Neutron scattering studies of the D-O and D-12 cyclohexane under high pressure

Abstract

Preliminary results of neutron diffraction and inelastic neutron scattering studies under high pressure of normal (D-O) and deuterated (D-12) cyclohexane are presented.     

Neutron diffraction studies of hydrogen-bonded ices at high pressure

Abstract

The development of the Paris-Edinburgh cell has extended the pressure range for neutron diffraction by a full order of magnitude to 30 GPa. This advance has provided a wealth of new information on the high-pressure behaviour of ices such as water-ice, ammonia, methane and hydrogen sulphide. We have now put in place technique developments to permit loading of water-gas mixtures into the Paris-Edinburgh cell.     

Neutron scattering study of the phase transition in s-triazine at high pressure

The elastic transition in s-triazine (C₃N₃H₃) from a trigonal ($\text{R}\text{3}\text{c}$) high temperature (low pressure) structure to a monoclinic (C2/c) low temperature (high pressure) phase has been investigated at pressures up to 5 kbar using neutron scattering techniques. Neutron diffraction was used to measure the pressure dependence of the order parameter and inelastic scattering to study the softening of the transverse acoustic phonon modes on three isotherms. In both cases the effect of pressure on the transition is found to be described primarily by that on the temperature of the transition.     

Neutron scattering studies of simple fluids

Summary Neutron scattering may be used in a variety of ways to study simple fluids, and four examples will be given. In the first the nature of the forces causing the structure of noble-gas fluids will be examined. Secondly the structure of homonuclear diatomic liquids will be considered including studies of molecular bonding. Thirdly, the structure of a relatively complicated atomic fluid, sulphur, will be reviewed. And in the fourth the properties of a model Lorentz gas—a dilute mixture of hydrogen in fluid argon—will be described. These examples show both the power and diversity of neutron results, especially when they are combined with accurate computer simulations of these fluids. Proposals for future experiments in this field will be discussed. Paper presented at the workshop ?Highlights on Simple Liquids?, held in Turin at ISI on 1–3 May, 1989.     

Dynamics of hydrogen-bonded water networks under high pressure: Neutron scattering and computer simulation

Abstract

Molecular dynamics simulation of proton-ordered high pressure ice modifications II and IX was performed. Dynamics of both isotope varieties, H₂O and D₂O, was simulated. Rectangular simulation box of ice II contained 576 and that of ice IX 768 molecules. The average kinetic energy corresponded to 82 and 201 K for ice II and to 87 and 203 K for ice IX. One-phonon densities of states were calculated via Fourier transformation of velocity autocorrelation functions and compared with those found experimentally from inelastic incoherent neutron scattering. This characteristic was calculated for all the molecules, as well as for the molecules of a particular crystallographic type. Both simulated ice modifications contain molecules of two different structural types. Dynamic characteristics of molecules of different types are slightly different. Splitting of the librational peak at about 60–70meV observed in the ice II experimental spectrum is mainly due to such difference. In the case of ice II simulated spectra reproduce experimental ones quite reasonably in the whole range of energies, while in the case of ice IX agreement with the experiment is worse.     

Light scattering studies of oleic acid under high pressure

Abstract

Elastic light scattering of oleic acid has been studied at room temperature and at pressures up to 0.5 GPa, where the strong scattering of radiation in the visible region occurs. For the application of pressure above 0.21 GPa the strong increase of the scattered light intensity has been observed some seconds after an application of pressure. More detailed analysis shows that Me theory is not sufficient to explain experimental results for size of Scattering particle greater than 18 μm.     

Neutron powder diffraction studies at high pressure using a pulsed neutron source

Abstract

High-pressure neutron-diffraction experiments using the time-of-flight (TOF) method are presented. We first introduce the helium-gas pressure system at Argonne's Intense Pulsed Neutron Source (IPNS) with the Special Environment Powder Diffractometer (SEPD). The results for TIBa₂CuO₆₊₈, and La_2?x, Sr_x, CuO_4–8 are briefly reviewed. We then introduce the present situation in Japan. We have been developing a high-pressure system on high-resolution powder diffractometer (HRP) which was much improved recently and renamed VEGA at KEK. The best resolution of 0.45% was obtained at VEGA. The preliminary results for YBa₂, Cu₄O₈, and HgBa₂, CuO₄₊₈, are presented and compared with the SEPD data.     

