X-ray crystallography is a widely used analytical technique for the structure solution of small molecules. Since the determination of the rock salt structure in 1913 by Henry and Lawrence Bragg the technique has developed allowing the solution of larger and more complex structures. The information that can be determined about these structures has increased as X-ray sources, detectors, and computational methods have improved. However, certain properties of molecules cannot always be directly determined from single wavelength X-ray diffraction. These include, inter alia: the site specific oxidation or spin state of an element in compounds where more than one state of the same element exist; discrimination between consecutive heavy elements in the periodic table. As the size of molecules being studied increases, reduced data resolution also becomes a problem. The aim of this research was to determine whether these problems can be addressed by measuring the changing anomalous scattering contribution of heavy atoms within structure through careful selection of the X-ray energy. Firstly, I report an investigation into the problemof discriminating oxidation state, spin state and elements of near identical scattering by exploiting their anomalous signal. I first present DetOx, a program written during the course of the project to deconvolute the fluorescence signal from materials containing more than one state of the same element into their respective spectra. This allows the calculation of anomalous scattering factors for both atomic states of an atom, which can subsequently be used to refine the occupancy of the different states at ambiguous sites within the crystal structure. The approach taken here, to determine differences due to relatively small anomalous signals, is analogous to the refinement of the Flack parameter whereby small changes in many hundreds or thousands of observations can be used to fit a parameter with a high degree of precision and accuracy. I show the application of this technique to the mixed oxidation state compound, GaCl₂, and the two-step spin crossover material, Fe(btr)₃(ClO₄)₂. Refinement of the occupancy of charged ions on multiple sites using data at a single, carefully selected wavelength proved successful for these compounds, although upon extension to materials containing a larger number of anomalous scatterers, the absorption became a major issue in the data along with problems associated with simultaneously refining occupancies at more sites in the structure. We have demonstrated that calculations can be made to select specific experimental data to collect in order to improve the measured signal. However, due to limitation of the current collision model on the diffractometer used we have not yet been able to construct data collection strategies to take advantage of this. I next present a new ratio refinement technique to overcome this absorption problem due to the increased number of scatterers. By using ratios between datasets close in energy, but below the absorption edge, we were able to exploit small changes in f' without encountering absorption problems associated with the increase in f''. These ratio values were then refined against a lab structure using a modified version of CRYSTALS to reveal the site specific occupancies of different atomic species within a given structure. For mixed-valence compounds, e.g. Mn₃ and Mn₆ clusters, the difference in anomalous signal between the different states proved too small for a stable least-squares refinement solution. However, we have shown that using a simulated annealing algorithm (to refine only occupancies), we can consistently obtain the expected structure. For mixed-metal structures e.g. the Mn₅Co₄ cluster, there was enough contrast in the data to refine occupancies with a least-squares approach, and these results were supported using simulated annealing. Lastly, I describe the application of structure solution techniques based on methods used in macromolecular crystallography to 'large' small molecules. Traditionally these have been reserved for non-centrosymmetric protein structures, however with the trend of synthesising larger and larger small molecules, problems encountered in macromolecular crystallography leading to low resolution datasets are becoming increasingly common. I have shown that it is possible to solve the structure of centrosymmetric structures by exploiting the anomalous signal in multiple wavelength diffraction experiments. The technique is applied successfully to two relatively small molecules, however the results are promising for moving to larger structures in the future.

