The spinel structure (general formula AB2O4) is widely occurring in natural and synthetic materials, and has a marked technological and scientific significance due to its magnetic, electric and multiferroic behaviours. The presence of transition metal cations with multiple oxidation state and the resulting charge, orbital and spin degrees of freedom of the partially occupied d-orbitals lead to uniquely ordered ground states. The coupling of all the three degrees of freedom can result in a structurally distorted ground state where the direct metal-metal interaction forms atomic clusters, or 'orbital molecules'. The Verwey phase of magnetite (Fe3O4), occurring below TV ~ 125 K, is driven by a cooperative bond distortion that forms linear Fe3+-Fe2+-Fe3+ arrangement (trimeron). The effect of non-stoichiometry and chemical modification on this complex structure has been investigated with a variety of samples through microcrystal synchrotron XRD. A mineral sample (Al, Si, Mg and Mn impurities, TV = 119 K) confirms the Verwey phase as the most complex long-range electronic order known to occur naturally; its relevance in space sciences is discussed. Moreover, the structural analysis of two synthetic magnetites (Fe3(1-δ)O4 with 3δ = 0.012 and TV = 102 K, Fe3-xZnxO4 with x = 0.03 and TV = 90 K) univocally confirmed the persistence of the transition, and its first order, at doping level > 1 %, contrary to previous reports. Moreover, the temperature evolution of the trimerons and their persistence above TV was probed through X-ray Pair Distribution Function analysis on pure Fe3O4: the data analysis between 90 K < T < 923 K show that the Verwey phase goes from long-range ordered (T < 125 K) to short-range ordered (T > 850 K). Magnetite can thus only be considered to have a regular cubic spinel structure above the Curie temperature (TC = 858 K). The pyrochlore lattice of B cations in a spinel gives the structure the potential for frustration upon antiferromagnetic ordering. Fe2GeO4 and γ-Fe2SiO4 were synthesised through conventional solid state routes, with the use of high-pressure synthesis for the latter. Magnetometry and heat capacity measurements highlighted two transitions (Tm1 = 8.6 K and Tm2 = 7.2 K, and Tm1 = 11.2 K and Tm2 = 7.5 K respectively). Powder neutron diffraction data between 2 K < T < 25 K showed that both materials stay undistorted below TN. Magnetic Rietveld refinement led to two highly unconventional magnetic structures, with incommensurate propagation vectors and modulation of the moment magnitude. γ-Fe2SiO4 also shows a spin-ice order below Tm2. The results are unique and unusual for transition metal oxides; the models are systematised by proposing a 'frustration wave' model, in which the degree of frustration is a spatial quantity that can be distributed through the structure in order to stabilise the ground state.

En 1920, le moteur Diesel marque l'histoire en se faisant une place dans le milieu de l'automobile. Toutefois, malgré la révolution que représente le moteur Diesel notamment en terme de technologie (moteur à combustion interne dont l'allumage n'est pas commandé mais spontané par phénomène d'auto-inflammation (absence de bougie d'allumage)), des inconvénients majeurs subsistent, tout particulièrement au niveau environnemental et sanitaire (émission de gaz à effet de serre, prélèvement accru d'énergie fossile, impact direct sur la santé). Afin de lutter contre ces émissions, l'Union Européen à mit en place les normes EURO (depuis 1993) incitant les constructeurs automobiles à concevoir des procédés d'élimination des particules carbonées et à apporter des évolutions au niveau des motorisations. C'est dans ce contexte qu'a vu le jour la technologie Filtre à Particules initié par Peugeot en 1999 pour évoluer d'années en années jusqu'à être considérées aujourd'hui comme une avancée majeur en terme de traitement des particules Diesel. Encore aujourd'hui les problèmes d'émanations demeurent en raison des imbrûlés générés par le moteur diesel (suies, HC aromatiques polycycliques, d'oxyde de soufre, d'oxyde d'azote…). Les dégagements de particules de suies fines demeurant un problème particulièrement important au niveau de la santé. Cette thèse s'inscrit dans l'optique d'optimisation du procédé FàP en proposant l'élaboration de membrane à base de SiC supportée. Plus généralement, notre étude concerne l'élaboration de céramiques poreuses (membranes supportées et mousses) à base de silicium pour application environnementale et sanitaire (Filtration des particules fines, dépollution et séquestration de CO2).Le Chapitre I traite du contexte général de l'étude. La problématique des émissions de particules est abordée d'un point de vue sanitaire et environnemental en précisant les normes en vigueurs pour leur contrôle. La technologie FàP est décrite avant d'introduire le SiC et la voie dite des « polymères précéramiques » (PDCs). L'aspect catalytique est ensuite abordé avant de développer le principe d'élaboration de membrane SiC et leur intérêt pour une application de dépollution automobile.Le Chapitre II traite de l'élaboration de membranes SiC supportées. L'étude concerne l'élaboration d'un procédé optimale pour déposer une membrane au sein de la porosité du FàP qui modifierait les caractéristiques de porosité de ce dernier sans pour autant engendrer des répercussions néfastes sur la filtration. Le polymère précéramique, précurseur de SiC, sera alors décrit et nous étudierons sa mise en forme par la technique dite de « trempage-tirage » (dip-coating) afin d'élaborer, après pyrolyse, une membrane SiC. Cette dernière sera caractérisée par de nombreux outils expérimentaux.Le Chapitre III reprend le procédé d'élaboration des membranes de SiC élaboré dans le Chapitre II mais il proposera d'aller plus loin avec la réalisation et l'étude de catalyseurs pour la combustion des suies, et leur intégration au sein d'une microémulsion de type SiC-MxOy utilisée pour revêtir les FàP.Le Chapitre IV propose une étude sur la préparation de mousses à base de SiC. Ce chapitre d'aspect plus fondamental consistera à développer des mousses cellulaires et à porosité hiérarchisée à base des éléments silicium (Si), bore (B), carbone (C) et azote (N). Cette phase de carbonitrure de silicium et de bore (Si/B/C/N) sera élaborée par couplage de la voie PDCs avec soit des agents sacrificiels soit par réplication. Une étude préliminaire sur la séquestration de CO2 sera alors décrite pour finir. / Since the 90's, Diesel engines are widely used though they are criticized because of the pollution emitted. The constant updates of the Europeans norms (since 1993) concerning the diesel emissions imply a perpetual improvement of filtration techniques. The Diesel Particles Filter (DPF) technology used by the car manufacturer PSA Peugeot Citroën is one of the best ways to fulfill the limitation for diesel emissions. However, particles emission issue is still a problem and future legislations more and stricter, so an improvement of the DPF process is required to respect them. In this context, we have considered the elaboration of two different types of porous membranes on the DPF channels. The first one was in SiC, and had the aim to enhance the filtration efficiency. In this way, the smallest particles matter could be locked in the filter. The second kind of membrane integrates a catalytic phase inside the ceramic matrix, so in addition to the filtration aspect, it could improve soot combustion during the regeneration step of the DPF.The first chapter of my thesis deals with the literature corresponding to the subject, i.e. the DPF technology, non-oxides Si-based ceramics, and in particular those obtained through polymer-derived ceramics route (also called PDCs route). Then, ceramic coatings and catalytic phases are also treated. In the second chapter, we have considered the PDCs route and preceramic polymers to elaborate a SiC coating inside the DPF channels. We employed the dip-coating technique to overlay the channel surface with the AHPCS precursor of SiC (allylhydridopolycarbosilane), then, a pyrolysis under argon allows obtaining a SiC coating, in order to decrease the average pore diameter of the DPF (keeping an efficient filtration while avoiding overpressure) to catch soot nanoparticles evolving from Diesel engine.The third part of my PhD deals with the elaboration of another kind of coating for the DPF channels including a catalytic phase in the ceramic membrane. For this purpose, the microemulsion synthesis has been considered to prepare SiC-MxOy membrane. Further, we incorporated various catalytic phases based on Ce, Fe and Pt as activators of soot combustion. By employing the dip-coating technique, we successfully covered the DPF channels of our monoliths with the aforementioned microemulsion and after a heat treatment under controlled atmosphere; a porous coating consisting of the catalytic phase and the ceramic matrix was obtained. From this film, the porosity has been modified by lowering the diameter of the initial pores, but also by getting an additional porosity due to the polymer conversion and the surfactant decomposition. Catalytic sites in the ceramic have improved the soot combustion by lowering the temperature of the combustion.The fourth chapter introduces the elaboration of porous SiBCN materials through two approaches, replication and warm-pressing with sacrificial template (polymethylmethacrylate, PMMA). The SiBCN ceramic is a promising material due to its high mechanical properties and its stability at high temperature (1700-1800°C). By coupling the PDCs way with those two techniques, we are able to elaborate SiBCN porous materials which features can be tuned according to the technological application envisaged.

