High-pressure radial X-ray diffraction study of osmium to 58 GPa

Nonhydrostatic compression behavior of osmium (Os) was investigated up to 58.2 GPa using radial X-ray diffraction (RXRD) together with lattice strain theory in a diamond-anvil cell. The apparent bulk modulus of Os derived from RXRD data varies from 262 GPa to 413 GPa, depending on Ψ, the orientation of the diffraction planes with respect to the loading axis. Fitting to the third-order Birch-Murnaghan equation of state, the RXRD data obtained at Ψ = 54.7° yields a bulk modulus K₀ = 390 ± 6 GPa with pressure derivative K^′ ₀ fixed at 4. The ratio of differential stress to shear modulus t/G ranges from 0.024 to 0.029 at the pressures of 15.7–58.2 GPa. The yield strength was observed to increase with compression and reach the value of 11.7 GPa at the highest pressure. This confirms that Os is the strongest known pure metallic material compared with the reported stiff elemental metals such as W, Mo and Re. It was found that the apparent c/a ratio changed with the nonhydrostatic compression, as well as the orientation Ψ in our experiments. Moreover, the aggregate moduli of Os at high pressure were determined from the RXRD measurements.     

High pressure X-ray diffraction experiments on NpS and PUS up to 60 GPa

Abstract

Neptunium and plutonium monosulfides were studied under high pressure up to ～60 GPa using a diamond anvil cell in an energy dispersive X-ray diffraction facility. The compounds, of cubic rock salt structure type at ambient pressure, do not show any crystallographic phase transition in the domain of investigation. From the pressure-volume relationship, we determined bulk moduli of 92 and 120 GPa with pressure derivatives of 4.6 and 4.1 for NpS and PUS respectively.     

In situ X-ray diffraction study of germanium at pressures up to 11 GPa and temperatures up to 950 K

In situ X-ray diffraction measurements on germanium were conducted in the pressure range of 5-11 GPa and temperatures up to 950 K. Using our data a better defined P-T diagram for germanium is presented. The coordinates of the triple point between Ge_I-Ge_II-Ge_L have been determined to a better degree of precision. The onsets of the Ge_I-Ge_II transition were found both under hydrostatic and non-hydrostatic conditions. Anisotropy of thermal expansion coefficient for the Ge_II is characterized from the c/a ratios in the temperature interval 473-823 K. Phases Ge_III and Ge_IV are shown to be metastable forms of germanium.     

X-ray diffraction study of molybdenum diselenide to 35.9 GPa

In situ high-pressure angle dispersive synchrotron X-ray diffraction studies of molybdenum diselenide (MoSe₂) were carried out in a diamond-anvil cell to 35.9 GPa. No evidence of a phase transformation was observed in the pressure range. By fitting the pressure-volume data to the third-order Birch-Murnaghan equation of state, the bulk modulus, K_0T, was determined to be 45.7±0.3 GPa with its pressure derivative, K′_0T, being 11.6±0.1. It was found that the c-axis decreased linearly with pressure at a slope of −0.1593 when pressures were lower than 10 GPa. It showed different linear decrease with the slope of a −0.0236 at pressures higher than 10 GPa.     

An X-ray diffraction study of liquid chlorine

The static structure of liquid chlorine at room temperature is investigated by means of X-ray diffraction. From comparisons with previous neutron and X-ray data, it appears that most of the discrepancies present in the liquid structure factor between neutron and X-ray studies have been removed. Some experimental difference in the two structure factors still remains and one possible explanation is that the electron density distribution of liquid chlorine differs from that derived within the independent atom approximation.     

Quasi-isentropic compression of liquid argon up to 500 GPa

The compressibility of liquid argon up to pressures ∼500 GPa has been investigated experimentally. The argon was compressed by a cylindrical shell accelerated by the detonation products of an explosive. The density was recorded by the gamma-graphic method and the pressure was determined from the gas-dynamic calculations. Comparing the experimental and computational results showed that the compression process studied is isentropic to a quite high degree. The compression of liquid argon up to a density of 7.3 g/cm³ did not show any clear anomalies associated with a structural transition or metallization. Zh. éksp. Teor. Fiz. 111, 2099–2105 (June 1997)     

X-ray study of xenon to 23 GPa

X-ray diffraction photographs of polycrystalline xenon in a diamond- anvil cell show that the crystal structure of Xe remains fcc up to at least 23 GPa at 298 K. The atomic volumes are in good agreement with earlier x-ray data at lower pressures and with calculated volumes along the O-K isotherm.     

Structure of liquid iron at pressures up to 58 GPa

We report structural data on liquid iron at pressures up to 58 GPa measured by x-ray scattering in a laser heated diamond anvil cell. The determined structure factor preserves essentially the same shape along the melting curve. Our data demonstrate that liquid iron at high pressures is a close-packed hard-sphere liquid. The results place important constraints on the thermodynamic and transport properties of liquid iron and the melting curve of iron.     

