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The family of phases Li3M2XO6 (M = Mg, Co, Ni; X = Nb, Ta, Sb) has a novel rock salt superstructure in which the X cations occupy one set of octahedral sites with the Li and M cations distributed over three octahedral sites in a non-random manner. The non-random site occupancies vary depending on M and X and appear to be an equilibrium feature of the structure. Li3Ni2NbO6 undergoes a continuous order-disorder transition and can be doped with Cr3+ by a number of mechanisms to give either ordered or disordered rock salt structures. The phase diagram of the solid solutions of general formula Li3-xNi2-xCr xNbO6 has been determined and shows solid solutions for both ordered and disordered polymorphs with a transition zone that is 200-300A°C wide. Li3Ni2TaO6 and Cr-doped Li3Ni 2NbO6 compositions are modest semiconductors. The alpha and beta structures of polymorphic Li2CuZrO 4 have been determined. Both structures are ordered rock salt with unique sites for Zr and one set of Li atoms; in the alpha polymorph, Cu and the other set of Li atoms are partially ordered over two sites, but in beta-Li 2CuZrO4 all cations are uniquely ordered. In the phase transition from alpha-Li2CuZrO4 to the low temperature beta polymorph, the CuO6 octahedra, which predominantly corner-share in alpha, become edge-sharing. Magnetic susceptibility data indicate that, in both polymorphs, there are both ferromagnetic and antiferromagnetic interactions. The predominant interactions in the alpha phase are antiferromagnetic whereas in the beta polymorph, they are ferromagnetic. Conductivity measurements show that beta-Li2CuZrO4 is a modest semiconductor.

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Crystallization is an increasingly important process in a variety of applications from drug development to single crystal X-ray diffraction structure determination. However, while there is a good deal of research into prediction of molecular crystal structure, the factors that cause a molecule to be crystallizable have so far remained poorly understood. The aim of this project was to answer the seemingly straightforward question: can we predict how easily a molecule will crystallize? The Cambridge Structural Database contains almost a million examples of materials from the scientific literature that have crystallized. Models for the prediction of crystallization propensity of organic molecular materials were developed by training machine learning algorithms on carefully curated sets of molecules which are either observed or not observed to crystallize, extracted from a database of commercially available molecules. The models were validated computationally and experimentally, while feature extraction methods and high resolution powder diffraction studies were used to understand the molecular and structural features that determine the ease of crystallization. This led to the development of a new molecular descriptor which encodes information about the conformational flexibility of a molecule. The best models gave error rates of less than 5% for both cross-validation data and previously-unseen test data, demonstrating that crystallization propensity can be predicted with a high degree of accuracy. Molecular size, flexibility and nitrogen atom environments were found to be the most influential factors in determining the ease of crystallization, while microstructural features determined by powder diffraction showed almost no correlation with the model predictions. Further predictions on co-crystals show scope for extending the methodology to other relevant applications.

