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This thesis is primarily concerned with the growth of bulk cadmium telluride (CdTe) crystals. The work relates to a modified physical vapour transport system which also enables the growth of ternary ll-VI compounds, in particular cadmium zinc telluride (CdZnTe). A computer simulation to model the CdTe growth process is developed, along with a system to measure the partial pressures in situ during growth. The modified Multi Tube Physical Vapour Transport (MTPVT) system, which is essentially a combination of the Markov annulus and Rosenberger flow restrictor designs, consists of two source tubes and a growth tube connected by a crossmember. This crossmember, which is optically heated, contains two capillaries which act as flow restrictors, and allows physical displacement, and therefore better thermal decoupling, of the source and growth areas. The growth of CdTe in the MTPVT system is investigated, in particular, factors affecting the optimal design for the growth tube. The design of the pedestal, on which the seed crystal is located, is of prime importance in preventing polycrystalline growth, and the size and shape of the seed crystal is also critical. Increasing the inner diameter of the growth tube from 32 mm to 52 mm reduces the effect of the annulus gap around the seed, and increases the controllability of the growth process. A computer simulation is used to model the vapour flow from the subliming source, through the capillary, and to the growing crystal and down the annulus. The trends predicted closely match those obtained experimentally, and imply, as expected, the growth process is controlled by the mass transport rate through the capillary. Further comparison with experiment gives an upper limit of 0.45 eV for the activation energy of the incorporation of atoms into the CdTe seed, although no lower limit can be set. The partial pressure - optical density relationships are derived for Cd(_g), Zn(_g) and Te2(_g), with the 214 nm absorption line of Zn(_g) observed to deviate significantly from Beer’s Law. Optical monitoring during CdTe growth by a computer controlled optical absorption measurement system allows in situ monitoring of the partial pressures. The measurements of the source side partial pressures match those predicted by the computer simulation, with the partial pressure ratio of around 1.7 also consistent with the model.

In the crystal packing of molecules wherein a single bond links aromatic groups, a change in the torsion angle can optimise close packing of the molecule. The improved intermolecular interactions, Uinter, outweigh the conformational energy penalty, ΔEintra, to give a more stable lattice energy, Elatt = Uinter + ΔEintra. This thesis uses this lattice energy model hierarchically in a new Crystal Structure Prediction (CSP) algorithm, CrystalPredictor version 1.6, which varies the low-barrier torsion angles at the start of generating hypothetical crystal structures. The crystal structure of 1-benzyl-1H-tetrazole was successfully predicted in an informal ‘blind test’ when given the chemical diagram and the number of molecules in the asymmetric unit cell. Then, the concept of whether specific molecular fragments favour polymorphism (i.e. polymorphophore) was investigated by analysing the crystal energy landscapes of the monomorphic fenamic acid and the polymorphic derivative tolfenamic acid. The CSP results show that the polymorphophore promotes but does not guarantee polymorphism and that the substituents on the polymorphophore fragment decide the relative energies of the crystal structures. Molecular Dynamics (MD) cannot use this lattice energy model because many ab initio calculations of ΔEintra on a single molecule are expensive. However, the examination of the physical origin of the torsional barrier in fenamates aided the derivation of an analytical model fo ΔEintra. This thesis develops codes for fitting analytical intramolecular force fields to ab initio conformational profiles of fenamates. An intramolecular exp-6 atom-atom term (for the non-bonded repulsion-dispersion contributions) plus a cosine term (that represents the changes to the Molecular Orbitals) accurately model the ab initio conformational energy surfaces of fenamic and tolfenamic acids. This thesis provides a first step in extending ΔEintra data generated from CSP studies to help MD on condensed phases of pharmaceutical-like organic molecules.

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This thesis is concerned with structural studies of ferroelectric crystals, in particular the tetragonal paraelectric and orthorhombic ferroelectric phases of KH2P04, KD2PO4 and RbH2P01 and the cubic paraelectric phase of Cu3B7013'Cl and Ni3B7013I, using both neutron powder and single-crystal diffraction techniques. The methods of constrained least-squares refinement and significance testing are discussed since these techniques were used to determine the significance of certain structural features of interest in the single-crystal studies. Structural studies of tetragonal KH2PO4, KD2P04 and RbH2PO4 at atmospheric pressure revealed marked isotope and temperature effects of many of the structural features tested. On increasing pressure the P04 groups in tetragonal KH2P04 and KD2PO4 rotate such that the O-H,D 0 distance does not, within error, change. This may well explain the unexpectedly small pressure-dependence of n, the tunnelling frequency of a proton (deuteron) in the absence of any coupling, and Slo, the collective tunnelling mode frequency, reported by Peercy. In the orthorhombic phase the H ions order onto one of their two possible sites in the tetragonal phase. The displacements of the K, P and 0 ions are related to the choice of site of the H ion on the 0-0 bond. P-O(2) is significantly longer than P-O(l), the difference is smallest in KD2PO4. It is shown that it is not necessary to postulate disorder to explain the crystal structure of Ni3B7013I at 77°K. For Cu3B7013CR boracite it was not possible, within the resolution of the experiment to distinguish between the tetrahedral disorder or anharmonic (123 third cumulant) models for the distribution of the halogen ion. Increasing anharmonic effects were found as X I -> CL.

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In this Thesis we use atomic force microscopy (AFM) and scanning white light interferometry (Zygo) to investigate the spatial heterogeneity of plastic flow in crystalline materials. To this end we analyze one-dimensional surface profiles obtained from surface maps of deformed Alkali Halide single crystals. The Alkali Halides investigated were potassium chloride (KCI) and lithium fluoride (LiF), both of high purity. In the case of LiF also irradiation hardened LiF was investigated. For the deformed surfaces of the pure Alkali Halides we find self-affine behaviour with an exponent of about ζ=0.7..0.8 indicating long-range correlated patterns in the underlying strain field. No intrinsic limit to the scaling is found and the combination of AFM and Zygo allow us to assess the exponent over almost 6 orders of magnitude, from the mm down to the nm scale. We also find power-law behaviour for the probability density function of the step height <i>p</i>(<i>s</i>) and the exponent found to describe the power-law best over about 3 orders of magnitude is about <i>_T=</i>-1.5. For <i>p</i>(<i>s</i>) an intrinsic limit is found towards larger surface step sizes. For irradiated LiF a different behaviour is observed for low deformations where strain softening leads to the emerging of characteristic step heights rather than scale-free behaviour. Towards larger strains the scale free behaviour with an exponent of <i>_T</i>=-1.5 is recovered.