NPLIN was studied in supersaturated solutions (S = 1.06) of potassium chloride (KCl) and bromide (KBr). The fraction of samples nucleated (f) follows a nonlinear dependence on peak power density that approaches f = 1 at higher incident powers. It is shown that a lower threshold power is required for nucleation using 532 nm laser pulses than at 1064 nm, and that a higher fraction of samples nucleate when exposed to 532 nm pulses at a given laser power. Comparison with KCl shows higher fractions of KBr samples nucleate with lower threshold values at both wavelengths. Samples of KCl of equal supersaturation at two different temperatures (23 and 33 °C) exposed to 1064 nm pulses show that those at 33 °C are significantly more labile to nucleation. The ratio of samples nucleated at 33 °C compared to those at 23 °C was 2.11 ± 0.47. A classical nucleation model based on activation of subcritical solute clusters accounts remarkably well for the experimental data and provides phenomological values of the crystal–solution interfacial tension (γ) at 23 °C for KCl and KBr of 5.283 and 4.817 mJ m-2. At 33 °C, the model yields a best-fit value of γ = 5.429 mJ m-2 for KCl. As an extension of this work the use of an evanescent wave (ew) generated by total internal reflection was investigated as a method to cause nucleation in supersaturated KCl solution. Evanescent wave NPLIN (ew-NPLIN) was shown to cause nucleation. The results showed a higher laser-power threshold required for nucleation and sample lability greater than that of bulk NPLIN. In a second approach to understanding NPLIN, the structures of concentrated solutions were probed by a series of laser scattering experiments. Evidence showing populations of particles in solution was provided by Rayleigh laser scattering (RLS) experiments. Scattering in solutions prepared to be nearly saturated (S = 0.95) was observed using a low magnification (×10) microscope objective; almost all solutions showed the presence of scattering objects moving freely in solution. For those that showed no particles, it was noted that the solutions were typically of higher solute concentration (> 11 mol% solute). Ammonium nitrate solution showed no particles using ×10 magnification, however particles were identified when higher magnification was used (×50 and ×100). Video footage of the Rayleigh scattering observed in aqueous solutions of glycine, urea and ammonium nitrate obtained using ×50 magnification were analysed using a custom nanoparticle tracking software. The results showed a population of particles in aqueous urea and glycine solutions with particle concentrations of the order 108 particles cm-3 and mean hydrodynamic diameter of approximately 267 ± 1 and 173 ± 2 nm respectively. Not enough particles were identified in ammonium nitrate solution to complete the tracking analysis; however a fluctuating background scatter suggested a population of particles with sizes below the limit of resolution of the optical system. Using aqueous urea solution as a model system the structure of the particles identified in solution was investigated using scanning microscopy. The second-harmonic scattering (SHS) signal measured in concentrated aqueous urea solution was measured as a function of solution concentration (C) over a range of saturation conditions from undersaturated (S = 0.15) to supersaturated (S = 1.86). The results show a non-linear increase in SHS signal with local maxima near S = 0.95 and 1.75 suggesting a change in solution structure near these points. Rayleigh scattering images indicate the presence of particles in nearly saturated (S = 0.95) urea solutions. Time-dependent SHS measurements indicate that signals originate from individual events encountered during scanning of the sample through the focal volume of the probe laser, consistent with second harmonic generation (SHG) from particles. SHG from aqueous dispersions of barium titanate (BaTiO3) nanoparticles with diameters < 200 nm, showed signals ~20 times larger than urea solutions. The results suggest the existence of a population of semi-ordered clusters of urea that changes with solution concentration.

