In 1915, Debye derived his well-known equation for the X-ray scattering from a sample of randomly orientated gas-phase molecules. He approximated the molecular scattering by adding the contributions of isolated atomic constituents. This is known as the Independent Atom Model (IAM). However, it omits the redistribution of valence electrons due to bonding, and is limited to the electronic ground state. The main proposition of this thesis is that it is worthwhile going beyond the IAM when interpreting X-ray scattering data. In part, this is motivated by the arrival of new X-ray sources called X-ray Free-Electron Lasers (XFELs). A new method called Ab Initio X-ray Diffraction (AIXRD) is introduced. It calculates the elastic X-ray molecular scattering factor directly from wave functions calculated by ab initio electronic structure theory, for instance Hartree-Fock or multiconfigurational self-consistent field. In this way, the valence electrons are correctly taken into account, and calculations based on electronically excited wave functions become possible. The wave functions must be constructed from spatial orbitals made up of Gaussian-Type Orbitals (GTOs), giving an analytical solution to the Fourier transform integrals involved, and is key to computationally efficient and accurate results. This is compared to a fast Fourier transform (FFT) method, where the electron density is computed on a 3D grid and an FFT algorithm is used to obtain the elastic X-ray molecular scattering factor. Inspired by post-crystallography experiments such as serial femtosecond crystallography and single-particle imaging at XFELs, the AIXRD method is expanded to allow accurate X-ray diffraction calculations from large molecules such as proteins. To make the underlying ab initio problem tractable, the molecule is split into fragments. In other words, the electron density is constructed by a sum of fragment contributions, as is the corresponding molecular form-factor. In this way, it is analogous to the IAM approach except that instead of isolated atoms, there are isolated fragments. A pairwise summation of fragment contributions is also used to account for fragment-fragment interactions. Various fragment definitions are compared based on their effect on the X-ray diffraction signal, and are compared to the IAM method. Finally, X-ray diffraction from molecules in specific quantum states is calculated, revealing a distinct quantum fingerprint in the X-ray diffraction, and a comparison to experiment is made. In particular, the elastic X-ray diffraction is calculated from gas-phase H2 pumped to various electronic, vibrational, and electronic states. This is expanded upon for polyatomic molecules using the harmonic approximation for the vibrational states.

This work presents findings from experiments carried out using the neutron Laue method in tandem with laboratory source X-ray diffraction to characterise a series of organic molecular complexes which exhibit interesting, and potentially “tunable”, temperature dependent charge transfer effects, such as proton migration and proton disorder within hydrogen bonded networks. These subtle processes are studied by variable temperature neutron diffraction studies, allowing the positional and anisotropic displacement parameters of the hydrogen atoms to be refined accurately and their evolution with temperature followed. The hydrogen atom behaviour is found to be influenced by the local environment, including weak intermolecular interactions in the vicinity of the hydrogen bond under study. Complexes of urea and methyl substituted ureas with small organic acids are presented, which show robust and reproducible structural motifs. In favourable circumstances, these contain short, strong hydrogen bonds (SSHBs) within which the proton may undergo temperature dependent migration. By synthesising a number of complexes containing SSHBs, potential routes to the design of proton migration complexes are found, which utilise crystal engineering principles and pKa matching. Variable temperature studies conducted on these complexes also show unusual thermal expansion properties and phase transitions in urea-acid complexes which do not display proton migration. Systems containing hydrogen bonded dimers of 3,5-dinitrobenzoic acid are also studied, and shown to contain temperature proton disorder within moderate strength hydrogen bonds linking the dimers. The presence and potential onset temperature of any disorder is found to be influenced by interactions around the acid dimers and potential routes to controlling proton disorder are discussed. Complexes of the proton sponge, 1,8-bis(dimethylamino)napthalene (DMAN), with organic acids are also presented, in which the structures have been determined using neutron diffraction. DMAN readily accepts a proton from the acid co-molecules used in forming the complexes, forming a strong intramolecular SSHB within the protonated DMAN. Strong intermolecular hydrogen bonds are also induced between the acid molecules in many cases. The neutron studies presented here investigate the effect of weak interactions on the behaviour of hydrogen atoms located within these SSHBs, and also indicate over what distance such interactions significantly affect the hydrogen atom behaviour.

