Stephen M. Bennington

Transmission electron microscopy, electron diffraction and hardness studies of high-pressure and high-temperature treated C60

TEM and electron diffraction investigations were performed on high-temperature and high-pressure treated polymeric C60 fullerene, prepared at 700, 773 and 800 K in the pressure range of 3-9 GPa, for one hour. A mixed phase of alternating rhombohedral lamellae and tetragonal domains is formed, even at high pressures, with the ratio of polymers being dependent on the pressure applied. At higher temperatures, 900-1173 K, and for a similar pressure range, graphitic `hard carbon phases formed, via an interpolymer interface instability and/or shear mechanism. This phase exhibited a hardness of 10-15 GPa and a high degree of elastic recovery under indentation. Treatment times of 0.5-30 minutes for the graphitic phase showed the kinetics of graphitization to be slow (~1 h) and proceeding via cross-linked intermediates containing trapped whole C60 cages. The influence of the cross-linking resulted in hardness values up to 33.5 GPa, for a twenty-minute treatment.

Tetrahedral order in homologous disaccharide-water mixtures.

The present work aims at evidencing the kosmotrope nature of trehalose through the analysis of inelastic neutron scattering measurements on trehalose and sucrose water solutions at different temperatures. Neutron spectra were collected by using the spectrometer MARI at the ISIS pulsed neutron source of the Rutherford Appleton Laboratory (Chilton, UK). To study the structural modifications induced on the tetrahedral hydrogen-bond network of water by homologous disaccharides, as a first step, the vibrational properties of pure water at different temperatures have been investigated. In particular, the temperature behavior of the intramolecular OH stretching mode has been analyzed. Successively, the vibrational properties for pure water have been compared with those of the sugar water solutions focusing the attention on the tetrahedral network-forming tendency. Finally, the obtained findings have been compared with previous Raman scattering evidences, and the results interpreted in the frame of recent molecular dynamics simulation works.

Two-Dimensional Spin Density Wave State in LaFeAsO

The parent compound of the Fe oxypnictide superconductor, LaFeAsO, has been studied by pulsed neutron powder inelastic scattering measurement. The inelastic scattering intensity along the Q -axis at T =140 K exhibits a prominent asymmetric peak ascribed to the (π,π, l ) magnetic rod, suggesting the two-dimensionality of the spin density wave above the magnetic transition temperature. It persists even in the tetragonal phase up to room temperature, which is well above the magnetic transition temperature, corresponding to two antiferromagnetic sublattices or domains with a strong effective antiferromagnetic interaction J 2 between next-nearest-neighbour Fe magnetic moments in a single Fe square lattice.

In situ diffraction measurement of the polymerization of C60 at high temperatures and pressures

In situ energy dispersive x-ray measurements were performed on C60 fullerene at pressures of 2.6, 5.7 and 12 GPa and temperatures between 300 and 1100 K. The polymerization process was followed in detail and the dynamics of the phase changes measured. This has enabled us to map the P-T phase diagram and look at the dynamics of the phase transformations.

Multiparticle states in the S = 1 chain system CsNiCl3.

A continuum of magnetic states has been observed by neutron scattering from the spin-1 chain compound CsNiCl3 in its disordered gapped one-dimensional phase. Results using both triple-axis and time-of-flight spectrometers show that around the antiferromagnetic point Qc = pi, the continuum lies higher in energy than the Haldane gapped excitations. At 6 K the integrated intensity of the continuum is about 12(2)% of the total spectral weight. This result is considerably larger than the 1%-3% weight predicted by the nonlinear sigma model for the three-particle continuum.

PHONON SOFTENING IN ICE IH

Abstract We present detailed phonon dispersion measurements along the (11.0) and (10.0) directions in a crystal of ice Ih, measured using neutron scattering. The softening of the TA and LA branches are measured as a function of temperature. We show that an instability in the TA-mode is responsible for the negative thermal expansion of ice below 50xa0K.

Study of the ferroelastic transformation in zirconia by neutron diffraction

Abstract Neutron diffraction studies of tetragonal zirconia polycrystals subjected to various applied uniaxial stresses indicate that a ferroelastic crystalline transformation occurs within the bulk of the material. The critical coercive stress is determined to be 1.65 ± 0.03 GPa. The results agree well with previous reports concerning the ferroelasticity of tetragonal zirconia, but they are the first direct evidence indicating that the anomalies observed using the first X-ray diffraction studies originate from the bulk of the ceramic and not solely from the surface. The preferred orientation observed in the tetragonal crystallites was measured as a function of applied uniaxial stress and interpreted via a resolved stress criterion. It is believed that the ferroelastic transformation can act as an energy-absorbing mechanism and hence toughen zirconia or ceramics containing dispersed zirconia.

In situ x-ray diffraction studies of three-dimensional C60 polymers

In situ investigations into the P/T field of C60 fullerene were performed using energy-dispersive x-ray diffraction techniques. Isobars were obtained at 11 and 9 GPa accompanied by isotherms at 750 and 800 K with pressure reaching 13 GPa. The P/T history and pressure isotropy were investigated with the aim of optimizing conditions for 3D polymer formation. Confirmation of the formation of 3D polymers was performed in situ; however, the reclaimed sample did not exhibit the expected high hardness value, due to depolymerization on pressure release. Isotropy in the pressure field promoted formation and retention of the face-centred-cubic structure.

Phonon dispersion curves in KOD-doped ice observed by neutron scattering

We measured the inelastic neutron scattering (INS) of a single crystal of KOD-doped D2O ice, and investigated the dispersion curves in a proton-ordered phase of ice Ih (ice XI) and ice Ih. The intensity maps of the INS show that ice XI and ice Ih have two phonon dispersion curves in 50–65 and 72–90 meV, which assigned to the librational vibrations of water molecules. The results indicate that the phonon dispersion curves caused by the collective motions of protons exist in ice, and that ice Ih has small domains with proton-ordered arrangements.

The structure and dynamics of hard carbon formed from C60 fullerene

Abstract Subjecting C60 to high pressure and temperature reveals a wide range of new carbon structures ranging from polymeric to amorphous diamond-like phases. When C60 is compressed at 3xa0GPa and heated to 700°C it produces a form of carbon that is semimetallic and has a hardness that is approximately two-thirds that of diamond. From elastic and inelastic neutron scattering measurements and transmission electron microscopy we propose that this phase is composed of regularly warped graphite planes, much like corrugated cardboard or iron.