The (1-x)BiFeO3-(x) (K0.5Bi0.5)TiO3 (BFO-KBT) system has been investigated with respect to composition and temperature. Powder samples have been synthesised by the traditional solid state method at regular intervals across the compositional range. High resolution powder x-ray diffraction measurements have been made on these powders between room temperature and up to 950ºC. The data from these measurements have been analysed by the Rietveld method, and the results of these refinements have been used to construct a phase diagram of the BFO-KBT system. It was found that there are no sharp transitions in the phase diagram, either with composition or with respect to temperature, and that aside from the BFO end member, all samples investigated were best fitted with a mixed phase model. At low mol% KBT, it was found that the mixed phase model that best described the system was a mix of rhombohedral R3c and cubic Pm3m. As the mol% KBT was increased, the cubic phase increased, becoming dominant between 15% KBT and 20% KBT. The rhombohedral phase diminished with increasing mol% KBT and was no longer a component in the mixed phase system beyond 60% KBT. At 70% KBT, the room temperature system could still not be modelled with a single phase. A mix of cubic Pm3m and monoclinic P1m1 was used to model the system. At 90% KBT, the model was again found to be best fitted by a different mix of phases, specifically a mix of monoclinic P1m1 and tetragonal P4mm phases were used to model the data. The tetragonal phase was found to become more dominant from this point with increasing mol% KBT, but even the KBT end member was found to be best modelled with a mix of monoclinic P1m1 and tetragonal P4mm phases. It was found that all samples become more cubic with increasing temperature. It was found that the temperature at which the change to a cubic state occurred decreased with increasing mol% KBT. It was also found that at high mol% KBT and high temperature, there was phase separation into a two-cubic mixed phase model instead of the expected single Pm3m cubic phase. This phase separation has been linked to the reported morphotropic phase boundary in the material. The BFO-KBT system was found to change little with respect to temperature below 500ºC, which makes it an attractive material for high temperature device applications if doped with a material with a stronger piezoelectric response. In addition, single crystal x-ray diffraction of BFO-KBT crystals has been undertaken with laboratory-based and synchrotron systems. It was found that the latter was necessary to obtain good results from a refinement of the data in SHELXL due to the absorption of the BFO-KBT system, minimised through the use of a much higher energy x-ray beam. It was found in JANA2006 that the BFO-KBT system was best fit with anharmonic atomic displacement parameters, which was linked to the difference between the short range and long range order in the material.

La microscopie champ proche micro-onde, qui fait partie de la famille des microscopies à sonde locale, est envisagée aujourd’hui dans de nombreux domaines d’applications de la physique, de la biologie et des micro et nanotechnologies. Dans ce manuscrit, le microscope micro-onde à champ proche qui est exploité est un instrument développé au laboratoire IEMN bénéficiant d’une grande sensibilité dans une large bande de fréquences de travail [2-18 GHz]. Le potentiel d’applications du microscope est démontré au travers de la caractérisation de liquides avec différentes modalités de caractérisation (sonde en contact, sans contact et en immersion). En particulier, cet outil est mis en œuvre pour la spectroscopie diélectrique de solutions aqueuses de glucose.Cet instrument qui offre une capacité d'imagerie sub-longueur d'onde est également testé pour différentes situations (imagerie de surface et de sub-surface). La résolution d'imagerie ainsi que la précision de mesure sont évaluées puis des méthodes de traitement d'images simples sont proposées pour améliorer la qualité de l'imagerie. Enfin, une piste pour une intégration plus grande de l’instrument, qui consisterait à remplacer l’analyseur de réseau par un dispositif plus compact (type réflectomètre six-ports) est explorée. / Near-field microwave microscopes, which belong to the local scanning probe microscopes family, are considered today as advanced characterization tools in many applications areas including physics, biology and micro and nanotechnologies. The near-field microwave microscope that is used in the work and described in this manuscript is an instrument developed at IEMN owning a great sensitivity in a wide operating frequency band [2-18 GHz]. The potential of the microscope in terms of applications is demonstrated through the characterization of liquids with different modalities of characterization (probe in contact, non-contact and immersed in a liquid). In particular, this instrument is investigated for dielectric spectroscopy of aqueous glucose solutions.This characterization tool that offers sub-wavelength imaging capability is also tested in different situations (surface and subsurface imaging). Imaging resolution and measurement accuracy are evaluated and easily implementable processing methods are proposed to improve the quality of imaging. Finally, a solution towards a larger compactness of the instrument is investigated through the replacement of the network analyzer by a more compact device (six-port reflectometer type).

