This thesis is a study of the structure and dynamics of crystalline benzene across its solid temperature range at ambient pressure, using the techniques of molecular dynamics simulation and neutron powder diffraction. The simulations cover a wide range of properties, making comparison with the large body of experimental data that is available for benzene. A rescaling of an <i>ab initio</i> potential from the literature is proposed, and this rescaled potential is shown to give structural parameters in good accord with experiment, especially for the temperature dependence. The melting point and elastic compressibilities are determined for the model and it is found that, in the simulations, the crystal can be superheated up to the temperature of an elastic lattice instability. The implementation of a molecular dynamics program on a large array of transputers is described. The neutron powder diffraction experiment was performed primarily to study the crystal structure close to the melting point, in order to investigate reported premelting phenomena. The results contradict a previous study, but effects are seen that can be explained, using the simulations, as precursors of a form of disordering transition. The simulations predict that the full transition should be observable experimentally by the application of pressure, as this would increase the temperature range over which the solid is stable. Librational spectra are obtained from the simulations, and these indicate an explanation for some experimental results. Finally, the nature of reorientational motion about the sixfold axis of the molecule is considered. The model is shown to reproduce well the experimentally determined temperature and pressure dependence of the rate. A detailed study of the correlation of reorientation events is made, in a much greater detail than is possible experimentally. Evidence is found for only a very weak local correlation, which is a subject over which there has been considerable debate.

This thesis presents an investigation of the diffraction of x-rays by the types of disordered grating structure which are commonly present on near-perfect crystalline surfaces. Models to calculate the diffraction are developed with the intention of interpreting experimental measurements in terms of both the macroscopic grating structure and the atomic structure. The observability of interference effects from grating structures illuminated by partially coherent radiation is discussed. General kinematical expressions are developed describing the three-dimensional x-ray scattering profile associated with vicinal surfaces with terrace length disorder. Scattering profiles are calculated for specific examples of vicinal surfaces with statistically independent terrace lengths and correlations between terraces are discussed. The analysis of crystal truncation rod (CTR) intensities from vicinal surfaces is considered. The diffraction of x-rays by a reconstructed vicinal surface is investigated for the specific case of the Ge(001) (2x1) surface. It is proposed that an interference modulation of the reconstruction scattering can occur due to ordering in the registration of the reconstructed unit cells on different terraces; a model is developed to simulate the scattering profile. Subdomain structure due to lower symmetry reconstructions is also treated. Data from several Ge(001) surfaces is presented and interpreted in terms of the scattering models. An experimental study of the order-disorder <I>c</I>(4x2) → (2x1) phase transition in the Ge(001) surface reconstruction is described.

No description available.

The work presented in this thesis applies advanced high-resolution powder diffraction techniques in combination with novel single-crystal diffraction techniques, recently developed in the Edinburgh High-Pressure Group, to studies of complex HP phases of group-V elements and Ga. The HP phases Bi-III and Sb-II, a subject of many experimental studies in the past and present, are shown to have an incommensurate host-guest structure, similar to that found in alkaline-earth elements. The basic host-guest structure is shown to be modulated, and a single-crystal diffraction study on Bi-III has revealed the true from of the modulation between the host and guest components. From HP powder diffraction studies of Sb, a new incommensurate host-guest phase Sb-II* is discovered, and a novel incommensurate to incommensurate phase transition is observed. The HP phase As-III is solved from powder diffraction data as a modulated incommensurate host-guest structure, and is shown to be the same as Sb-II*. Four-dimensional formalism is applied to the crystallographic description of these incommensurate phases. The group-III element Ga is known to have a complex HP behaviour with metastable effects and several phase transitions. The structure of the HP phase Ga-II was long believed to be cubic with 12 atoms in the unit cell but is shown from single-crystal diffraction data to be orthorhombic with 104 atoms in the unit cell. A new HP phase is discovered to be stable in Ga above the Ga-II phase, and its structure is solved from powder diffraction techniques as rhombohedral with 6 atoms in the unit cell, which represents a new structure type of an element. A revised phase diagram of Ga is proposed.

