and have an ordered, garnet-like crystal structure with cadmium occupying the dodecahedral and octahedral sites, and germanium the octahedral and tetrahedral sites. The crystal structure (a = 12.406 +/- 1 angstroms, c = 12.256 +/- 1 angstroms, and space group 14,/a) has been refined by least-squares analysis to an R (discrepancy index) of 0.073. Two high-pressure phases of CaGeO(3) were synthesized, one isotypic with tetragonal CdGeO(3) (a = 12.514 +/- 3 angstroms, c = 12.358 +/- 3 angstroms), and the other isotypic with perovskite.     

Potassium-sodium ratios in aqueous solutions and coexisting silicate melts

Silicate melts were equilibrated with aqueous chloride solutions at temperatures between 770 degrees and 880 degrees C and a total pressure of 1.4 to 2.4 kilobars. The ratio of potassium to sodium in the aqueous phase was (0.74 +/- 0.06) times the corresponding ratio in the coexisting melt over the entire range of temperature and pressure for all chloride concentrations between 0.2 and 4.2 moles per kilogram.