Corine Anna Bickley. / Includes bibliographical references (p. 124-132). / Research partially supported by the National Institute of Neurological and Communicative Disorders and Stroke. Research partially supported by a Whitaker Health Sciences Fellowship and a C.J. LeBel fellowship.

In the first part of this study are presented the synthesis and results of structural investigations of mixed metal sulfosalts. Included are the structures of five new lead, bismuth and antimony sulfosalts; phase GZ1 (Bi4.39 Pb1.4 Sb3.21 S13), phase GZ2 (Bi1.74 Pb2.62 Sb2.15 S8), phase GZ3 (Bi0.87 Pb1.22 Sb0.42 S3), phase GZ4 (Sb4.66 Pb3.97 Bi1.37 S13) and phase GZ5 (Sb6 Pb14 S23), and the revaluation of the known phases zinckenite (Pb1.49 Sb3.64 S7), stibnite (Sb2 S3), lillianite (Pb3 Bi2 S6) and galenobismutite (Pb Bi2 S4). With the exception of stibnite and galenobismutite these phases are characterised by mixed occupancies, and the site populations were assigned via a combination of microprobe analyses, polyhedral distortion and bond valence analysis results. To this end two FORTRAN programs were written, OCCGEN96 and VOLUME96, the former to calculate the occupancy of a site based on the bond valence contributions of its constituents, and the latter to calculate its polyhedral volume. Phase GZ1 exhibits a monoclinic structure with a central region reminiscent of stibnite, and its distribution of occupancies indicate the presence of a supercell produced by ordering. Phases GZ2 and GZ3 are hexagonal with features in common with zinckenite. GZ3 possesses the smallest hexagonal cell yet found for a sulfosalt and its structure contains three-fold metallic columns, in addition it displays a metal coordination environment not previously seen in this type of structure. The positional disorder, and partial occupancies (manifest as poorly defined hexagonal channels) exhibited by zinckenite and the other hexagonal sulfosalts are here explained for the first time in terms of alternately occupied configurations arranged on a superlattice. Phase GZ5 resulted from an attempt to crystallise the mineral semseyite whose structure is unknown, but possibly closely related to that of GZ5. The poorly defined phases from the literature have all been refined with better discrepancy indices and in the case of lillianite the results differ significantly from those previously reported. Additional investigations of phase GZ1 were carried out using transmission electron microscopy, and these lead to the identification of a number of related phases and also revealed the presence of disordered and composite structures. In the second part of the study diamond inclusions of lower mantle origin are investigated. In common with the sulfosalts they are characterised by mixed occupancies, however the distribution of metal species within mineral silicates is better understood than in sulfosalts, and they provided a good base from which to study the latter. The presented results include the crystal structure of the new mineral TAPP (Mg0.76 Fe0.15) (Al1.76 Cr0.16 Mn0.06) (Mg1.88 Fe0.08) Si1 (Si1.92 Al0.08) O12, which is of great significance in lower mantle genesis theory; the structures of two novel pyroxenes (Mg0.46 Al0.43 Fe0.06 Cr0.05) (Mg0.39 Na0.31 Ca0.16 Fe0.06 Mn0.04) (Si1.91 Al0.09) O6 and (Mg0.73 Al0.23 Cr0.03) (Mg0.70 Na0.06 Ca0.03 Fe0.16 Mn0.04) (Si0.83 Al0.17) SiO6 and the only sapphire (Al1.98Fe0.01Si0.01O3) to be found as a diamond inclusion. The structure of TAPP is particularly interesting as it has a garnet composition but is tetragonal not cubic. In the past the only non cubic garnets have been so only by a few angstroms at the most, while the cell of TAPP has a c parameter nearly three times that of a.