Diffraction techniques - incorporating both x-rays and neutrons, single-crystals and powders - have been developed to allow crystal structures to be determined at high pressures with high accuracy. For single-crystal neutron-diffraction studies, an optimised data collection strategy has been developed for use with a sapphire-anvil pressure cell and a position-sensitive detector and, combined with a newly developed clamp-type pressure cell, has allowed both the scope and accuracy of high-pressure structural studies using neutron-diffraction techniques to be extended. Application of the new techniques to high-pressure structural studies of H-ordering systems of the KH<SUB>2</SUB>PO<SUB>4</SUB>-type, strongly suggests that the H-atom site separation, δ, is a strong determinant of the ordering temperature T_c such that a hydrogenous material and its deuterated analogue have the same T_c, within error, at the same δ. The results also suggest that the differences in T<SUB>c</SUB> between different H-ordering systems are determined by the differences in δ, and that δ in all systems tends to im0.22pr and T<SUB>c→OK. For single-crystal x-ray diffraction studies using a diamond-anvil cell, analysis has shown that removal of intensity from either the incident or diffracted beams by simultaneous diffraction of the diamonds can reduce the intensity of sample reflections by up to 50%. A data collection strategy to detect this effect has been developed. The use of tungsten as a gasket material has also been investigated, and provides a possible solution to the long-standing problem of using AgK</SUB>α radiation for single-crystal studies. Application of these newly-developed techniques to a structural study of the high-T<SUB>c</SUB> superconductor YBa<SUB>2</SUB>Cu<SUB>4</SUB>O<SUB>8</SUB>, illustrates the improved accuracy now available using single-crystal x-ray diffraction techniques and AgKα radiation. The results of this study cast considerable doubts on the accuracy of previous high-pressure structural studies of high-T_c superconductors. Work has also been done on developing powder-based techniques. These show considerable promise for accurate structural studies to very high pressures in the future.

This work describes and compares the crystal structures of diaslereomeric pairs of compounds which could result from resolutions of neutral amino acids by both mandelic acid (MAN) and camphor-10-sulphonic acid (CSA). Chapter 2 details experimental methods used. Tables list the outcomes of NMR and mp analysis of crystals to suggest whether or not they were the desired products. Section 2.13 describes the conventions employed for the labelling of atoms. Chapter 3 describes the determination of crystal structures of six MAN-amino acid complexes, and experimental data which suggested whether a 1:1 compound had been made in these and other attempts. Crystallographic data and tables listing atomic coordinates and thermal parameters are included in each of the six sections describing structures. Chapter 4 details evidence that MAN molecules were undissociated. The molecular conformations are compared with those of other known structures, and the molecular packing arrangements; hydrophobic interactions; and hydrogen bonding schemes are given. Only 2 pairs of diastereomcric complexes were obtained. Tables listing bond lengths, angles and torsion angle are given at the end of the chapter. Chapter 5 adopts the same format as chapter 3 and describes 5 simple salts and 4 amino acid salts of CSA. Progress towards the unsuccessful preparation and structure determination of other amino acid-CSA salts is described in section 5.10. The absolute configuration of (+ )-CSA was verified as described in section 5.1. Chapter 6, like chapter 4, is about molecular conformations, molecular packing arrangements and hydrogen bonds. A single pair of diastereomcric salts was prepared successfully to allow comparison of their crystal structures (MET/CSA). The structure of one other salt, PHG/CSA, was particularly important, since the molecular recognition that takes place is quite clear cut and results in the absolute preference for one diastcreomcr, R-PHG-1S-4R-CSA, over the other, and indeed makes CSA a particularly good resolving agent for PHG. The source of conformational comparisons between CSA molecules was the simple salts prepared in this work. Tables listing bond lengths, angles and torsion angles are to be found at the end of the chapter. Packing diagrams are included at the end of section 6.3 and part packing diagrams showing only the hydrophilic parts of the structures in section 6.4. Chapter? analyses some resolution attempts, in which powder diffraction was the major tool. Photocopies of powder photographs are included. The results of resolutions were examined in the light of an understanding of the numbers of hydrogen bonds and the packing arrangemenls determined in the single crystal work.

The martensitic transition in zirconium is a first order solid-to-solid transition which transforms from body-centred-cubic (bcc) to hexagonal close-packed (hcp) structure. By using a Finnis-Sinclair type many-body potential derived for zirconium and molecular dynamics (MD) methods, a large number of simulations have been studied with the implementation of Nosé-Hoover thermostat and Parrinello-Raman scheme. We found that the transition is a result of the instability of a transverse N-point phonon in the bcc lattice which can be stabilised by the large fluctuation of the anharmonic effect above the transition temperature via extra vibrational entropy. The transition temperature in our calculations is 1,333K. With the concept of 'local atomic structure', the microstructure can be studied at the atomic level. The kinetics of the transition is dominated by the strain energy. Stripes of (10?1) twins are formed as a consequence. The twins contain some stacking faults. Stacking faults play a major role in the splitting of partial dislocations in twins. After the martensitic microstructure is deformed, we found that the twinning deformation occurs by the aid of partial dislocations and the interchange of neighbouring atoms. The interchange of neighbouring atoms leads to the term 'plastic atoms'. We believe that these plastic atoms cause a microscopic irreversible process and the absent of the shape memory effect in zirconium.

