Search

The atomic structure of a germanium doped phosphorous selenide glass of composition Ge2.8P57.7Se39.5 is determined as a function of pressure from ambient to 24 GPa using Monte-Carlo simulations constrained by high energy x-ray scattering data. The ambient pressure structure consists primarily of P4Se3 molecules and planar edge shared phosphorus rings, reminiscent of those found in red phosphorous as well as a small fraction of locally clustered corner-sharing GeSe4 tetrahedra. This low-density amorphous phase transforms into a high-density amorphous phase at ~6.3 GPa. The high-pressure phase is characterized by an extended network structure. The polyamorphic transformation between these two phases involves opening of the P3 ring at the base of the P4Se3 molecules and subsequent reaction with red phosphorus type moieties to produce a cross linked structure. The compression mechanism of the low-density phase involves increased molecular packing, whereas that of the high pressure phase involves an increase in the nearest-neighbor coordination number while the bond angle distributions broaden and shift to smaller angles. The entropy and volume changes associated with this polyamorphic transformation are positive and negative, respectively, and consequently the corresponding Clapeyron slope for this transition would be negative. This result has far reaching implications in our current understanding of the thermodynamics of polyamorphic transitions in glasses and glass-forming liquids.

The effect of pressure on the crystal structure of white phosphorus has been studied up to 22.4 GPa. The ?alpha phase was found to transform into the alpha' phase at 0.87 +- 0.04 GPa with a volume change of 0.1 +- 0.3 cc/mol. A fit of a second order Birch- Murnaghan equation to the data gave Vo = 16.94 ? 0.08 cc/mol and Ko = 6.7 +- 0.5 GPa for the alpha phase and Vo = 16.4 +- 0.1 cc/mol and Ko = 9.1 +- 0.3 GPa for the alpha' phase. The alpha' phase was found to transform to the A17 phase of black phosphorus at 2.68 +- 0.34 GPa and then with increasing pressure to the A7 and then simple cubic phase of black phosphorus. A fit of a second order Birch-Murnaghan equation to our data combined with previous measurements gave Vo = 11.43 +- 0.05 cc/mol and Ko = 34.7 +- 0.5 GPa for the A17 phase, Vo = 9.62 +- 0.01 cc/mol and Ko = 65.0 +- 0.6 GPa for the A7 phase and , Vo = 9.23 +- 0.01 cc/mol and Ko = 72.5 +- 0.3 GPa for the simple cubic phase.

The laser heating system for the diamond anvil cell at endstation 2 of beamline 12.2.2 of the Advanced Light Source in Berkeley, CA has been constructed and is available for in-situ high-pressure high-temperature x-ray experiments. The endstation couples a high-brilliance synchrotron x-ray source with an industrial strength laser to heat and probe samples at high pressure in the diamond anvil cell. The system incorporates an 80 watt Nd:YLF (cw) laser operated in TEM01* mode. Double-sided heating is achieved by splitting the laser beam into 2 paths that are directed through the opposing diamond anvils. X-ray transparent mirrors steer the laser beams coaxial with the x-ray beam from the superconducting bending magnet (energy range 6-35 KeV) and direct the emitted light from the heated sample into two separate spectrometers for temperature measurement by spectroradiometry. Objective lenses focus the laser beam to a size of 25 micron diameter (FWHM) in the sample region. An x-ray spot size of 10 micron diameter (FWHM) has been achieved with the installation of a pair of focusing Kirkpatrick-Baez mirrors. A unique aperture configuration has produced an x-ray beam profile that has very low intensity in the tails. The main thrust of the program is aimed at producing in-situ high-pressure high-temperature x-ray diffraction data, but other modes of operation, such as x-ray imaging have been accomplished. Technical details of the experimental setup will be presented along with initial results.

A tensonometer for stretching metal foils has been constructed for the study of strain broadening in x-ray diffraction line profiles. This device, which is designed for use on the powder diffractometer in Station 2.3 at Daresbury Laboratory, allows in-situ measurements to be performed on samples under stress. It can be used for data collection in either transmission or reflection modes using either symmetric or asymmetric diffraction geometries. As a test case, measurements were carried out on a 18mum thick copper foil experiencing strain levels of up to 5 percent using both symmetric reflection and symmetric transmission diffraction. All the diffraction profiles displayed peak broadening and asymmetry which increased with strain. The measured profiles were analysed by the fundamental parameters approach using the TOPAS peak fitting software. All the observed broadened profiles were modelled by convoluting a refineable diffraction profile, representing the dislocation and crystallite size broadening, with a fixed instrumental profile pre-determined using high quality LaB6 reference powder. The de-convolution process yielded "pure" sample integral breadths and asymmetry results which displayed a strong dependence on applied strain and increased almost linearly with applied strain. Assuming crystallite size broadening in combination with dislocation broadening arising from fcc a/2 111 dislocations, we have extracted the variation of mechanic al property with strain. The observation of both peak asymmetry and broadening has been interpreted as a manifestation of a cellular structure with cell walls and cell interiors possessing high and low dislocation densities.

