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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.

The high-pressure, high-temperature behavior of iron has been investigated to 161 GPa and 3000 K by in situ synchrotron X-ray diffraction with double-side laser-heated diamond anvil cells. We found that only α-, γ- and e-Fe can be clearly verified as the stable solid phase in the P–T range studied. Only e-Fe is observed from deep lower mantle (∼1500 km) to outer core conditions. Within the P–T range examined, we did not observe a significant change with pressure or temperature on the c/a ratio of e-Fe. The melting curve of iron has been determined to 105 GPa. A Lindeman law fit gives a melting point of iron at the inner core boundary of 5800 (±200) K, which provides an upper bound on the temperature at that depth. We also examine numerous experimental factors that may complicate the analysis of high P–T diffraction data, and discuss the effects of sample stress on the X-ray diffraction results.

In situ high-pressure synchrotron X-ray diffraction and infrared absorption experiments for gibbsite were performed at room temperature up to 53 and 25 GPa, respectively. A phase transition was confirmed at about 2.5 GPa. The high-pressure phase is indexed as an orthorhombic structure, rather than a triclinic structure as reported in previous studies. The compressibility of gibbsite and its high-pressure polymorph were studied, and their bulk moduli K 0 were determined to be 49 and 75 GPa, respectively with K 0′ as 4. The in situ high-pressure infrared absorption spectra revealed the gradual disordering of hydrogen substructure above 15 GPa in quasihydrostatic compression.

The phase transitions and equation of state of cobalt oxide (CoO) have been studied at ambient temperature under non-hydrostatic and hydrostatic high pressures, with the diamond-anvil cell technique. We discovered that under hydrostatic pressure conditions the CoO structure transformed from the low-density rhombohedral to a high-density rhombohedral phase at 90 ± 1 GPa with the molecular volume decreasing by about 2.7%. This phase transition with volume discontinuity may be related to magnetic collapse, which was predicted by theory. The pressure of the phase transition of CoO from face-centred cubic B1 to rhombohedral structure was 43 ± 2 GPa. The decompression spectrum data of CoO under non-hydrostatic pressure showed that these phase transitions are reversible.

A new high-pressure polymorph of merrillite with the structure of trigonal γ-Ca 3 (PO 4 ) 2 was found in the shock-produced veins of the Suizhou meteorite, where it coexists with ringwoodite, majorite, NaAlSi 3 O 8 -hollandite, and majorite-pyrope garnet. The crystallographic nature of this natural γ-Ca 3 (PO 4 ) 2 phase was characterized by Raman spectroscopy and X-ray diffraction, and all data compare favorably to the same data obtained from γ-Ca 3 (PO 4 ) 2 synthesized at 14 GPa and 1400°C. The cell parameters of this new high-pressure mineral are a = 5.258(1) angstroms and c = 18.727(3) angstroms, space group R-3m, and density = 3.447 (g/cm 3 ), where the number in parentheses are standard deviations in the last significant digits. The natural occurrence of the γ-Ca 3 (PO 4 ) 2 phase together with other high-pressure minerals constrains the pressure of the shock veins at about 23 GPa. The Suizhou meteorite provides the first naturally occurring example of γ-Ca 3 (PO 4 ) 2 polymorph.

Solid methane (CH(4)) was compressed up to 202 GPa at 300 K in a diamond-anvil cell. The crystal structure and equation of state over this entire range were determined from angle dispersive X-ray diffraction results. CH(4) undergoes phase transitions from rhombohedral to a simple cubic phase at 19 GPa and from simple cubic to a higher pressure cubic phase at approximately 94 GPa. This higher pressure cubic phase was stable to the maximum pressure investigated. Combined with previous optical measurements, it was found that at room temperature compressed CH(4) remains an insulator with cubic structure to 202 GPa. (C) 2009 Elsevier B. V. All rights reserved.

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.