Este trabajo se ha enfocado hacia la obtención, optimización, estudio y caracterización microestructural de cerámicas superconductoras texturadas másicas de REBCO siendo RE; Y, Tm y Xenotima*, así como sus soldaduras. La técnica de crecimiento cristalino empleada ha sido la de solidificación direccional inducida por semilla, Top Seeding Melt Textured Growth (TSMTG). Con el objetivo de mejorar las propiedades microestructurales, físicas y mecánicas se han optimizado las diferentes variables en el crecimiento y la composición por medio del uso de aditivos secundarios (Y211, óxidos de Zr: BaZrO3 y YBa2ZrO5,5, RE, Ag, CeO2). Se han establecido diferentes modelos de crecimiento para muestras texturadas (TSMTG), ya que se ha producido una modificación en los mecanismos de crecimiento generada principalmente por los aditivos secundarios. Por un lado se han producido espectaculares procesos de arrastre/atrapamiento de partículas en los monodominios superconductores generados por la presencia de óxidos de Zr. Hecho que ha permitido obtener novedosas muestras de referencia (monodominios texturados maclados superconductores de YBCO libres de inclusiones de Y211 y de características comparables a un monocristal). Por medio de la adición de Ag a los materiales TSMTG con óxidos de Zr se han mejorado las propiedades mecánicas y superconductoras de estas muestras de referencia, al disminuir la densidad y propagación de grietas. De nuevo se ha obtenido una nueva muestra de referencia con inclusiones de partículas de Ag con una morfología elongada casi cilíndrica pero libre de inclusiones (Y211 y óxidos de Zr). Por otro lado se ha producido una espectacular redistribución iónica entre partículas atrapadas y la matriz superconductora en muestras REBCO texturado siendo RE: Tm y Xenotima. Generándose, de nuevo, novedosas muestras de referencia. Finalmente debido a la demanda de materiales texturados monodominio de grandes dimensiones y geometrías diversas que tengan una densidad de corriente crítica (Jc) elevada, buenas propiedades superconductoras y también mecánicas se han realizado innovadores procesos de uniones superconductoras complejas utilizando compuestos (TmBCO, compuestos de plata (YBCO/Ag (TSMTG) e YBCO/AG policristalino) como agentes soldantes puesto que su punto de fusión es inferior al del material REBCO. Ello ha permitido establecer un nuevo proceso de soldadura que permite obtener morfologías complejas. *Xenotima es una mezcla natural de tierras raras obtenida de Brasil. El contenido típico de esta mezcla natural es (% mol): 54% Y2O3, 14% Yb2O3, 11% Lu2O3, 10% Re2O3, 7% Dy2O3, 2% Tm2O3, 2% Ho2O3. / This research work has been focused on obtaining Superconducting Melt Textured Composites REBCO being RE; Y, Tm and Xenotime*, growth process optimization and microstructural characterization has been analyzed and a new welding procedure has been developed leading to high quality joints. The crystal growth technique used has been Top Seeding Melt Textured Growth (TSMTG). In order to improve the microstructural, physical and mechanical properties has been optimized the different growth parameters and the initial composition of the precursor mixture by adding secondary additives (Y211, Zr-oxide: BaZrO3 and YBa2ZrO5,5, RE, Ag, CeO2). Different growth models for Melt Textured samples (TSMTG) have been established because it has been a change in growth mechanisms mainly generated by secondary additives. MTG YBCO composites free of Y211 precipitates homogeneously distributed within the matrix have been prepared by the addition of a Zr-oxide. This addition generates a spectacular drag effect/trapping of particles in the growth front. Has allowed obtain a novel reference samples (twinned superconductors MTG YBCO single domains free of Y211 inclusions and comparable to a single crystal characteristics). Through the addition of Ag to TSMTG Zr-oxides materials, mechanical and superconducting properties of these reference samples have been improved with decreasing density and crack propagation. Again a new reference sample with inclusions of Ag particles with an elongated cylindrical morphology but free of inclusions (Y211 and Zr-oxides) has been obtained. On the other hand there has been an important ionic redistribution, trapped particles in a superconducting matrix of MTG REBCO being RE: Tm and Xenotime. Generating, again, a novel reference sample. Finally due to the demand of large textured single domain materials with diverse geometries having high critical current density (Jc), good mechanical and superconducting properties have been made innovative processes of complex junctions using superconducting compounds (TmBCO, silver compounds (YBCO / Ag (TSMTG) and YBCO / polycrystalline Ag) as welding agents since its melting point smaller than the REBCO material. This has enabled a new welding process for obtaining complex morphology. * Xenotime is a natural mixture of rare earth from Brazilian origin. The typical content of this natural mixture is (mol %): 54% Y2O3, 14% Yb2O3, 11% Lu2O3, 10% Re2O3, 7% Dy2O3, 2% Tm2O3, 2% Ho2O3.