An alternate family of “high” voltage (where the equilibrium voltage lies between 3.6 V and 4.2 V) polyanion cathode materials is reported in this thesis with the objective of improving specific energy density (Wh/kg) and developing a better understanding of polyanion electrochemistry. The electrochemical properties, synthesis and the structure of novel fluorosulfate materials crystallizing in the tavorite and the triplite type mineral structures are described. These materials display highest discharge voltages reported for any Fe2+/Fe3+ redox couple. LiFeSO4F was prepared in both the tavorite and the triplite polymorphs using inexpensive and scalable methods. Complete structural characterization was performed using X-ray and neutron based diffraction methods. A rapid synthesis of fluorosulfates can be achieved by using microwave heating. The local rapid heating created by the microwaves generates nanocrystalline LiFeSO4F tavorite with defects that induce significant microstrain. To date, this is unique to the microwave synthesis method. Phase transformation to the more stable triplite framework, facilitated by the lattice defects which include hydroxyl groups, is therefore easily triggered. The formation of nanocrystalline tavorite leads to nanocrystalline triplite, which greatly favors its electrochemical performance because of the inherently disordered nature of the triplite structure. Direct synthesis of the electrochemically active triplite type compound can be carried out either by extending the duration of the solvothermal reactions or by the partial substitution of Fe by Mn to produce LiFe1-xMnxSO4F. This study, overall, has led to a better understanding of the transformation of tavorite to the triplite phase. To examine Li and the Na ion conduction and their correlation with the electrochemical performance of 3-D, 2-D and 1-D ion conductors, atomistic scale simulations have been used to investigate tavorite type LiFeSO4F, NaFeSO4F, olivine type NaMPO4 (M= Fe, Mn, Fe0.5Mn0.5) and layered Na2FePO4F. These calculations predict high mobility of the Li-ion in the tavorite type LiFeSO4F but sluggish Na-ion transport in iso-structural NaFeSO4F. High mobility of the Na-ion is predicted for phosphate layered and olivine structures. Finally, the synthesis and structural details of NaMSO4F (M=Fe, Mn) and NH4MSO4F (M=Fe, Mn) are presented in the last chapter to show the structural diversity present in the fluorosulfate family.

DC and RF magnetron co-sputtering techniques are one of the most suitable techniques in fabrication of thin films with different compositions. In this work, Al2O3 and Si/Al2O3 thin films were fabricated by using magnetron co-sputtering technique. For Al2O3 films, the stoichiometric, optical and crystallographic analyses were performed. For Si contained Al2O3 films, the formation conditions of Si nanocrystals were investigated. To do so, these thin films were sputtered on Si (100) substrates. Post annealing was done in order to clarify the evolution of Al2O3 matrix and Si nanocrystals at different temperatures. Crystallographic properties and size of the nanocrystals were investigated by X-ray diffraction (XRD) method. The variation of the atomic concentrations and bond formations were investigated with X-ray photoelectron spectroscopy (XPS). The luminescent behaviors of Si nanocrystals and Al2O3 matrix were investigated with photoluminescence (PL) spectroscopy. Finally, the characteristic emissions from the matrix and the nanocrystals were separately identified.

An alternate family of “high” voltage (where the equilibrium voltage lies between 3.6 V and 4.2 V) polyanion cathode materials is reported in this thesis with the objective of improving specific energy density (Wh/kg) and developing a better understanding of polyanion electrochemistry. The electrochemical properties, synthesis and the structure of novel fluorosulfate materials crystallizing in the tavorite and the triplite type mineral structures are described. These materials display highest discharge voltages reported for any Fe2+/Fe3+ redox couple. LiFeSO4F was prepared in both the tavorite and the triplite polymorphs using inexpensive and scalable methods. Complete structural characterization was performed using X-ray and neutron based diffraction methods. A rapid synthesis of fluorosulfates can be achieved by using microwave heating. The local rapid heating created by the microwaves generates nanocrystalline LiFeSO4F tavorite with defects that induce significant microstrain. To date, this is unique to the microwave synthesis method. Phase transformation to the more stable triplite framework, facilitated by the lattice defects which include hydroxyl groups, is therefore easily triggered. The formation of nanocrystalline tavorite leads to nanocrystalline triplite, which greatly favors its electrochemical performance because of the inherently disordered nature of the triplite structure. Direct synthesis of the electrochemically active triplite type compound can be carried out either by extending the duration of the solvothermal reactions or by the partial substitution of Fe by Mn to produce LiFe1-xMnxSO4F. This study, overall, has led to a better understanding of the transformation of tavorite to the triplite phase. To examine Li and the Na ion conduction and their correlation with the electrochemical performance of 3-D, 2-D and 1-D ion conductors, atomistic scale simulations have been used to investigate tavorite type LiFeSO4F, NaFeSO4F, olivine type NaMPO4 (M= Fe, Mn, Fe0.5Mn0.5) and layered Na2FePO4F. These calculations predict high mobility of the Li-ion in the tavorite type LiFeSO4F but sluggish Na-ion transport in iso-structural NaFeSO4F. High mobility of the Na-ion is predicted for phosphate layered and olivine structures. Finally, the synthesis and structural details of NaMSO4F (M=Fe, Mn) and NH4MSO4F (M=Fe, Mn) are presented in the last chapter to show the structural diversity present in the fluorosulfate family.