Vibrational spectra of high- and low-density amorphous ice,ice Ic,and ice Ih by neutron scattering

Abstract

The vibrational spectra of high- and low-density amorphous ice made ice Ic and Ice Ih have been obtained at 15 K using inelastic incoherent neutron scattering     

Nature of the structural transformations in B2O3 glass under high pressure

We study high-pressure polyamorphism of B2O3 glass using x-ray diffraction up to 10 GPa in the 300-700 K temperature range, in situ volumetric measurements up to 9 GPa, and first-principles simulations. Under pressure, glass undergoes two-stage transformations including a gradual increase of the first B-O (O-B) coordination numbers above 5 GPa. The fraction of boron atoms in the fourfold-coordinated state at P<10 GPa is smaller than was assumed from inelastic x-ray scattering spectroscopy data, but is considerably larger than was previously suggested by the classical molecular dynamics simulations. The observed transformations under both compression and decompression are broad in hydrostatic conditions. On the basis of ab initio results, we also predict one more transformation to a superdense phase, in which B atoms are sixfold coordinated.     

Lattice vibrational spectra of vitreous silica densified by pressure

The results of the measurement of Raman scattering spectra and infra-red absorption spectra of densified vitreous silica are reported. Raman active-low frequency vibrations are largely changed by compression.     

Neutron scattering studies of magnetic properties of actinide systems

In this article we review neutron scattering studies of the magnetic properties of actinide systems over the last 15 years, with particular emphasis on the work since 1984 and the reviews by Rossat-Mignod et al. and Buyers and Holden. In section 2 the results obtained on the actinide dioxides UO₂, NpO₂ and PuO₂ at spallation sources and the recent observation of intermultiplet transitions in UPd₃ and UPt₃ are presented. In section 3 a discussion is given of magnetization densities, starting with the almost localized system such as PuSb and continuing to more recent work on itinerant intermetallic systems such as UNi₂, UFe₂ and PuFe₂. In these latter systems the ratio μ_L/μ_S of the orbital and spin moments may be determined by neutrons and compared to theory. In section 4 we present work on compounds with the NaCl crystal structure. This includes complex magnetic phase diagrams, as found for example in UAs and NpAs, the discussion of the results of critical scattering experiments, and neutron inelastic experiments on ferromagnets such as PuSb and UTe. In section 5 the work on single crystals of U-containing heavy fermions is presented. In these systems neutron scattering has been able to characterize the small magnetic moments and the nature of the magnetic correlations, which are frequency dependent. A summary of the significant progress and problems remaining is given in section 6.     

Neutron scattering studies of the reentrant spin-glass system PdFeMn

Small angle neutron scattering (SANS) and inelastic neutron scattering studies were performed on a polycrystalline sample of (Pd_{9 9.65}Fe_0.35)₉₅Mn₅. This system exhibits a sequence of phase transitions from a paramagnetic to ferromagnetic state and then a transition to spin-glass-like behavior near T_SG = 3.0 K. The temperature dependence of the SANS exhibits a peak at T_c = 9.0 K and then a Q-dependent increase at low temperatures. This low temperature behavior contrasts sharply with that observed at low temperatures for a normal ferromagnet. No spin waves were observed within the ferromagnetic phase for the momenta values probed. Instead, a broad (FWHM ～ 5.0 meV) quasielastic distribution which was almost temperature independent was observed. This scattering was interpreted as a measure of the distribution of the magnetic energies of the Mn²⁺ ions.     

Chemical transformations of benzene under high pressure and at high temperature

Abstract

In order to understand the chemical transformations of organic compounds composed of aromatic rings under high pressure and at high temperature, we have studied the benzene behavior up to 25 GPa and 550°C by Raman scattering. The chemical transformations and the phase diagram have been established. This point was worth investigating because the phases of benzene before the chemical transformations influence the characteristics of the recovered compound at atmospheric pressure.     

DFT studies on the vibrational and electronic spectra of acetylsalicylic acid

The following is a theoretical and experimental study on the vibrational and electronic properties of acetylsalicylic acid (ASA). Vibrational information was obtained by FT-IR and Raman spectroscopy which agree well with harmonic vibrational frequency calculations. The calculations were carried out using density functional theory B3LYP methods with 6-311G** and LANL2DZ basis sets. The vibrational assignments were calculated by Gaussview. Absorption UV-Vis experiments of ASA reveal three maximum peaks at 203, 224 and 277 nm, which are in agreement with calculated electronic transitions using TD-B3LYP/6-311G**.     

Investigations of pressure induced structural phase transformations in pentaerythritol

We have investigated the pressure induced structural changes in pentaerythritol {2,2-bis-(hydroxymethyl)-1,3-propanediol} with the help of X-ray diffraction studies. Our results show that this compound undergoes transformations to a lower symmetry phase between 5.2-5.9 GPa. It further undergoes phase transformations at ∼8.5 and ∼11 GPa; eventually evolving to a disordered phase beyond 14-15 GPa in agreement with our earlier Raman studies. On release of pressure from 18.5 GPa, the compound transforms back to the initial tetragonal phase.