El 129I està considerat un radionúclid perillós a causa del seu llarg temps de vida mitjana, la baixa interacció que presenta amb la majoria dels materials geològics i la seva rellevància biològica. L'activitat nuclear ha generat al voltant de 5068 kg de 129I, apareixent molt diluït en les deixalles aquoses de les centrals nuclears. Els hidròxids dobles laminars (HDLs) de Mg/Al han estat estudiats com materials adsorbents de iodur, fent proves amb diferents ràtios i diferents anions interlaminars. La capacitat d'adsorció es veu seriosament afectada per les propietats texturals del material, i els òxids mixtes derivats de les HDLs de carbonat amb ràtio Mg/Al 3/1 tenen la major capacitat d'adsorció, la qual s'incrementa amb l'aplicació d'ultrasons. HDLs de diferents composicions es van estudiar també com a possibles candidats per a la immobilització de iode (com iodat o com iodur) utilitzant diferents mètodes d'incorporació: coprecipitació, intercanvi aniònic i reconstrucció. La reconstrucció dóna la major incorporació de iodat, ca. El 54% enfront del ca. 40% que va aconseguir per coprecipitació. També es va estudiar l'estabilitat tèrmica dels materials i la pèrdua de iodat en contacte amb una solució. La pèrdua al escalfar a 453 K les mostres va ser molt significativa i també ho va ser la pèrdua de iodat en contacte amb aigua Milli-Q o salmorra. En l'etapa actual de la investigació es poden considerar els HDLs com a aptes per adsorbir iodur d'una solució però no són adequats per a un emmagatzematge a llarg termini de iode radioactiu. / El 129I está considerado un radionucleido peligroso debido a su largo tiempo de vida media, la baja interacción que presenta con la mayoría de los materiales geológicos y su relevancia biológica. La actividad nuclear ha generado alrededor de 5068 kg de 129I, apareciendo muy diluido en las desechos acuosos de las centrales nucleares. Los hidróxidos dobles laminares (HDLs) de Mg-Al con diferentes ratios y diferentes aniones interlaminares han sido estudiados como materiales adsorbentes de yoduro. La capacidad de adsorción se vió fuertemente afectada por las propiedades texturales del material, y los óxidos mixtos derivados de las HDLs de carbonato con ratio Mg/Al 3/1 tienen la mayor capacidad de adsorción, la cual se incrementa con la aplicación de ultrasonidos. HDLs de diferentes composiciones se estudiaron también como posibles candidatos para la inmovilización de yodo (como yodato o como yoduro) usando diferentes métodos de incorporación: coprecipitación, intercambio aniónico y reconstrucción. La reconstrucción da la mayor incorporación de yodato, ca. 54% frente al ca. 40% que alcanzó por coprecipitación. También se estudió la estabilidad térmica de los materiales y la pérdida de yodato en contacto con una solución. La pérdida del al calentar a 453 K fue muy significativa y también lo fue la pérdida de yodato en contacto con agua Milli-Q o salmuera. En la etapa actual de la investigación se pueden considerar a los HDLs como aptos para adsorber yoduro de una solución pero no son adecuados para un almacenamiento a largo plazo de yodo radiactivo. / 129I is considered to be a very hazardous radionuclide due to its long half-life, the low interaction that it presents with most geological materials and its biological relevance. The nuclear activity has generated around 5068 kg of 129I, present very diluted in liquid wastes from nuclear power plants. Mg-Al Layered double hydroxides (LDHs) with different ratios and interlayer anion have been studied as iodide adsorbent materials. Adsorption capacity was strongly affected by material textural properties, and the carbonate LDHs-derived mixed oxides with Mg/Al ratio of 3/1 showed the largest adsorption capacity, increased applying ultrasound. LDH of different compositions were studied as potential candidates for the immobilization of iodine (as iodate or iodide) using different incorporation methods: coprecipitation, anionic exchange and reconstruction. The reconstruction gives the highest iodate incorporation, ca. 54 % vs. ca. 40 % reached by coprecipitation. The thermal stability of the materials and the anionic release in contact with a solution was also studied. The iodine loss by heating at 453 K was very significative and the release of iodine in contact with Milli-Q water and brine solution. At the present stage of the research we may conclude, that LDHs can be considered as iodine scavengers for a short-term storage, but they are not suitable for a long-term storage of radioactive iodine.