Layered lithium transition-metal oxides (LMOs) are used as the positive electrode material in rechargeable lithium-ion batteries. Because transition metals undergo redox reactions when lithium ions intercalate in and disintercalate from the lattice, the selection and composition of transition metals largely influence the electrochemical performance of LMOs. Recently, a Ni-rich compound, LiNi0.8Co0.1Mn0.1O2 (NCM811), has drawn much attention. It is expected to replace its state-of-the-art cousins, LiCoO2 (LCO) and LiNi1/3Co1/3Mn1/3O2 (NCM111), because of its higher capacity, lower cost, and reduced toxicity. However, the excess Ni, as a transition-metal element in NCM811, can cause structural and cycling instability. Starting from NCM811, I modified the composition of transition metals by two approaches: 1) introducing cobalt deficiency and 2) substituting Ni, Co, and Mn with Zr. Their influences on the phase, structure, cycling performance, rate capability, and ionic transport were investigated by a variety of characterization techniques. I found that cobalt non-stoichiometry can suppress Ni2+/Li+ cation mixing, but simultaneously promotes the formation of oxygen vacancies, leading to rapid capacity fade and inferior rate capability compared to pristine NCM811. On the other hand, Zr can reside on and expand the lattice of NCM811, and form Li-rich lithium zirconates on their surfaces. In particular, 1% Zr substitution can increase the stability of NCM811 and facilitate Li-ion transport, resulting in enhanced cycling durability and high-rate performance. My studies help improve the understanding of the effects of transition metals on the degradation of the Ni-rich layered positive electrode material and provide modification strategies to enhance its performance and durability for Li-ion battery applications.

<p>The subject of the present thesis is theoretical first principles electronic structure calculations on designer hard materials such as the transition metal carbides and oxides. The theoretical investigations have been made in close collaboration with experimental research and have addressed both bulk electronic properties and surface electronic properties of the materials.</p><p>Among the bulk studies are investigations on the effects of substoichiometry on the relative phase stabilities and the electronic structure of several phases of MoC and the nature of the resulting vacancy peaks. The changes in phase stabilities and homo-geneity ranges in the group IV to VI transition metal carbides have been studied and explained, from calculations of the T=0 energies of formation and cohesive energies. The anomalous volume behavior and phase stabilities in substoichiometric TiC was studied including effects of local relaxations around the vacancy sites. The vacancy ordering problem in this compound was also studied by a combination of electronic structure calculations and statistical physics.</p><p>The studies of the surface electronic properties include research on the surface energies and work functions of the transition metal carbides and an investigation on the segregation of transition metal impurities on the TiC (100) surface.</p><p>Theoretical studies with the aim to facilitate the realization of novel designer hard materials were made, among these a survey of means of stabilizing potentially super-hard cubic RuO₂, studying the effects of alloying, substoichiometry and lattice strains. A mechanism for enhancing hardness in the industrially important hard transition metal carbides and nitrides, from the discovery of multi-phase/polytypic alloys, has also been predicted from theoretical calculations.</p>

Two framework oxide materials of the MO��� network type have been synthesized and structurally characterized by synchrotron and X-ray powder diffraction and the Rietveld method in the temperature range 25~500 K. The results show one of them to be a low thermal expansion material. Theoretical studies of negative thermal expansion (NTE) in framework oxides were conducted with two methods, geometrical modeling by Rigid Unit Mode (RUM) method and lattice dynamic calculations by free energy minimization (FEM) method, the results are compared with each other as well as with experimental observations. RUM analysis of all five types of framework oxide structures negates any simple and direct correlation between presence or absence of RUMs in a structure and the sign of its thermal expansion. Instead, results suggest that NTE of a crystalline solid can not be explained by pure geometrical considerations over its structure alone, and for a better understanding of structure-relationship in negative thermal expansion structures, specific interatomic interactions present in each one must be brought in explicitly. FEM calculation of two negative thermal expansion structures indicates on a structure by structure basis NTE could be predicted and understood within the Gruneisen model, which attributes NTE of a structure to special vibration modes in a structure that softens when the lattice shrinks. The soft NTE modes are, however, not necessarily RUM or RUM like vibration motions. / Graduation date: 2003