X-ray diffraction and infrared data have been collected from a sample of ettringite from ambient pressure to 6.4 GPa. The sample was found to reversibly transform to an amorphous phase at 3 GPa. The isothermal bulk modulus of ettringite was found to be 27(7) GPa and the incompressibilities of the lattice parameters were found to be 71(30) GPa along a and 108(36) GPa along c.

A new facility for high-pressure diffraction and spectroscopy using diamond anvil high-pressure cells has been built at the Advanced Light Source on Beamline 12.2.2. This beamline benefits from the hard X-radiation generated by a 6 Tesla superconducting bending magnet (superbend). Useful x-ray flux is available between 5 keV and 35 keV. The radiation is transferred from the superbend to the experimental enclosure by the brightness preserving optics of the beamline. These optics are comprised of: a plane parabola collimating mirror (M1), followed by a Kohzu monochromator vessel with a Si(111) crystals (E/DE ~; 7000) and a W/B4C multilayers (E/DE ~; 100), and then a toroidal focusing mirror (M2) with variable focusing distance. The experimental enclosure contains an automated beam positioning system, a set of slits, ion chambers, the sample positioning goniometry and area detectors (CCD or image-plate detector). Future developments aim at the installation of a second end station dedicated for in situ laser-heating on one hand and a dedicated high-pressure single-crystal station, applying both monochromatic as well as polychromatic techniques.

The effects of pressure on the OH stretching frequencies of natural talc and two samples of synthetic 10-[Angstrom], phase have been measured using a diamond-anvil cell and a synchrotron infrared source. The 10-[Angstrom] phase was synthesized at 6.0-6.5 GPa, 600[degrees]C for 46 hours (sample 10[Angstrom],-46) and 160 hours (10[Angstrom]-160). Spectra were collected up to 9.0 GPa (talc), 9.9 GPa (10[Angstrom]-46), and 9.6 GPa (10[Angstrom]-160). The OH stretching vibration of [Mg.sub.3]OH groups in talc occurs at 3677 [cm.sup.-1] at ambient pressure, and increases linearly with pressure at 0.97(2) [cm.sup.-1] G[Pa.sup.-1]. The same vibration occurs in 10-[Angstrom] phase, but shows negligible pressure shift up to 2 GPa, above which the frequency increases linearly to the maximum pressure studied, at a rate of 0.96(3) [cm.sup.-1] G[Pa.sub.-1] (10[Angstrom]-46) and 0.87(3) [cm.sup.-1] G[Pa.sup.-1] (10[Angstrom]160). Two other prominent bands in the 10-[Angstrom], phase spectrum are suggested to be due to stretching of interlayer [H.sub.2]O, hydrogen-bonded to the nearest tetrahedral sheet. These bands also show little change over the first 2 GPa of compression, as most of the compression of the structure is taken up by closing non-hydrogen bonded gaps between interlayer [H.sub.2]O and tetrahedral sheets. Between 2 and 4 GPa, changes in band intensities suggest a rearrangement of the interlayer [H.sub.2]O. Keywords: Talc, 10-[Angstrom] phase, high-pressure studies, IR spectroscopy

An asymmetric binary Margules formulation for excess Gibbs energy, enthalpy, and entropy is adequate to describe the 1 bar halite-sylvite solvus in NaCl-KCl (Thompson and Waldbaum 1969). However, a binary Margules formulation is not adequate to describe excess volumes of single-phase chlorides in P-V-T-X from ambient P-T to ~20 kbar and 700 [degrees]C. Excess volumes across NaCl-KCl increase with temperature, decrease with pressure, and show systematic deficits in the consolute region. These patterns can be explained by the importance of a third component--vacancy defects that relieve the lattice stresses of K-Na size mismatch. New, two-phase observations in P-V-T-X allow delineation of the excess Gibbs energies to high pressures where the excess enthalpies and entropies do not depend on Tat each P, but show significant variation between 1 bar and ~20 kbar. The volume, entropy, and enthalpy of K-Na mixing become more ideal at high pressure. But the solvus expands with pressure because entropy approaches ideality faster than enthalpy and, therefore, Gibbs energy of K-Na mixing becomes less ideal with pressure. The consolute temperature rises about 80 [degrees]C in 17 kbar, with little change in consolute composition. The binary Margules equation of state provides a prediction of the rise of the solvus that is impressively convergent with the new observations. This convergence is especially impressive given the clear inadequacies of the binary excess volume formulation on which the prediction is based.