High-pressure energy dispersive X-ray diffraction of wustite has been obtained with three types of diamond cells; each one is designed to optimize a type of in situ study, namely single-crystal X-ray diffraction, deviatoric strain measurement, or simultaneous high-P-T experimentation. The results demonstrate that above 17 GPa at 300 K wustite undergoes a displacive transition from B1 to a rhombohedral structure, and above 90 GPa at 600 K, a second transition to the NiAs structure. Many of the earlier inconsistencies concerning the structure and properties of wustite at high pressures can be attributed to the extreme softening of the C44 elastic modulus at pressures above 10 GPa.

Using in situ high-pressure x-ray diffraction methods, the compressibility of 30nm 3C–SiC nanocrystals was studied under hydrostatic conditions while helium was used as pressure transmitting medium, as well as under nonhydrostatic conditions without pressure medium. No threshold pressure phenomenon was observed for the compressibility of the nanocrystals during compression in hydrostatic conditions, while the critical pressure around 10.5GPa was observed during nonhydrostatic compression. These indicate that the threshold pressure phenomena, recently reported that the nanocrystals initially exhibited much higher bulk modulus below the threshold pressure during compression [Appl. Phys. Lett. 83, 3174 (2003); J. Phys. Chem. 107, 14151 (2003)], were mainly caused by the nonhydrostatic effect instead of a specific feature of nanocrystals upon compression. The bulk modulus of 3C–SiC nanocrystals is estimated as 220.6±0.6GPa based on the hydrostatic compression data.

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.

Effects of pressure on lattice parameters and molar volume of diaspore were studied to 25.5 GPa at 300 K by polycrystalline X-ray diffraction techniques. The pressure-volume data fit with a second-order Birch-Murnaghan equation of state give a bulk modulus K 0 = 167.5 GPa by Ruoff and previously reported, but unsubstantiated, value of K 0 = 85 GPa by Ruoff and Vanderborgh [ Phys. Rev. Lett. 66, 754 (1991)]. Thermodynamic calculation indicates that hydrogen and ruby are stable relative to diaspore and aluminum to pressures above 200 GPa.

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.

Undoped La 2 CuO 4 oxide ceramic single crystal and polycrystalline materials were studied up to 25.0 GPa in a diamond-anvil cell (DAC) apparatus. La 2 CuO 4 undergoes a phase transformation from an orthorhombic to a tetragonal structure at about 3.4 GPa, and this result is different from the data previously published by others. Compression of this material is only 9% at 25.0 GPa.

Here, we report on the in situ synchrotron inelastic x-ray scattering spectra of Na-borate glasses at high pressure up to 25 GPa. The pressure-induced boron coordination transformation from $^{[3]}\text{B}$ to $^{[4]}\text{B}$ is linear with pressure characterized by a single value of ${(\ensuremath{\partial}\text{ }^{[3]}\text{B}/\ensuremath{\partial}P)}_{T}$. Previous studies of Li-borate and pure-borate glasses show a nonlinear transformation with multiple ${(\ensuremath{\partial}\text{ }^{[3]}\text{B}/\ensuremath{\partial}P)}_{T}$ values for different pressure ranges, revealing the important role cation field strength plays in densification and pressure-induced structural changes. Considering the distribution of the energy difference between low- and high-pressure states $(\ensuremath{\Delta}\ensuremath{\epsilon})$ in the energy landscape and the variance of the ratio $\ensuremath{\Delta}\ensuremath{\epsilon}$ to its pressure gradient ${(\ensuremath{\partial}\ensuremath{\Delta}\ensuremath{\epsilon}/\ensuremath{\partial}P)}_{T}$ as a measure of network flexibility with pressure, an amorphous system with a large variance in $\ensuremath{\Delta}\ensuremath{\epsilon}$ at 1 atm and/or a small ${(\ensuremath{\partial}\ensuremath{\Delta}\ensuremath{\epsilon}/\ensuremath{\partial}P)}_{T}$ may undergo a gradual coordination transformation (e.g., Na borates). In contrast, a system with the opposite behavior (e.g., Li borates) undergoes an abrupt coordination transformation. The results and concepts of this study thus can shed light on opportunities to study the effect of composition on the nature of densification in low-$z$ oxide and other archetypal glasses and melts.