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La technique de diffraction de monocristaux est la plus utilisée pour déterminer une structure tridimensionnelle de protéine, permettant ainsi la compréhension de sa fonction biologique.Cependant, cette technique nécessite l'obtention d'un monocristal.La détermination d'une structure par diffraction de poudre ne requiert que l'obtention d'un précipité cristallin, souvent obtenu lors de la recherche d'une condition de cristallisation.Nous présentons dans cette thèse plusieurs protéines étudiées par diffraction de poudre. Le développement de nouvelles méthodes pour la préparation des échantillons et l'acquisition des données sont présentées, étant donnée leur importance cruciale dans ces études.Nous avons étudié le polymorphisme de l'urate oxidase par la détermination des différentes phases cristallines résultant des modifications des conditions de cristallisation. Cette étude a débouché sur l'identification d'une forme cristalline nouvelle d'intérêt pharmaceutique. Une des phases d'urate oxidase à permis l'obtention d'un cliché de diffraction de qualité suffisante à la redétermination et l'affinement de sa structure.Un protocole pour le refroidissement cryogénique de poudre de protéineest présenté, offrant une durée de vie accrue de l'échantillon lors de l'acquisition de données.Ce protocole a permis l'affinement de deux structures d'insuline humaine. Nous présentons également la détermination d'une structure préliminaire du domaine macro du virus Mayaro, basé uniquement sur des données acquises sur un unique échantillon polycristallin en forme d'oursin. Une étude de la matrice de protection de deux baculovirus illustre les limites auxquelles se heurte actuellement la technique de diffraction de poudre de protéines.En annexes, les étapes nécessaires pour la préparation et l'analyse des données sont présentées. / Knowledge of the structure of proteins helps in understanding theirbiological function.The main technique used, single crystal diffraction, requires thelimiting step of growing a single crystal.On the other hand, powder diffraction requires only a crystallineprecipitate made of many microcrystals such as those often discardedduring the search for suitable single crystal growth conditions.We present in this thesis different studies of proteins by powder diffraction.Development of new methods for sample preparation and data acquisitionare also presented, as they have been crucial steps to obtain highquality diffraction data.We studied the polymorphism of Urate oxidase by observing thedifferent crystallographic phases resulting from the changes in thecrystallisationconditions.A crystallographic phase of pharmaceutical interest has been identified.Also one phase of urate oxidase complexed with its inhibitor gave apowder pattern sufficient to re-determine and refine its structure.A protocol for cryocooling protein powder samples has been found,extending the lifespan of the sample in the intense X-ray beam.This allowed the refinement from powder data of two forms ofcryocooled human insulin.We present also the determination of a preliminary structure of themayaro virus macro domain, based on a powder diffraction patternobtained on a single urchin-like bundle of needles.A study of the protective protein matrix of two baculoviruses ispresented, showing some current limits of the method.In annexes, the steps for preparing and analysing protein powders are described.

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No / A macrocycle, 6, has been synthesized in high yield from 2,5-di(pyrazol-1-yl)hydroquinone and 1,1`-fc[B(Me)NMe2]2 {fc = Fe(C5H4)2}. The molecule incorporates two redox-active 1,1`-ferrocenylene units in its backbone and contains four chiral boron centers, each of them possessing the same configuration. It is demonstrated that crystal structures of organometallics of moderate complexity can be solved from high-resolution X-ray powder diffraction patterns, once the connectivity between the functional groups is known.

Demands for new, energy-efficient appliances have greatly increased in response to our growing need for a more environmentally friendly society. Magnetic refrigeration is a technique that utilizes the magnetocaloric effect, with possible energy savings of up to 30% compared to commercial gas compression refrigerators. A material appropriate for commercial magnetocaloric devices should be both cheap and non-toxic; it should also exhibit a first-order magnetic transitions close to room temperature. The magnetic properties of Fe2P-related materials can be relevant in this context, since their magnetic properties can be finely tuned through the substitution of Fe by Mn and P by Si, As, Ge or B to meet the general requirements for a magnetocaloric device. An in-depth study has therefore here been made of the structural and magnetic properties of the (Fe,Mn)2(P,Si)-system. The phase diagram of the FeMnP1-xSix-system has been carefully re-examined. It is found to contain two single-phase regions: an orthorhombic Co2P-type structure (x < 0.15) and a hexagonal Fe2P-type structure (0.24 ≤ x < 0.50). Selected compounds within the Fe2P-type region of the phase diagram have been shown to exhibit potential for use in magnetic refrigeration applications. Neutron powder diffraction has here been used to determine the magnetic structures of selected crystalline compositions within the FeMnP1-xSix-system to gain a better understanding of its magnetic properties. The Fe2P-type region is mainly ferromagnetic, but an incommensurate antiferromagnetic structure has also been identified close to the Co2P/Fe2P-type phase border for x ≈ 0.25. The so-called ''virgin effect'' in the Fe2P-type region of the FeMn(P,Si) phase diagram is found to be accompanied by an irreversible structural phase transition induced by magnetostriction. This new phase is found to be preserved during successive cooling-heating cycles. Furthermore, the magnetic properties of the substituted Fe2P-type structure changes significantly for metal:non-metal ratios away from 2:1. Such deviations could well explain the apparently conflicting structure-property relationships described in earlier literature for the FeMnP1-xSix-system.

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