Biomedical subjects
Publications and source records attributed to R J Nelmes.
We present a neutron diffraction study of liquid water to 6.5 GPa and 670 K. From the measured structure factors we determine radial and angular distributions. It is shown that with increasing density water approaches a local structure common to a simple liquid while distorting only a little the tetrahedral first-neighbor coordination imposed by hydrogen bonds that remain intact.
After a pressure decrease to ambient, the high-pressure SC16 phase of GaAs is found to transform to the hexagonal wurtzite structure. This has been suggested for GaAs in calculations but never previously observed experimentally. Wurtzite-GaAs is found to be stable at ambient pressures at temperatures up to 473 K, with a structure that is only slightly distorted from ideal. On recompression, the ratio is constant with pressure and wurtzite-GaAs transforms to the orthorhombic phase at 18.7(9) GPa.
We present a neutron diffraction study of the transition between low-density and high-density amorphous ice (LDA and HDA, respectively) under pressure at approximately 0.3 GPa, at 130 K. All the intermediate diffraction patterns can be accurately decomposed into a linear combination of the patterns of pure LDA and HDA. This progressive transformation of one distinct phase to another, with phase coexistence at constant pressure and temperature, gives direct evidence of a classical first-order transition. In situ Raman measurements and visual observation of the reverse transition strongly support these conclusions, which have implications for models of water and the proposed second critical point in the undercooled region of liquid water.
Ga-II, the stable phase of Ga between 2 and 10 GPa at room temperature, is shown to have a complex 104-atom orthorhombic structure. A new phase, Ga-V, is found between 10 and 14 GPa, with a rhombohedral hR6 structure. Ga-II has a modulated layer structure like those recently reported for Rb-III and Cs-III, with similar 8- and 10-atom a-b layers stacked along the c axis in the sequence 8-10-8-8-10-8-8-10-8-8-10-8. The cI16 structure of Li and Na can be understood as a stacking of very similar 8-atom layers. It is suggested that a Hume-Rothery mechanism contributes to the occurrence of these complex structures in such different metals.
The Bragg peaks from the structure formed by the guest chains in the incommensurate composite structure of Rb-IV are all found to broaden strongly at pressures below 16.7(1) GPa. This signals a loss of the interchain correlation. At the lowest reachable pressure before the transition to Rb-III, 16.2 GPa, the correlation length is only approximately 30 A, or 4 times the interchain distance. There is also evidence of a loss of long-range order within each chain. The chains thus exhibit the onset of the characteristics of an ordered 1D liquid.
We report in situ neutron diffraction studies of high-density amorphous ice (HDA) at 100 K at pressures up to 2.2 GPa. We find that the compression is achieved by a strong contraction ( approximately 20%) of the second neighbor coordination shell, so that at 2.2 GPa it closely approaches the first coordination shell, which itself remains intact in both structure and size. The hydrogen bond orientations suggest an absence of hydrogen bonding between first and second shells and that HDA has increasingly interpenetrating hydrogen bond networks under pressure.
The crystal structure of Rb-III, stable between 13 and 17 GPa, has been determined from quasi-single-crystal x-ray diffraction data. It is orthorhombic, space group C222(1), with 52 atoms in the unit cell, and has an 8-10-8-8-10-8 stacking of 8- and 10-atom layers. The recently reported 84-atom structure of Cs-III can be understood as an 8-8-10-8-8-8-8-10-8-8 stacking of the same layers. These represent a new class of modulated elemental structures.
The structure of Cs-III, stable between 4.2 and 4.3 GPa at room temperature, has been determined from single-crystal x-ray diffraction data. Rather than the simple fcc structure previously reported [Hall et al., Science 146, 1297 (1964)], the data yield a complex new type of elemental structure which is orthorhombic (space group C222(1)) with 84 atoms in the unit cell. No evidence could be found for the fcc form reported previously, even in a further experiment, conducted under conditions close to those used by Hall et al., which also yielded the 84-atom structure.
The structure of a new methane hydrate has been solved at 3 GPa from neutron and x-ray powder diffraction data. It is a dihydrate in which a 3D H-bonded network of water molecules forms channels surrounding the methane molecules. The network is closely related to that of ice-Ih and the methane-water system appears to be the first in which a cage clathrate hydrate is transformed into an ice-related hydrate (a "filled ice").
Rb-IV is found to have an incommensurate composite structure, comprising a tetragonal host framework and a simple body-centered tetragonal guest. This does not have the unexpectedly short Rb-Rb distances of the previously reported structure [U. Schwarz et al., Phys. Rev. Lett. 83, 4085 (1999)]. The ratio of the c-axis lattice parameters is strongly pressure dependent and approaches the commensurate value of 5/3 at the transition to phase V. A reversible broadening of the guest structure is observed below 16.5(2) GPa.
Methane hydrate is thought to have been the dominant methane-containing phase in the nebula from which Saturn, Uranus, Neptune and their major moons formed. It accordingly plays an important role in formation models of Titan, Saturn's largest moon. Current understanding assumes that methane hydrate dissociates into ice and free methane in the pressure range 1-2 GPa (10-20 kbar), consistent with some theoretical and experimental studies. But such pressure-induced dissociation would have led to the early loss of methane from Titan's interior to its atmosphere, where it would rapidly have been destroyed by photochemical processes. This is difficult to reconcile with the observed presence of significant amounts of methane in Titan's present atmosphere. Here we report neutron and synchrotron X-ray diffraction studies that determine the thermodynamic behaviour of methane hydrate at pressures up to 10 GPa. We find structural transitions at about 1 and 2 GPa to new hydrate phases which remain stable to at least 10 GPa. This implies that the methane in the primordial core of Titan remained in stable hydrate phases throughout differentiation, eventually forming a layer of methane clathrate approximately 100 km thick within the ice mantle. This layer is a plausible source for the continuing replenishment of Titan's atmospheric methane.
A novel approach to the detection and modelling of preferred orientation is presented, based on the interpretation of two-dimensional powder patterns. A simple graphical construction is introduced to aid interpretation, and the application of this construction to some standard diffraction geometries is discussed. It is also shown in outline how a standard preferred-orientation model can be adapted to describe two-dimensional data.
The sample volume in diamond-anvil pressure cells suitable for X-ray powder diffraction studies is very small (</= 100 mum across). The resulting low signal-to-noise ratio has made it very difficult to obtain useful results with monochromatic angle-dispersive techniques, and the alternative white- beam energy-dispersive techniques have limited resolution and generally give unreliable peak intensities. The situation has been transformed recently by the introduction of the image-plate two-dimensional detector, which allows angle-dispersive methods to be used with a greatly increased signal and improved powder averaging. A short review is given of this development, the experimental techniques, and the principal advantages, particularly as found in results obtained at SRS Daresbury over the past two or three years.