We envisage a fundamental study of the physical mechanisms (dislocation slip versus deformation twinning) involved in plastic deformation of hexagonal close-packed (HCP) metals like titanium and magnesium. A novel combination of X-ray imaging and diffraction techniques, termed X-ray diffraction contrast tomography (DCT), will be used to investigate details of the deformation process in the bulk of polycrystalline specimen. DCT provides access to the position, 3D shape, (average) orientation and elastic strain tensor of grains in polycrystalline sample volumes containing up to 1000 grains and more. Ultimately, an extension of the X-ray DCT technique is associated with a section topography methodology on the same instrument. This combination enables the measurement of local orientation and elastic strain tensors inside selected bulk grains. A very preliminary study of this approach is carried out on a magnesium alloy, underlying the current limitations and possible improvements of such approach. In this thesis, the data acquisition and analysis procedures required for this type of combined characterisation approach have been developed. The work is supported by the use of neutron diffraction, for an in-situ loading experiments, and two-dimensional electron backscatter diffraction (EBSD), for the initial microstructure of the materials and cross-validation of the results obtained with the X-ray DCT technique.

No description available.

No description available.

This study focuses on various aspects of nanocrystals embedded in a dielectric matrix. In the first part of this work, a new approach with the use of Fourier Transform Infrared spectroscopy (FTIR) in the nanocrystal analysis was developed and presented. Si and Ge nanocrystals embedded in SiO2 matrix were mainly studied. This new approach is based on the analysis of structural variations of SiO2 matrix during the formation of semiconductor nanocrystlas. It is shown that the chemical and structural variations of the host matrix are directly related to the precipitation of nanocrystals in it. This correlation provides valuable information about the presences of nanocrystals in the matrix. In the second part of this work, fabrication of SiC films with and without Si nanocrystals inclusions was studied. With this aim, stoichiometric SiC and Si rich SiC thin films were fabricated by using magnetron co-sputtering and Plasma Enhanced Chemical Vapor Deposition (PECVD) techniques. For SiC films, the structural and optical analyses were performed. For Si rich SiC films, the formation conditions of Si nanocrystals were investigated. Post annealing studies were carried out to track the evolution of the SiC matrix and formation of Si nanocrystals at different temperatures. Chemical and structural properties of the SiC host matrix were investigated with FTIR spectroscopy. Optimum conditions for the fabrication of stoichiometric SiC layers were determined. The crystallography of the nanocrystals was investigated by X-Ray Diffraction (XRD). The variation of the atomic concentrations and bond formations were investigated with X-Ray Photoelectron Spectroscopy (XPS). Raman spectroscopy and Transmission Electron Microscopy (TEM) were used to verify the formation of Si nanocrystals. We have shown that both single and multilayer Si nanocrystals can be fabricated in the amorphous SiC matrix for applications such as light emitting diodes and solar cells.

