Stephen A. Gramsch

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.

Structure, metal-insulator transitions, and magnetic properties of FeO at high pressures

Abstract The high-pressure behavior of rocksalt-structured FeO has been investigated using the LDA + U method, a first-principles computational technique that allows treatment of correlated electrons with strong localized repulsions. Within the local density approximation (LDA) FeO is predicted to be a metal, but with LDA + U, an insulating state is obtained at zero pressure. Electronic and magnetic behavior, the equation of state, and lattice strain are determined for three values of the Coulomb repulsion U. We find two self-consistent solutions, one with rhombohedral and one with monoclinic electronic symmetry. For U = 4.6 eV, the monoclinic solution becomes more stable than the rhombohedral solution at 110 GPa, leading to an insulator-metal transition; with increasing U, metallization occurs at higher pressures. Results from the LDA + U calculation suggest that the high-spin magnetic state should persist to pressures greater than 300 GPa. The method gives improved agreement with experiments for ground state properties as compared to LDA and GGA methods that do not explicitly include a local Coulomb repulsion.

Optical study of H2O ice to 120GPa: 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.

A diamond gasket for the laser-heated diamond anvil cell

Advances in laser heating techniques with diamond anvil cells have enabled direct investigations of materials under extreme pressure-temperature conditions. The success of uniform heating to the maximum temperatures at megabar pressures relies critically on maximizing the gasket thickness which in turn depends upon the shear strength of the gasket. We have used diamond powder, the strongest possible material, to formulate a gasket for in situ x-ray diffraction with double-sided laser heating. The increase in gasket thickness allows increases in sample and insulator thickness, thereby improving the quality and pressure-temperature range of the measurement. We did not observe any pressure difference within 40 μm of the center of the sample chamber and the temperature distribution across the sample itself is within ±47 K. These improvements as well as the fact that the diamond gasket can allow the sample to remain in good condition after high P–T processing make it an extremely useful technique in diamond ce...

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 O2 molecules interact predominantly through the half-filled 1πg* orbital <10 GPa. Enhanced intermolecular interactions develop because of increasing overlap of the 1πg* orbital in the low-pressure phases, leading to electron delocalization and ultimately intermolecular bonding between O2 molecules at the transition to the ε-phase. The ε-phase, which consists of (O2)4 clusters, displays the bonding characteristics of a closed-shell system. Increasing interactions between (O2)4 clusters develop upon compression of the ε-phase, and provide a potential mechanism for intercluster bonding in still higher-pressure phases.

Two-dimensional energy dispersive x-ray diffraction at high pressures and temperatures

Diffraction studies at extreme pressure-temperature conditions encounter intrinsic difficulties due to the small access angle of the diamond anvil cell and the high background of the diffraction peaks. Energy-dispersive x-ray diffraction is ideal for overcoming these difficulties and allows the collection and display of diffracted signals on the order of seconds, but is limited to one-dimensional information. Materials at high pressures in diamond anvil cells, particularly during simultaneous laser heating to temperatures greater than 3000 K often form coarse crystals and develop preferred orientation, and thus require information in a second dimension for complete analysis. We have developed and applied a diamond cell rotation method for in situ energy-dispersive x-ray diffraction at high pressures and temperatures in solving this problem. With this method, we can record the x-ray diffraction as a function of χ angle over 360°, and we can acquire sufficient information for the determination of high P–T phase diagrams, structural properties, and equations of state. Technical details are presented along with experimental results for iron and boron.

Pressure-induced transformations in deep mantle and core minerals

Abstract Recent experimental and theoretical studies provide new insight into the variety of high-pressure transformations in minerals that comprise the Earth’s deep mantle and core. Representative examples of reconstructive, displacive, electronic and magnetic transformations studied by new diamond-anvil cell techniques are examined. Despite reports for various transitions in (Mg,Fe)SiO3-perovskite, the stability field of the orthorhombic phase expands relative to magnesiowüstite + SiO2 with increasing pressure and temperature. The partitioning of Fe and Mg between Mg-rich silicate perovskite and magnesiowüstite depends strongly on pressure, temperature, bulk Fe/Mg ratio, and ferric iron content. The soft-mode transition in SiO2 from the rutile- to CaCl2-type structure, originally documented by X-ray powder diffraction, Raman scattering, and first-principles theory has been explored in detail by single crystal diffraction, and transitions to higher-pressure forms have been examined. The effect of H on the transformations of various nominally anhydrous phases and transitions in dense hydrous Mg-silicates are also examined. New studies of the phase diagram of FeO include the transition to rhombohedral and higher-pressure NiAs polymorphs, and provide prototypical examples of coupled structural, electronic, and magnetic transitions. High-spin/low-spin transitions in FeO have been examined by high-resolution X-ray emission spectroscopy to 150 GPa, and the results are compared with similar studies of Fe2O3 and FeS. Finally, laser-heating studies to above 150 GPa and 2500 K show that (hcp) ε-Fe has a large P-T stability field. Radial XRD measurements carried out at room temperature to 220 GPa have constrained the elasticity, rheology and sound velocities of ε-Fe at core pressures.

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.

High-pressure Raman spectroscopic studies of ulvöspinel Fe2TiO4

Abstract We report in situ Raman spectroscopic studies of ulvöspinel in a diamond-anvil cell under hydrostatic conditions up to 57 GPa at room temperature. Two modes near 493 and 681 cm-1 are observed clearly at 1 GPa. In the cubic spinel structure, the lower frequency peak can be assigned to a mode of F2g symmetry and the higher frequency peak can be assigned to a mode with A1g symmetry. The remaining three modes could not be observed unambiguously in the measurements, although there are five Raman-active modes (A1g+Eg+3F2g) in the Fd3̅m space group of the spinel structure according to factor group analysis. The peak positions and shapes in the Raman spectra agree well with those measured under ambient conditions. With increasing pressure, the frequencies of the A1g and F2g modes increase continuously up to 9 GPa with pressure derivatives of 2.5 and 2.1 cm-1/GPa, respectively. There is no obvious degradation of crystal symmetry or structural change observed within this pressure range. Upon increasing pressure to ~20 GPa, the F2g mode splits into B1g+Eg modes, and then into B1g+B2g+B3g modes. The intensities of the Raman bands gradually decrease due to the tetragonalorthorhombic phase transition. This mode completely disappears at a pressure of 29 GPa. The most striking characteristic of the Raman spectrum of ulvöspinel is that compression leads to the extinction of the Raman-active mode derived from F2g symmetry. Only one peak resulting from the A1g mode can be observed continuously up to 57 GPa. The peak shift derived from the A1g mode and its full-width at half maximum (FWHM) values suggests another phase transition occurring around 30 GPa. The Raman spectrum of ulvöspinel is in good agreement with the spectra of ZnCr2O4 and ZnFe2O4 spinels.

High P–T Brillouin scattering study of H2O melting to 26 GPa

The properties of solid and liquid phases of H2O at high pressure and temperature remain an active area of research. In this study, Brillouin spectroscopy has been used to determine the temperature dependence of sound velocities in H2O as a function of pressure up to 26 GPa through the phase field of ice VII and into the liquid to a maximum temperature of 1200 K. The Brillouin shift of the quasi-longitudinal acoustic mode moves to lower frequencies upon melting at each pressure. As a test of the method, measurements of the melting of Ar by Brillouin scattering at several pressures show a similar behavior for the acoustic mode, and measured melting points are consistent with previous results. The results of H2O melting are consistent with previously reported melting curves below 20 GPa. The data at higher pressure indicate that ice melts at a higher temperature than a number of previous studies have indicated.