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Thesis (Ph. D.)--University of Washington, 1997. / Vita. Includes bibliographical references (leaves [161]-163).

With the overriding goal of developing functional multiferroic systems with technological potential, this thesis focuses on the role of orbital and charge ordering in coupling magnetism and ferroelectricity in synthetic quadruple perovskites. Using x-ray diffraction as the primary characterisation tool, modulations to crystal ordering have been interpreted in terms of orbital occupation and charge variation. Expanding on previous magnetic structure studies and polarisation measurements, structural analysis of CaMn₇O₁₂ has led to the experimental realisation of a new mechanism for multiferroicity, resulting from a "magneto-orbital helix". Motivated by the idea of tuning multiferroic properties through varying manganese valence, the doped system CaCu_xMn_7-xO₁₂ has been studied. Structural models considering the possibility of domain formation and multiple coexisting modulations have been tested against x-ray diffraction data. Finally, motivated by theoretical predictions of ferroelectric phases and multiferroicity in doped, simple, manganite perovskites, a structural model for the low temperature phase of NaMn₇O₁₂ has been developed, based upon theoretical predictions for orbital ordering and the experimentally determined magnetic structure. This model has been tested against previously measured neutron diffraction data. The importance of understanding crystal formation and domain structures when applying theoretical models has been highlighted, and has prompted the consideration of future work involving viewing and manipulating twin formation.

Submitted by Rafael Santana (rafael.silvasantana@ufpe.br) on 2017-07-10T19:30:23Z No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Dissertacao - Pedro Negrão.pdf: 2417925 bytes, checksum: 563fc37dbb3bf794487bc4e3c8fc102a (MD5) / Made available in DSpace on 2017-07-10T19:30:23Z (GMT). No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Dissertacao - Pedro Negrão.pdf: 2417925 bytes, checksum: 563fc37dbb3bf794487bc4e3c8fc102a (MD5) Previous issue date: 2016-03-29 / CAPES / As argilas compõem um grupo de matérias-primas muito importante devido a suas múltiplas funções industriais. Em especial o caulim, possui propriedades físicas e químicas que possibilitam a sua utilização no processo industrial de uma grande gama de produtos, desde matéria-prima para a indústria cerâmica, de carga e cobertura de papel, entre outras. Esta pesquisa objetivou descrever, através da caracterização mineralógica, análise química, análise granulométrica, ensaios tecnológicos e de plasticidade, as características do material arenocaulinítico sondado no município de Igarassu – PE, ocorrência localizada próxima ao bairro de Nova Cruz 2. O material foi coletado através de sondagem a trado, tendo as amostras passado por análise mineralógica por difração de raios X, análise química por fluorescência de raios X, análise granulométrica, ensaios de plasticidade e foram confeccionados corpos de prova para a realização de ensaios tecnológicos. Os resultados obtidos foram comparados com as características de caulins de outros jazimentos descritos na literatura e com as especificações dos caulins aproveitados industrialmente. / Clays form a group of so important raw materials due to its multiple industrial functions. In particular, the kaolin has physical and chemical properties which allow its use at the industrial process a wide range of products from a raw material for ceramic industry, filler and paper coating, among others. This research aimed to describe through mineralogical characterization, chemical analysis, particle size analysis, technological tests and plasticity characteristics of sandy-kaolinitic materials collected in the city of Igarassu - PE, occurrence located near the Nova Cruz 2. The material was got by auger boring, the samples submitted to a mineralogical analysis through X ray diffraction, chemical analysis through X ray fluorescence, particle size analysis, plasticity tests and was prepared specimens to be performed for the technological tests. The results obtained were compared with the characteristics of kaolin of other deposits described in the literature and with the specifications of kaolin exploited industrially.

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