Systems using Ferroelectric (F) Liquid Crystal over Silicon (LCoS) Spatial Light Modulators (SLMs) have been limited, in the past, by the poor optical performance of the devices. The high degree of backplane flatness and hence LC layer thickness uniformity required have been very difficult to achieve due to stresses induced in the silicon wafer as a consequence of CMOS processing. Mirror quality and LC alignment are also major factors in determining the (F)LCoS SLM optical efficiency and contrast ratio. A process of forming "thin" high-quality mirrors was developed, using an intermediate metal chemical mechanical polishing process (CMP). An associated process was also developed in which the thin mirrors were fabricated flush with the surrounding oxide, using a novel interpixel gap fill process (flat at 2.5nm) which allowed a major improvement to be made in the liquid crystal flow front, during cell filling. We have successfully demonstrated a technique for reducing the backplane warpage from 3.0λ down to λ/8, over a die, thus improving the LC layer thickness uniformity. The robust silicon dioxide spacers used in the die flattening experiments, above, were fully characterized and the deposition and patterning process optimized to consistently provide spacer layer thickness uniformities of <1% over a 75mm wafer. Issues relating to the transparent ITO layer were addressed. We have shown that the surface roughness of the granular "in-house" deposited ITO can be reduced by ~40% (on a submicrometer scale) using a slight variant of the oxide CMP process. The optical performance of LC devices was also found to be improved by optimizing the ITO and SiO layer thickness. Reflections from the ITO coated glass have been reduced by approximately 10% by optimizing the ITO layer thickness for a particular wavelength.

Homogeneous alignment of liquid crystal (LC) molecules is fundamental to the fabrication and optimum operation of LC devices. As a consequence of employing alignment layers with directional properties, the alignment of LC molecules is promoted. The most commonly used technique to fabricate LC alignment layers is by mechanically rubbing polymer films. However, reductions in production costs and further advancements in LC technology are now being hindered by the rubbing technique and so either a new technique or monitoring of the current technique is required. In these studies, a monitor of the rubbing technique and of the potential replacement techniques is demonstrated. By constructing a surface sensitive technique, traditionally used to monitor semiconductor growth, and applying it to alignment layers, the first uses of reflection anisotropy spectroscopy (RAS) to monitor the fabrication of LC alignment layers are presented. This technique, in conjunction with atomic force microscopy (AFM), has been successfully used to study a number of different variables of the rubbing technique and as an <i>in-situ</i> real time monitor of the photoalignment and atomic beam etching techniques. Of these, the etching technique has shown the most potential to replace mechanical rubbing yet it is probably the least understood. An interesting by-product of these alignment layer studies has been the introduction of an extension to the normal RAS technique. By using examples such as doubly rubbed alignment layers, angular dependent RAS (ADRAS) has been shown to be capable of isolating and monitoring optical anisotropy from multiple sources within a single system. To increase understanding of the etching technique, the complex polymer surfaces were replaced by a model system: a Copper single crystal. The complexity of these studies was then increased by the introduction of vicinal surfaces and the adsorption of chiral molecules onto the Cu(110) surface. Both of these changes alter the characteristic spectra but the latter also has potential to allow further uses of ADRAS as the chiral molecule is known to lie at a specific angle to the rows of Copper atoms.

I consider three different, but important, types of anisotropic material: First, for transverse isotropic media with a vertical axis of symmetry (TIV), I investigate the effects of dip on the travel time for imaging purposes. Secondly, transverse isotropic media with a horizontal axis of symmetry (TIH) are used as a model for a vertically fractured materials; I extend existing techniques for fracture strike estimation to dipping layers. Finally, for a combination TIV and TIH media, which represents orthorhombic symmetry, analysis of the AVO behaviour leads to an inversion procedure for crack density. For a dipping TIV layer I show that the travel time can be decomposed into structure- and anisotropy-independent parts. A thorough investigation using synthetic seismograms reveals that this separation is valid for up to 15% P-wave anisotropy and dip angles of up to 20 degrees. I propose an additional processing step to enhance seismic data if anisotropy and dip both are present. I extend an estimation method for the fracture strike in a TIH medium to dipping layers. An analytical analysis shows that the application of a non-linear global optimization scheme is required to invert the strike direction satisfactorily. This new method is applied to a real data set, and the general trend of the regional strike direction is confirmed. The third type of anisotropy (orthorhombic), is investigated its dynamic response. A derivation of all pure and mode-converted reflection coefficients is followed by a separation approach to extract the crack-dependent contribution. I demonstrate that a special acquisition geometry can lead to the inversion of the parameter crack density.