In situ proton NMR spectra and ${T}_{1}^{\phantom{\rule{0.2em}{0ex}}\ensuremath{-}1}$ and ${T}_{2}^{\phantom{\rule{0.2em}{0ex}}\ensuremath{-}1}$ relaxation rates of filled-ice hydrogen hydrates at pressure reveal fast translational motion of the ${\mathrm{H}}_{2}$ molecules within the ice frameworks. The NMR spectra to $3.6\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ gave surprisingly narrow resonances of the ${\mathrm{H}}_{2}$ guests. Pressure effects on ${T}_{1}^{\phantom{\rule{0.2em}{0ex}}\ensuremath{-}1}$ and ${T}_{2}^{\phantom{\rule{0.2em}{0ex}}\ensuremath{-}1}$ of the ${\mathrm{H}}_{2}$ indicate that molecular rotation and diffusion contribute together to the relaxation, from which correlation times ${\ensuremath{\tau}}_{\mathrm{rot}}$ and ${\ensuremath{\tau}}_{\mathrm{dif}}$ were separately determined. Liquidlike fast diffusion of the ${\mathrm{H}}_{2}$ with little pressure sensitivity was deduced from ${\ensuremath{\tau}}_{\mathrm{dif}}$, indicating that the ice framework allows active guest translation even in extensively compressed states.

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.

A natural chromite single crystal was compressed in a helium pressure medium to just above 35 GPa at ambient temperature and studied with energy-dispersive x-ray diffraction. The volume compression of the cubic, spinel-type structure was fit to a third-order Birch-Murnaghan equation of state with parameters a0= 8.338(4) Å, V0 = 579.6(9) Å3, K0 =179(10) GPa, and K' = 3.9(9) up to 29 GPa. A distortive phase transition was discovered at higher pressures to a CaAl2O4-type orthorhombic structure, with two cubic unit-cell axes increasing and the third decreasing with increasing pressure. The transition can be fit to a Landau-type strain-order formulism with approximately 5% volume decrease from the cubic phase at 35 GPa. The transition may be triggered by electronic or magnetic transitions in the 3d elements Fe and Cr cations.

International audience; Physical properties including the equation of state, elasticity, and shear strength of pyrite have been measured by a series of X-ray diffraction in diamond-anvil cells at pressures up to 50 GPa. A Birch-Murnaghan equation of state fit to the quasihydrostatic pressure-volume data obtained from laboratory X-ray source/film techniques yields a quasihydrostatic bulk modulus K0T=133.5 (-5.2) GPa and bulk modulus first pressure derivative K'0T=5.73 (-0.58). The apparent equation of state is found to be strongly dependent on the stress conditions in the sample. The stress dependency of the high-pressure properties is examined with anisotropic elasticity theory from subsequent measurements of energy-dispersive radial diffraction experiments in the diamond-anvil cell. The calculated values of K0T depend largely upon the angle ‚ between the diffracting plane normal and the maximum stress axis. The uniaxial stress component in the sample, t=†3m†1, varies with pressure as t=m3.11+0.43P between 10 and 30 GPa. The pressure derivatives of the elastic moduli dC11/dP=5.76 (-0.15), dC12/dP=1.41 (-0.11) and dC44/dP=1.92 (-0.06) are obtained from the diffraction data assuming previously reported zero-pressure ultrasonic data (C11=382 GPa, C12=31 GPa, and C44=109 GPa).