This thesis is concerned with the propagation of elastic waves in polycrystalline materials. In particular, in establishing a relationship between the statistical properties of the wavefield and the statistical properties of the material via a correlation function. Here the study of elastic waves has been restricted to surface acoustic waves (SAWs), mainly because they are readily accessible using an optical scanning acoustic microscope (OSAM)

This thesis describes the effect of pressure on crystal structures that are in some way unusual. The aim was to investigate whether pressure could be used to force these ‘structural oddities’ to conform to more conventional behaviour. In many cases pressure-induced phase transitions were observed, and the driving forces of these are considered. L-serine monohydrate crystallises with layers of hydrogen bonded serine molecules. Layers are linked together by H-bonds from the donor atoms of water molecules. The orientation of the water molecules between the layers is uncommon for other layered hydrates in the CSD. A single crystal of serine hydrate undergoes a pressure-induced phase transition at 5 GPa, which is characterised by a rotation of the water molecules to an orientation which is more frequently observed. PIXEL calculations show that the transition is driven by the PV term in the equation G = U - TS + PV. An attempt to reproduce the transition in another layered hydrate with a similar topology was partially successful in the compression of S-4-sulfo-L-phenylalanine monohydrate, which undergoes a similar phase transition at 1 GPa. Methyl 2-(9H-carbazol-9-yl)benzoate crystallises unusually with eight molecules in the asymmetric unit (Z’ = 8). Compression of a single crystal results in a phase transition at ca. 5 GPa to give a Z’ = 2 polymorph. The PV term is an important contributor to the driving force of the transition. The geometries of the molecules in phase-II are significantly less stable than in phase-I, and as pressure is released on phase-II the need to adopt a more stable molecular conformation eventually outweighs the PV advantage. The Z’ = 8 structure is eventually re-established at 4.6 GPa. This work illustrates how low Z’ polymorphs of the same structure are not always the thermodynamically more stable forms. When recrystallised in situ from a 4:1 by volume solution of methanol and ethanol, a new polymorph of salicylamide is obtained at 0.2 GPa. The ambient pressure phase appears in the CSD to contain a number of abnormally short H…H contacts. We find this model to be incorrect, and have re-determined the structure to find no short H…H contacts. PIXEL and DFT calculations indicate that the high-pressure polymorph is favoured over the ambient phase by the PV term, the zero point energy and entropy. Low completeness that often occurs as a result of shading from the high-pressure cell was improved by the inclusion of multiple crystals within the sample chamber. Bianthrone changes colour from yellow to green on grinding, though this does not occur when subjected to hydrostatic pressure to 6.5 GPa. There is, however, a subtle colour change from bright yellow to dark orange as pressure is applied, and it is likely that this is caused by changes in the - stacking distances. This work highlights how a system can react differently to hydrostatic and non-hydrostatic conditions.

