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The structural stability of methane hydrate under pressure at room temperature was examined by both in-situ single-crystal and powder X-ray diffraction techniques on samples with structure types I, II, and H in diamond-anvil cells. The diffraction data for types II (sII) and H (sH) were refined to the known structures with space groups Fd 3 m and P 6 3 / mmc , respectively. Upon compression, sI methane hydrate transforms to the sII phase at 120 MPa, and then to the sH phase at 600 MPa. The sII methane hydrate was found to coexist locally with sI phase up to 500 MPa and with sH phase up to 600 MPa. The pure sH structure was found to be stable between 600 and 900 MPa. Methane hydrate decomposes at pressures above 3 GPa to form methane with the orientationally disordered Fm 3 m structure and ice VII ( Pn 3 m ). The results highlight the role of guest (CH 4 )-host (H 2 O) interactions in the stabilization of the hydrate structures under pressure.

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 O 2 molecules interact predominantly through the half-filled 1π g * orbital g * orbital in the low-pressure phases, leading to electron delocalization and ultimately intermolecular bonding between O 2 molecules at the transition to the ε-phase. The ε-phase, which consists of (O 2 ) 4 clusters, displays the bonding characteristics of a closed-shell system. Increasing interactions between (O 2 ) 4 clusters develop upon compression of the ε-phase, and provide a potential mechanism for intercluster bonding in still higher-pressure phases.

Lattice strains were measured as a function of the angle ψ between the diffracting plane normal and the stress axis of a diamond anvil cell in a layered sample of molybdenum and gold. The sample was compressed over the range 5–24 GPa and the lattice strains were measured using energy-dispersive x-ray diffraction. As ψ is varied from 0° to 90°, the mean lattice parameter of molybdenum increases by up to 1.2% and that of gold increases by up to 0.7%. A linear relationship between Q(hkl), which is related to the slope of the measured d spacing versus 1−3 cos2 ψ relation, and 3Γ(hkl), a function of the Miller indices of the diffracting plane, is observed for both materials as predicted by theory. The pressure dependence of the uniaxial stress t for gold from this and other recent studies is given by t=0.06+0.015P, where P is the pressure in GPa. The uniaxial stress in molybdenum can be described by t=0.46+0.13P. Using gold as an internal pressure standard, the equation of state of molybdenum depends strongly ...

The equations for the lattice strains produced by nonhydrostatic compression are presented for all seven crystal systems in a form convenient for analyzing x-ray diffraction data obtained by newly developed methods. These equations have been used to analyze the data on cubic (bcc α-Fe) and hexagonal (hcp e-Fe) systems. The analysis gives information on the strain produced by the hydrostatic stress component. A new method of estimating the uniaxial stress component from diffraction data is presented. Most importantly, the present analysis provides a general method of determining single crystal elastic constants to ultrahigh pressures.

The equations for the lattice strains produced by non-hydrostatic compression of the specimen are presented for all seven crystal systems in a form suitable for the analysis of experimental data. Examples of cubic (FeO and bcc iron) and hexagonal (hcp iron) systems are given to demonstrate that the analyses of the data give the estimates of single crystal elastic constants under high pressures. As the lattice strains can be measured at high pressures up to a few megabars using a diamond anvil cell, this approach offers an unique method of estimating the elastic constants at such high pressures. Further, this is the only method of estimating elastic constants of a phase stabilized under pressure. The article also discusses some other interesting specimen-material properties that can be derived at such high pressures.

Compressed under ambient temperature, graphite undergoes a transition at approximately 17 gigapascals. The near K-edge spectroscopy of carbon using synchrotron x-ray inelastic scattering reveals that half of the pi-bonds between graphite layers convert to sigma-bonds, whereas the other half remain as pi-bonds in the high-pressure form. The x-ray diffraction pattern of the high-pressure form is consistent with a distorted graphite structure in which bridging carbon atoms between graphite layers pair and form sigma-bonds, whereas the nonbridging carbon atoms remain unpaired with pi-bonds. The high-pressure form is superhard, capable of indenting cubic-diamond single crystals.

Compressing glassy carbon above 40 GPa, we have observed a new carbon allotrope with a fully $s{p}^{3}$-bonded amorphous structure and diamondlike strength. Synchrotron x-ray Raman spectroscopy revealed a continuous pressure-induced $s{p}^{2}$-to-$s{p}^{3}$ bonding change, while x-ray diffraction confirmed the perseverance of noncrystallinity. The transition was reversible upon releasing pressure. Used as an indenter, the glassy carbon ball demonstrated exceptional strength by reaching 130 GPa with a confining pressure of 60 GPa. Such an extremely large stress difference of $g70\text{ }\text{ }\mathrm{GPa}$ has never been observed in any material besides diamond, indicating the high hardness of this high-pressure carbon allotrope.

The formation of substitutional alloys has been restricted to elements with similar atomic radii and electronegativity. Using high-pressure at 298 K, we synthesized a face-centered cubic disordered alloy of highly dissimilar elements (large Ce and small Al atoms) by compressing the Ce 3 Al intermetallic compound >15 GPa or the Ce 3 Al metallic glass >25 GPa. Synchrotron X-ray diffraction, Ce L 3 -edge absorption spectroscopy, and ab initio calculations revealed that the pressure-induced Kondo volume collapse and 4 f electron delocalization of Ce reduced the differences between Ce and Al and brought them within the Hume-Rothery (HR) limit for substitutional alloying. The alloy remained after complete release of pressure, which was also accompanied by the transformation of Ce back to its ambient 4 f electron localized state and reversal of the Kondo volume collapse, resulting in a non-HR alloy at ambient conditions.