Properties of shock-compressed liquid krypton at pressures of up to 90 GPa

The following quantities of shock-compressed liquid krypton are measured behind a plane shock front at pressures up to 90 GPa: compressibility up to densities of 7 g/cm³, brightness (color) temperatures of 6000–24000 K, and electrical conductivities of 40–60000 (Ω·m)⁻¹. X-t diagram methods are used to estimate sound speeds of up to 5.5 km/s at pressures of 30–75 GPa. The optical absorption coefficients in the violet and red (30–300 cm⁻¹) are measured at pressures of 20–90 GPa from the rise in brightness of the shock front luminosity. The optical reflection coefficient of the shock front (∼13%) at a pressure of 76.1 GPa is measured for the first time. Zh. éksp. Teor. Fiz. 116, 551–562 (August 1999)     

X-ray diffraction study of phase transition of ferroelectric liquid crystal DOBAMBC

Cu₆PS₅Br superionic crystals were implanted by sulphur ions. The effect of ion implantation on the phase transition (PT) temperatures in Cu₆PS₅Br is observed from isoabsorption studies of the optical absorption edge. The dependences of the ferroelastic PT temperature on the ion fluence are analysed.     

Equation of state of 7LiD up to 6 GPa from neutron powder diffraction experiments

Abstract

The equation of state of ⁷LiD has been measured at 300 K up to 6GPa by 90° time-of-flight neutron scattering methods, in a novel cell with toroidal anvils. The increased precision over previous data allows to check theoretical calculations on lithium hydrides.

L'équation d'état de ⁷LiD a été établie à 300 K jusqu'à 6 GPa par des mesures de durée de vol de neutrons diffractés à 90°, dans une cellule à enclumes toroidales de conception originale. La prkision de ces mesures permet un test significatif des calculs théoriques sur les hydrures de lithium.     

High-pressure Raman study of solid hydrogen up to 300 GPa

The high-pressure behavior of solid hydrogen has been investigated by in situ Raman spectroscopy upon compression to 300 GPa at ambient temperature. The hydrogen vibron frequency begins to decrease after it initially increases with pressure up to 38 GPa. This softening behavior suggests the weakening of the intramolecular bond and the increased intermolecular interactions. Above 237 GPa, the vibron frequency softens very rapidly with pressure at a much higher rate than that of phase III, corresponding to transformation from phase III into phase IV. The phase transition sequence has been confirmed from phase I to phase III and then to phase IV at 208 and 237 GPa, respectively. Previous theoretical calculations lead to the proposal of an energetically favorable monoclinic C2/c structure for phase III and orthorhombic Pbcn structure for phase IV. Up to 304 GPa, solid hydrogen is not yet an alkali metal since the sample is still transparent.     

X-ray diffraction study of a sequence of phase transitions in Cs2HgCl4 crystals

The temperature behavior of lattice parameters and diffraction patterns of the reciprocal lattice in Cs₂HgCl₄ crystals is studied by x-ray diffraction in the temperature range from 4.2 to 300 K. A sequence of phase transitions is observed and attributed to the evolution of incommensurate and commensurate modulations along the crystallographic a and c axes of a unit cell in the initial Pnma structure.     

X-ray magnetic circular dichroism measurements in Ni up to 200 GPa: resistant ferromagnetism

The structural stability of fcc Ni over a very large pressure range offers a unique opportunity to experimentally investigate how magnetism is modified by simple compression. K-edge x-ray magnetic circular dichroism (XMCD) shows that fcc Ni is ferromagnetic up to 200 GPa, contradicting recent predictions of an abrupt transition to a paramagnetic state at 160 GPa. Density functional theory calculations point out that the pressure evolution of the K-edge XMCD closely follows that of the p projected orbital moment rather than that of the total spin moment. The disappearance of magnetism in Ni is predicted to occur above 400 GPa.     

Application of X-ray resonant diffraction to structural studies of liquid crystals

Liquid crystals are soft materials that combine the fluidity of disordered liquids and the long range orientational or positional order of crystalline solids along one or two directions of space. X-ray scattering is widely and generally successfully used to investigate and characterize the microscopic structure of most liquid crystals. In many cases however, the Bragg reflections are forbidden by special symmetries of the unit cell and the low dimensional structure of the liquid crystalline phases are out of reach of conventional X-ray experiments. We show in this paper that this problem can be overcome by resonant scattering of X-rays as it reveals the anisotropy of the tensor structure factor. We review various examples in which the restored forbidden reflections reveal unambiguously the hidden structure of liquid crystalline phases. Moreover, we show that in some cases, a fine analysis of the polarization of the Bragg reflections enables one to discriminate between different structural models. These studies solved long standing questions about biaxial liquid crystal structures and provided new insights into physical phenomena such as supercritical behaviour or commensurate-incommensurate transitions.     

X-ray diffraction studies of N2 up to 43. 9 Gpa

Abstract

High pressure x-ray diffraction studies at room temperature indicate the δ → ? phase transition at 16. 3 GPa. The room temperature isotherm is determined and compared with the results of a Priori calculations.     

Magnetic neutron diffraction under pressures up to 43 GPa. Study of the EuX and GdX compounds

We present some recent results on magnetic neutron studies at pressures above 40 GPa. We used diamond and sapphire anvil pressure cells for low temperature neutron studies and a powder diffractometer specialized for high pressure studies. Focusing systems and low-background conditions allowed us to study samples with volumes as small as 0.001 mm³ in a diamond anvil cell at a pressure of 50 GPa. This technical progress allowed us to obtain new results in studies of “textbook” magnetic insulators EuX (X = S, Se, Te) and GdX (X = As, Sb, Bi). We found new magnetic phases. In the Eu-chalcogenides we observed a strong anion-dependent increase of the ferromagnetic exchange with decreasing lattice constant. In contrast, in the GdX compounds the antiferromagnetic order remains stable up to a pressure of 43 GPa. We show the importance of our results to understand the origin of indirect exchange interactions in magnetic semiconductors with well localized magnetic shells. This revised version was published online in July 2006 with corrections to the Cover Date.