This thesis describes the growth, X-ray diffraction assessment and tensile deformation properties of dislocation-free copper single crystals. As such it has been possible to conveniently section the work carried out into these three main areas within this thesis. Consequently, each chapter may be read almost independently of the others with references and further work suggestions being incorporated at the end of each chapter. This format, it is felt, does not disjoint the work: rather, it enables the central theme (i.e. the title of this thesis) to be developed in a much more continuously clear way than is normally apparent in a thesis where conclusions, further work suggestions and references are not drawn together until the end of the volume. Chapter I opens with a brief outline of the crystal growth methods which could have been used to produce low dislocation density (< 10⁶ cm/cm³) crystals by utilising the three fundamental phase transitions, i.e. solid to solid, vapour to solid and liquid to solid. The Czochralski method is then discussed in detail since it was used by the author to produce dislocation-free copper single crystals. The technological problems in obtaining such crystals are extensively enumerated and solutions presented. For instance, melt surface vibrations were eliminated by using a continuous flow of cooling water, by standing the complete crystal puller on a bed of foam, and by rigidly clamping the R.F. coil. It is emphasised that these technological problems must be solved before the scientific aspects of the growth of dislocation-free crystals can be studied. It is further shown that the author's modified crystal growing technique can be reliably used to grow dislocation-free crystals of copper each time, providing adequate care is taken over growth rates (1.2 cm/hr); specimen shape (a long thin "double-neck" must precede the required crystal to eliminate dislocations propagating from the seed and to act as a heat flow resistance); and crystal cooling rates (a long "tail" allowed the crystal to reach the ambient temperature slowly thereby minimising dislocation by thermal stresses and/or vacancy condensation). In the future, it is suggested, an automated crystal pulling system would be advantageous and a study of crystal growth in a synchrotron X-ray beam could be potentially definitive experiment on crystal growth. Chapter II looks at the theoretical aspects of Czochralski crystal growth with the aid of a new model which has been analysed on the Oxford University Computer. The model assumes a crystal-neck-seed configuration to be "sitting" on a liquid and examines the influence of geometric changes of the seed-neck-crystal and of radiation changes on the temperature gradients, primarily at the growth front. Four elements were chosen for study in this way: Si, Ge, Ag, Cu, i.e. four elements which have been grown dislocation-free. It was found that the seed size and shape played a small part in determining the interfacial temperature gradients, DT_c(0), of all the elements. The neck, however, could have a marked influence on DT_c(0) values in metals but not so much in semiconductor crystals. By geometric control alone it was found that the best way to reduce DT_c(0) values was to grow a large diameter crystal. The influence of radiation losses was found to be marked for semiconductor crystals but not for metal crystals. Finally, it is concluded that to minimise DT_c(0) values then the seed and neck must be long and thin and the crystal fat. These results fit in well with experimental knowledge. Further work to be carried out could consider the influence of a varying ambient temperature and convective heat losses on the interfacial temperature gradients. Chapter III is concerned with the interaction of X-rays with perfect crystals. The Lang-Borrmann X-ray topography technique is examined, and the experimental methods used to obtain X-ray topographs taken throughout this work are discussed. The major part of the chapter takes up the discussion of the theoretical interaction of X-rays with a perfect crystal set to diffract such X-rays. It is demonstrated that for a plane-wave incident on a cylindrical crystal, for the boundary condition to be satisfied the dispersion surface tie-points are displaced as the crystal traverses the incident X-ray beam. Thus the crystal wave-vectors no longer exactly satisfy the Bragg condition. This effect, was never unambiguously monitored experimentally because of the incident beam divergence. It is suggested that a future study