Recent high-pressure studies have shown that an electronic spin transition of iron in ferropericlase, an expected major phase of Earth's lower mantle, results in changes in its properties, including density, incompress- ibility, radiative thermal conductivity, electrical conduc- tivity, and sound velocities. To understand the rheology of ferropericlase across the spin transition, we have used in situ radial X-ray diffraction techniques to examine ferro- periclase, (Mg0.83,Fe0.17)O, deformed non-hydrostatically in a diamond cell up to 81 GPa at room temperature. Compared with recent quasi-hydrostatic studies, the range of the spin transition is shifted by approximately 20 GPa as a result of the presence of large differential stress in the sample. We also observed a reduction in incompressibility and in the unit cell volume of 3% across the spin transition. Our radial X-ray diffraction results show that the {0 0 1} texture is the dominant lattice preferred orientation in ferropericlase across the spin transition and in the low-spin state. Viscoplastic self-consistent polycrystal plasticity simulations suggest that this preferred orientation pattern is produced by {1 1 0}\1-10( slip. Analyzing our radial X-ray diffraction patterns using lattice strain theory, we evaluated the lattice d-spacings of ferropericlase and Mo as a function of the w angle between the compression direc- tion and the diffracting plane normal. These analyses give the ratio between the uniaxial stress component (t) and the shear modulus (G) under constant stress condition, which represents a proxy for the supported differential stress and elastic strength. This ratio in the mixed-spin and low-spin states is lower than what is expected from previous studies of high-spin ferropericlase, indicating that the spin transi- tion results in a reduced differential stress and elastic strength along with the volume reduction. The influence of the spin transition on the differential stress and strength of ferropericlase is expected to be less dominant across the wide spin transition zone at high pressure-temperature conditions relevant to the lower mantle.

When subjected to high pressure and extensive x-radiation, water (H 2 O) molecules cleaved, forming O–O and H–H bonds. The oxygen (O) and hydrogen (H) framework in ice VII was converted into a molecular alloy of O 2 and H 2 . X-ray diffraction, x-ray Raman scattering, and optical Raman spectroscopy demonstrated that this crystalline solid differs from previously known phases. It remained stable with respect to variations in pressure, temperature, and further x-ray and laser exposure, thus opening new possibilities for studying molecular interactions in the hydrogen-oxygen binary system.

Polycrystalline rhenium in the form of a powder contained in a two-stage gasket was compressed in a diamond anvil cell. No pressure medium was used to ensure that maximum nonhydrostatic stresses that rhenium can support were produced. The pressure was increased in steps to a maximum of 250 GPa and the diffraction pattern recorded at each pressure using energy dispersive technique. The differential stress t, a measure of the compressive strength, was determined at any given pressure from the offset between the measured unit cell volume and volume computed from the pressure-volume relation under hydrostatic pressure. The data suggest that t is 2.5 GPa at a pressure of 5 GPa and increases linearly to 26 GPa at 250 GPa. The present data agree well with those obtained from the radial diffraction data to 37 GPa in an earlier study but differ significantly from the results of another study. Though comparable with the theoretical strength of ideal solids, the observed strengths are extremely large for a polycrystalline aggregate of a solid. The pure pressure effect on the strength described by the shear modulus scaling is inadequate to account for such a large increase of strength with pressure . It is suggested that the major contribution to strength comes from the strain hardening of rhenium that arises due to plastic deformat ion of the sample during nonhydrostatic compression in a diamond anvil cell.

The high pressure behavior of titanium nitride (TiN) was investigated using synchrotron radial x-ray diffraction (RXRD) under nonhydrostatic compression up to 45.4 GPa in a diamond-anvil cell. We obtained the hydrostatic compression equation of state of TiN. Fitting to the third-order Birch–Murnaghan equation of state, the bulk modulus derived from nonhydrostatic compression data varies from 232 to 353 GPa, depending on angle ψ, the orientation of the diffraction planes with respect to the loading axis. The RXRD data obtained at ψ=54.7° yield a bulk modulus K0=282±9 GPa with pressure derivative K0′ fixed at 4. We have analyzed the deformation mechanisms by analyzing the (111), (200), (220), (311), and (222) peaks in the x-ray diffraction under pressures. The ratio of uniaxial stress component to shear modulus t/G ranges from 0.007–0.027 at the pressure of 6.4–45.4 GPa. It was found that the TiN sample could support a maximum uniaxial stress component t of 8.6 GPa, when it started to yield at 45.4 GPa under uniaxial compression. And the aggregate elastic moduli of TiN at high pressure were determined from the synchrotron RXRD measurements.

X-ray diffraction measurements have been carried out on cesium iodide (CsI) to 302 gigapascals with a platinum pressure standard. The results indicate that above 200 gigapascals CsI at 300 K has a hexagonal close-packed crystal structure with the ideal c / a ratio of 1.63 ± 0.01. The crystal structure and pressure-volume relations converge at high pressure with those of solid xenon, which is isoelectronic with CsI. The results indicate a significant loss of ionic bonding in the hexagonal close-packed metallic phase of CsI at ultrahigh pressure.