Unit-cell volumes of four single-phase intermediate halite-sylvite solid solutions have been measured to pressures and temperatures of ~28 kbar and ~700 [degrees]C. Equation-of-state fitting of the data yields thermal expansion and compressibility as a function of composition across the chloride series. The variation of the product [[alpha].sub.0]*[K.sub.0] is linear (ideal) in composition between the accepted values for halite and sylvite. Taken separately, the individual values of [[alpha].sub.0] and [K.sub.0] are not linear in composition. [[alpha].sub.0] shows a maximum near the consulute composition (XNacl = 0.64) that exceeds the value for either end-member. There is a corresponding minimum in [K.sub.0]. The fact that the [[alpha].sub.0]*[K.sub.0] product is variable (and incidentally so well behaved as to be linear across the composition series) reinforces the significance of the complementary maxima and minima in [[alpha].sub.0] and K0 (significantly, near the consolute composition). These extrema in [[alpha].sub.0] and [K.sub.0] provide an example of intermediate properties that do not follow simply from values for the end-members. Cell volumes across this series show small, well-behaved positive excesses, consistent with K-Na substitution causing defects through lattice mismatches. Barrett and Wallace (1954) showed maximum defect concentrations in the consolute region. Defect-riddled, weakened structures in the consolute region are more easily compressed or more easily thermally expanded, providing an explanation for our observed [[alpha].sub.0] and [K.sub.0] variations. These compliant, loosened lattices should resist diffusive transfer less than non-defective crystals and, hence, might be expected to show higher diffusivities. Tracer diffusion rates are predicted to peak across the consolute region as exchange diffusion rates drop to zero.

Volatile-sensitive electrical soundings are becoming more widely adopted, with large nationwide arrays currently being acquired globally. This boom in new data is despite several key uncertainties relating to the electrical responses of a wide range of minerals that make up crustal regions. Complications include the influence of mineral chemistry, hydrous or nominally hydrous phases, and oxygen fugacity on charge-carrying ion activity within a mineral substrate. Feldspars are the most abundant mineral group in the Earth's crust, comprising about 60% of its mineral assemblages and are particularly prevalent within subduction zones and lower crustal sequences. These areas are known locations where ore systems are commonly rooted, making them among the most widely studied regions in the Earth. To date, few studies exist that cover the electrical behavior of the feldspar mineral albite. To help address some of these issues and complications, we have undertaken electrical conductivity investigations on a single crystal of gem-quality albite (Ab49An48Or3) from Nuevo Casas Grande, Chihuahua, Mexico. Electrical conductivity measurements using impedance spectroscopy were performed at a pressure of 1 GPa over a temperature range of 373–1273 K in a multi-anvil high-pressure apparatus. Experiments were carried out using different metal electrodes: molybdenum, nickel, and rhenium to vary the oxygen fugacity during the experiments. FTIR measurements of the starting and final materials confirm that the initial samples are completely dry but absorb an average of 67 ppm H2O by mass during the experiments from the surrounding pressure medium materials. We observe no correlation in the amount of water absorbed in the feldspar to the oxygen fugacity under water-undersaturated conditions. Our investigations show that the activation enthalpy increases from ~0.77 to ~1.0 eV from the nominally hydrous to the completely dry feldspar. The activation enthalpy decreases with increasing oxygen fugacity for comparable water contents. An oxygen fugacity exponent of –0.069 is calculated at the nominal water content measured in the experiments, indicating an electrical conductivity mechanism that also involves the mobility of hydrogen. [ABSTRACT FROM AUTHOR]