could consider this problem in more detail. The Takagi-Taupin-Uragami generalised X-ray diffraction theory is reviewed and then used to calculate the intensity in the diffracted beam of a traverse topograph from a perfect cylindrical copper crystal. This computer simulation is also compared to an experimental condition. In both cases it is found that the Bragg surface of the crystal produces a very intense reflection whilst the remainder of the crystal gives a much reduced diffracted intensity in comparison. This asymmetric profile is interpreted in terms of absorption mechanisms which are strong at the centre of the crystal but not at the surface. The general agreement between theory and experiment is considered to be good although a few discrepancies arose, e.g. the lateral extent of the Bragg surface peak was found to be larger for the experiment than for the theory. Continued research in this area must explore further these small discrepancies; and it is suggested that a possible line of future study would be to examine theoretically and experimentally the X-ray diffraction from dislocation-free cylindrical crystals which possess low absorption coefficients (e.g. aluminium, silicon) for harder radiations (e.g. M_oK₁, Ag_K₁). It should then be possible to produce interference fringes and then it should be possible to examine the effects of a strain gradient on fringe spacing and visibility. Chapter IV sets out to discuss the tensile deformation behaviour of [1-2-3] growth axis, dislocation-free, chromium plated copper single crystals. This is done using the results from two sets of coupled experiments: an Instron deformation study and a synchrotron deformation investigation. It is first shown that chromium plating can destroy the perfection of the crystal unless care is exercised over plating temperatures and times. It was finally found that after plating for 10 seconds at 55C (1 μm of Cr deposited) that there was no indication of lattice dislocation. The stress-strain curve of a chromium plated dislocation-free copper crystal (Cr thickness 1 μm) is shown to exhibit a yield point commensurate with that for a non-plated, dislocation-free copper crystal, i.e. 90 g/mm². The work hardening curve is shown to be comprised of three distinct regions: an initial rapid work hardening rate; a transitionary work hardening rate; and a stabilised work hardening rate up to a shear strain of 6%. In straining the crystal through these regions the work hardening rate progressively decreased. For a plated crystal which is dislocation-free initially it is shown that serrations in the flow curve occur, whilst for all other crystals these are shown to be absent. Such serrations are argued to occur by a source suppression and new source operation mechanism during the early deformation stages (1% shear strain), and then by a crack formation mechanism at the copper-chromium interface during the later stages of deformation. The initial work hardening rates (18 kg/mm²) were found to be independent of coating thickness (0.5 μm and 1 μm) but in the latter two regions the thicker coating imparted a higher work hardening rate to the crystal than the thinner layer, e.g. for a 0.5 μm chromium layer the work hardening rate was 1.65 kg/mm² in region iii and for a 1 μm chromium layer it was 2.97 kg/mm² in region iii. It is tentatively suggested that the chromium layer affected mobile dislocation motion rather than dislocation generation. The synchrotron work produced no evidence to support the argument that dislocation motion up the elastic line took place. Yielding was found to occur at a stress level similar to that measured in the Instron work. The complex stress system imposed by the deformation jig and the lack of resolution rendered it impossible to decide where the dislocation sources were located. Further increases in the load on the crystal produced double slip and this was argued to prematurely occur because of the combination of torsion, bending and tension which the crystal experienced. Certain interfacial dislocation activity was registered but this was not readily analysed in terms of surface sources. The yielding behaviour was inhomogeneous and appeared to be remote from the few slip bands induced by specimen transport. After yielding evidence was found for dislocation pile-up at the centre of the crystal. It is pointed out that future studies should use a better design of deformation jig so as to apply only a tensile stress to the crystal. Plating thickness and specimen size could be further explored.