For an electronic device well-designed interfaces are critical for the performance. Studies of interfaces down to an atomic level are thus highly motivated both from a fundamental and technological point of view. In this thesis, a surface science approach has been employed to study the formation of interfaces in systems relevant for transistor and solar cell applications. Surface science methodology entails ultra high vacuum environment, single crystalline surfaces, submonolayer control of deposited material, surface sensitive spectroscopy and atomic resolution microscopy. The primary experimental method for characterization is electron spectroscopy. This is a family of very powerful experimental techniques capable of giving information on the atomic level. Additionally, studies have been performed using scanning tunnelling microscopy. Combined these two methods can provide an atomic level characterisation of the geometric and electronic properties of the surface. The emphasis of this work is placed on ultra thin TiO2 and ZrO2 films grown on silicon substrates by means of ultra-high vacuum metal-organic chemical vapour deposition. ZrO2 has also been grown on SiC and FeCrAl. Deposition has been performed with different process parameters. The interface region of each film has been characterised. The band alignment, a most important issue with regard to the development of new transistor devices, for the ZrO2/Si(100) system has been explored. Decomposition pathways of the metal organic precursors have been studied in detail. Changing process parameters is shown to alter both the precursor decomposition pathway and the nature of the interface region, thus opening the possibility to tailor the material function. The titanium dioxide films grown in situ have shown to be excellent models of nanostructured electrode materials. In this spirit, interfaces of model systems for the solid-state dye-sensitized solar cell have been studied. Links between device performance and interface structure have been elucidated.

The subject of the present thesis is theoretical first principles electronic structure calculations on designer hard materials such as the transition metal carbides and oxides. The theoretical investigations have been made in close collaboration with experimental research and have addressed both bulk electronic properties and surface electronic properties of the materials. Among the bulk studies are investigations on the effects of substoichiometry on the relative phase stabilities and the electronic structure of several phases of MoC and the nature of the resulting vacancy peaks. The changes in phase stabilities and homo-geneity ranges in the group IV to VI transition metal carbides have been studied and explained, from calculations of the T=0 energies of formation and cohesive energies. The anomalous volume behavior and phase stabilities in substoichiometric TiC was studied including effects of local relaxations around the vacancy sites. The vacancy ordering problem in this compound was also studied by a combination of electronic structure calculations and statistical physics. The studies of the surface electronic properties include research on the surface energies and work functions of the transition metal carbides and an investigation on the segregation of transition metal impurities on the TiC (100) surface. Theoretical studies with the aim to facilitate the realization of novel designer hard materials were made, among these a survey of means of stabilizing potentially super-hard cubic RuO2, studying the effects of alloying, substoichiometry and lattice strains. A mechanism for enhancing hardness in the industrially important hard transition metal carbides and nitrides, from the discovery of multi-phase/polytypic alloys, has also been predicted from theoretical calculations.