Submitted by Maryse Santos (maryseeu4@gmail.com) on 2016-08-26T14:49:04Z No. of bitstreams: 1 Dissertacao de Mestrado-Carla de Albuquerque Dias.pdf: 13161482 bytes, checksum: 7dae370ff2aeed1ca51c5f46b1701ffe (MD5) / Approved for entry into archive by Divisão de Documentação/BC Biblioteca Central (ddbc@ufam.edu.br) on 2016-08-29T13:45:00Z (GMT) No. of bitstreams: 1 Dissertacao de Mestrado-Carla de Albuquerque Dias.pdf: 13161482 bytes, checksum: 7dae370ff2aeed1ca51c5f46b1701ffe (MD5) / Approved for entry into archive by Divisão de Documentação/BC Biblioteca Central (ddbc@ufam.edu.br) on 2016-08-29T13:47:30Z (GMT) No. of bitstreams: 1 Dissertacao de Mestrado-Carla de Albuquerque Dias.pdf: 13161482 bytes, checksum: 7dae370ff2aeed1ca51c5f46b1701ffe (MD5) / Made available in DSpace on 2016-08-29T13:47:30Z (GMT). No. of bitstreams: 1 Dissertacao de Mestrado-Carla de Albuquerque Dias.pdf: 13161482 bytes, checksum: 7dae370ff2aeed1ca51c5f46b1701ffe (MD5) Previous issue date: 2016-04-15 / The motivation for this research was the synthesis and production of nanostructured materials ZnO (oxide of zinc) starting from Zn (pure zinc) using the high-energy mechanochemical technique (mechanochemistry or mechanochemical), applying the glycerin (C3H8O3) and the stearic acid (C18H36O2) as controlling agents of the process (PCA’s), both considered materials of interest in green chemistry. Through the measuring of X-Ray Diffraction (DRX) allied to the Rietveld Method it was possible to observe and to analyze in function of the times of grinding (0 min, 225 min, 525 min, 825 min, 855 min, 885 min, 915 min, 945 min, 975 min and 1005 min) the structural evolution of the sample. After 525 min a single phase of ZnO was synthesized, with a hexagonal structure of wurtzite type (or B4 type) with an average size of nanometric crystallites. It was also observed that the mechanochemical process does not modify the solubility of Zn according to the equilibrium diagram phase of the material. With the increase of milling time it was observed a polymorphic transition from the hexagonal structure of wurtzite type for a cubic rocksalt structure type (type B1 and type NaCl). The kinetics of the transition was measured by linewidth analysis and variation of the crystallographic parameters obtained in this study. / A motivação deste trabalho foi a síntese e produção de material nanoetruturado ZnO (óxido de zinco) a partir do Zn (zinco puro) utilizando-se a técnica mecanoquímica de alta energia (mechanochemistry ou mechanochemical), empregando-se como agente s controladores de processo (ACP’s) a glicerina (C3H8O3) e o ácido esteárico (C18H36O2), ambos considerados materiais de interesse na química verde. Através da realização de medidas de Difração de Raios-X (DRX) aliadas ao Método de Rietveld foi possível observar e analisar a evolução estrutural da amostra em função dos tempos de moagem (0 min, 225 min, 525 min, 825 min, 855 min, 885 min, 915 min, 945 min, 975 min e 1005 min). Após 525 min foi sintetizada uma fase única de ZnO, com estrutura hexagonal do tipo wurtzita (ou tipo B4) com tamanho médio de cristalitos nanométricos. Foi observado ainda que o processo de mecanoquímica não altera a solubilidade do Zn, conforme diagrama de fases em equilíbrio do material. Conforme houve o aumento do tempo de moagem observou- se uma transição polimórfica da estrutura hexagonal do tipo wurtzita para uma estrutura cúbica do tipo rocksalt (ou tipo B1 ou tipo NaCl). A cinética da transição foi quantificada pela análise de largura de linha e variação dos parâmetros cristalográficos obtidos no estudo.

This work has been devoted to the study of phase transformations involving chemical ordering and magnetic properties evolution in bulk Fe-Pt alloys composed of nanometer-sized grains. A comprehensive study of phase transformations and ordering in Fe-Pt alloys is performed by a combination of in-situ neutron powder diffraction and thermal analysis. The dependence of ordering processes on the alloy composition and initial microstructure (homogeneous A1 phase or multilayer-type) is established. Through the use of mechanical alloying and subsequent heat treatment it has been possible to achieve the formation of chemically highly ordered L10 FePt and, in the case of the Fe-rich and Pt-rich compositions, L12 Fe3Pt and FePt3 phases, respectively. Whereas in Pt-rich alloys the decoupling effect of the FePt3 phase leads to coercivity improvement, in Fe-rich nanocomposites a peculiar nanometer scale multilayer structure gives rise to remanence enhancement due to large effects of exchange interactions between the crystallites of the phases. The structure, magnetic properties and magnetisation reversal processes of these alloys are investigated. Experimentally observed phenomena are understood on the basis of a simple two-particle interaction model. Neutron diffraction has also been used for the investigation of the magnetic structure of ordered and partially ordered nanocrystalline Fe-Pt alloys. It has been shown that the magnetic moment of Fe atoms in L10-type Fe Pt alloys is sensitive to the compositional order. The results are compared to density functional calculations.