Compressibility of solids generally drops as compression proceeds. Bonds shorten and stiffen with density increase in the same structure. However for the B1-B2 phase transition in many measured alkali halides, bond lengths increase in going to the denser phase. And IR mode frequencies decrease (Hofmeister 1997), leading to the counter-intuitive expectation that compressibility should increase in going to the denser B2 phase. Past volume measurements have been equivocal at best in recognizing any such compressibility increase. New thermal equations of state for B1 and B2 KCl from in-situ X-ray diffraction measurements of phase volumes on Station 16.4 of the CLRC Daresbury Laboratory are no less equivocal about the issue of whether compressibility increases or decreases across the transition. In contrast, the new volume measurements show an easily resolved thermal expansion increase in going from B1 to B2 KCl. The product [alpha]*[KAPPA], which is better known than either [alpha] or [KAPPA], increases from 0.0195 [+ or -] 0.0005 kbar/[degrees]C in B1 to 0.0275 [+ or -] 0.0009 kbar/[degrees]C in B2 KCl. Yagi (1978) demonstrated a similar increase for KF, also supported mainly by increases in [alpha]. This increase can also be seen in RbCl (Walker et al. 2001). Bond weakening indicated by the thermal expansion increase is consistent with the elusive compressibility increase that is expected across the B1-B2 transition but which is not resolved from volume measurements. The thermal effects are more visible than the compressional effects on [alpha]-[KAPPA] across the transition. Bond tightening upon decompression reduces [alpha], increases solid viscosity, and hence decreases the Rayleigh number. An upwelling of material undergoing a decompression phase change with decrease of coordination number may have its convective friskiness damped at such a transition.

The hydrothermal crystallization of gyrolite was studied dynamically at 190-240 [degrees] C using synchrotron-based in situ Energy Dispersive Powder Diffraction (EDPD). The reaction mechanism involves the initial crystallization of a calcium silicate hydrate (C-S-H) gel, which has a sheet structure with well ordered Ca(O,OH) layers and disordered silicate layers. This is followed by the intermediate formation of Z-phase which finally transforms to gyrolite. This process involves ordering of the silicate layers and an increase in the order along c. Kinetics data for all stages of the crystallization process were determined by analyzing the growth and decline of various diffraction peaks with time. The activation energy ([E.sub.a]) (nucleation) for Z-phase is ~39 kJ/mol while that for gyrolite is ~56 kJ/mol. [E.sub.a] (crystallization) of gyrolite is higher at ~80 kJ/mol. The reaction occurs via a two-dimensional, diffusion-controlled mechanism and is a continuous process that suggests that Z-phase is an unstable, transient phase.

KCl[O.sub.3] thermal decomposition has been studied from 29-76 kilobars using a multianvil high-pressure device and in-situ energy-dispersive X-ray diffraction and off-line quenching experiments. The rhombohedral form of KCl[O.sub.3] was found to decompose to the B2 form of KCl and [O.sub.2] via an orthorhombic KCl[O.sub.4] intermediate over this pressure interval. The decomposition temperature was found to vary only slightly with pressure. The online experiments gave decomposition temperatures between 500 and 580 [degrees] C. Further off-line quenching experiments using sealed gold tubes determined the equilibrium decomposition boundary to be 550 [+ or -] 15 [degrees] C over this pressure range. Unit-cell parameters and volumes were determined for the high-pressure phases of KCl[O.sub.3] and KCl from the diffraction data. The partial molar volume of [O.sub.2] was calculated from the difference in the solid volumes. Oxygen fluid volumes were then calculated along the decomposition boundary and vary from 10.6 [+ or -] 0.2 c[m.sup.3]/mol at 29 kbar to 9.6 [+ or -] 0.1 c[m.sup.3]/mol at 76 kbar. These volumes are 30 to 50% less than previous estimates determined from shock wave data, and imply that oxygen can be more easily stored in Earth's mantle and core than previously believed. The thermal equation of state of the B2 form of KCl was investigated online using NaCl as an internal pressure standard. KCl was then used as an internal pressure calibrant for the online KCl[O.sub.3] decomposition experiments. The mechanical behavior of the multianvil high-pressure device was also studied and load vs. force characteristics are presented here.

Presents a study which investigated the thermal decomposition of KClO[sub 3] using a multianvil high-pressure device and in-situ energy-dispersive X-ray diffraction and off-line quenching experiments. Experimental techniques; Characterization of the multianvil device; Pressure calibration.

Abstract: The formation of a hexagonal rhenium carbide phase, Re2C, from the elements has been studied in a laser heated diamond anvil cell in a P, V range of 20–40GPa and 1000–2000K. No indication for the existence of cubic rhenium carbide, as suggested in the literature, or any other phase was found and Re2C is the only phase formed in the Re–C system up to around 70GPa and 4000K. A fit of a 3rd-order Birch–Murnaghan equation of state to the Re2C P, V-data results in a bulk modulus of GPa (). The linear compressibility of Re2C along [001] ( GPa) is significantly larger than the compressibility in the (001) plane ( GPa ). Based on the observations for Re2C, we predict the structure and elastic properties of an analogous Os2C phase, which is, at least in the athermal limit, more stable than any other osmium carbide studied previously by density functional theory based calculations. [Copyright &y& Elsevier]