Single-crystal brucite, Mg(OH)2, was studied to 14 GPa in a quasi-hydrostatic pressure medium using a diamond anvil cell and energy-dispersive synchrotron x-ray diffraction. The parameters of a third-order Birch-Murnaghan equation of state fit to the data are: K OT=42(2) GPa, and (∂K OT/∂P)T= 5.7(5). The bulk modulus is significantly lower than that obtained in recent shock compression and powder x-ray diffraction experiments under non-hydrostatic conditions. No evidence was found for a transition involving the Mg -O sub-structure over the pressure range of these experiments. This implies that the structural change previously identified at high pressure by Raman spectroscopy probably involves rearrangement of hydrogen atoms, leaving the Mg — O substructure largely unaffected.

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.

Attention is given to the lattice parameters of beta phase (Mg(0.84)Fe(0.16))2SiO4 determined by X-ray diffraction using synchrotron radiation under simultaneous high-pressure and high-temperature conditions. The experiments were conducted up to a pressure of 26 GPa and a temperature of 900 K. High pressures were generated in a Mao-Bell type diamond anvil cell using neon gas as a pressure medium. The sample was heated with an external Ni80Cr20 wire heater. Gold was used as an internal high-pressure calibrant at high temperature. The experimental data indicated the anisotropic behavior of the beta phase at high pressure and temperature, i.e., the c axis is about 35-percent more expansible and about 25-percent more compressible than the a and b axes. A value of 5.1 +/-0.8 was found for the Anderson-Grueneisen parameter. The derived thermodynamic parameters for the beta phase are summarized.

High-pressure, high-temperature properties of MgSiO3, (Fe0.1Mg0.9)SiO3, and (Fe0.2Mg0.8)SiO3 perovskites have been investigated using a newly developed X ray diffraction technique involving monochromatic synchrotron radiation. The first direct measurements of unit cell distortions and equation-of-state parameters of the orthorhombic perovskite as functions of composition and simultaneous high pressure and high temperature were obtained. The experiments were conducted under hydrostatic pressure up to 30 GPa, into the stability field of the perovskite. The results demonstrate that the perovskite is elastically anisotropic, with the lattice parameter b being 25% less compressible than a and c. Under increasing pressures the orthorhombic perovskite is distorted further away from the ideal cubic structure in agreement with theoretical predictions. The 298-K isothermal equations of state of the three perovskites are indistinguishable within the uncertainty limits of the experiment. The zero-pressure bulk modulus KT0 = 261 (±4) GPa with its pressure derivative KT0′ = 4 is close to that determined in previous static high pressure measurements. The thermal expansion obtained from the high P - T experiments are consistent with previous measurements carried out at zero pressure but shows a strong volume dependence. The temperature derivative of the isothermal bulk modulus at constant pressure (∂KT/∂T)p is −6.3(±0.5)×10−2 GPa/K. Analyses of the high-temperature data give a value for the Anderson-Gruneisen parameter δT of 6.5–7.5, which is significantly higher than that used in recent lower mantle models.

Results are reported on a room temperature static compression study of iron and the Fe(0.8)Ni(0.2) alloy to above 260 GPa, which provide direct pressure-volume measurements on geophysically important materials at the conditions close to those at the inner core boundary. The diffraction patterns obtained by XRD indicate that pure iron remains in the hcp structure to 304 GPa and that Fe(0.8)Ni(0.2) is stable to at least 255 GPa. The results of this study, in conjunction with work at higher temperatures, will make it possible to address directly the question of the composition of the inner core with a level of certainty that were previously applicable only to the mantle.

The crystal structure of one phase of superconductor Y−Ba−Cu−O Compound is analysed by RASA-5RP Four-circle single crystal diffractometer. It is found that the crystal is tetragonal with parametersa=3.8614 A,b=3.8614 A,C=11.6803 A. In the crystal structure Ba situates at the Wyckoff (0, 0, 0.3094) position, Y at the special position (0, 0, 0), Cu ions occupy another special position (0.5, 0.5, 0.5) and the Wyckoff position (0.5, 0.5, 0.1391) with some substitution by Y and some Oxygen vacancies surrounding Cu and Y.

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.