Organic molecules adsorbed on inorganic substrates are the topics of interest in this thesis. Interfaces of this kind are found in dye sensitized solar cells that convert solar energy to electricity, a promising environmentally friendly energy source which might provide a route to replace fossil fuels. Another field where these interfaces play a role is in molecular electronics, an approach to solve the down scaling in the ever increasing hunt for miniaturized electronic devices. The motivation for this work lies among other in these applications and surface science is a suitable approach to investigate the electronic and morphologic properties of the interfaces as it provides detailed knowledge on an atomic level. Phthalocyanines are the organic molecules investigated and the inorganic substrates range from wide band gap via narrow band gap semiconductors to metals. Photoelectron and X-ray spectroscopy experiments are performed to shed light on the electronic properties of the adsorbed molecules and the substrate, as well as the chemical interaction between adsorbate and substrate at the interface. The ordering of the adsorbate at the interface is important as ordered molecular thin films may have other properties than amorphous films due to the anisotropic electronic properties of the organic molecules; this is investigated using scanning tunneling microscopy. We find that the phthalocyanines are affected by adsorption when the substrate is TiO2 or Ag, where charge transfer from the molecule occurs or an interface state is formed respectively. The molecules are adsorbed flat on these surfaces giving a large contact area and a relatively strong bond. On Ag, ordered structures appear with different symmetry depending on initial coverage. The reactivity of the TiO2 surface is not ideal in the solar cell application and by modifying the surface with a thin organic layer, the negative influence on the adsorbed phthalocyanine is reduced. ZnO is not as reactive as TiO2, thanks maybe to the upright adsorption mode of the phthalocyanines. The semiconductor InSb is less reactive leading to self-assembled molecular structures on the (001) surface, either homogenously distributed in a one monolayer thick film or in strands along the reconstruction rows. InAs on the other hand has a larger influence on the adsorbed molecules resulting in a metallic film upon thermal treatment. / QC 20100812

This work presents a study of the electronic structure of four transition metal oxides (TMOs) using spectroscopic data and a variety of theoretical models. TMOs are a class of materials made from d-block metals in the periodic table, and one or more oxygen atoms. The nature of d-electrons is examined and theoretical models used to treat d-electron systems are tested against experimental data. Background theory of condensed matter physics is outlined. An overview of density functional theory (DFT) as a theoretical model for calculating the electronic structure of materials is presented. A variety of exchange-correlation (XC) functionals used within the DFT framework are outlined and tested for their applicability to the TMO systems in question. X-ray spectroscopy is briefly outlined and used to test the validity of the different XC functionals. All four compounds, AgO, Ag2O, CuO, and Cu2O require a Hubbard U term in the XC functional to most accurately reproduce experimental results. The effects of varying the value of U is examined in depth. The oxygen K-edge X-ray emission spectra (XES) exhibits a“two peak” structure for all compounds; the effect of varying the U value is to change the intensity ratio of the two peaks. The ratio of the two peaks as a function of U shows a linear trend in all compounds. A simple line is fit to the peak ratio vs. U curve. A common line between all compounds would provide an important metric with which to predict the appropriate U value needed in similar materials based on simple experimental data. However, the parameters of the fitted line were not common between the four compounds and any metric derived from this method would be system-dependent and not widely applicable to other systems. There are, however, interesting trends in the data when the U value is varied that provide subjects for future research. A number of fundamental quantities are determined both from experiment and theoretical calculations. Calculated bandgap values are shown to be lower than the experimental values for most functionals tested. This is not unexpected as DFT methods are known to predict much smaller bandgaps than expected. The Heyd-Scuseria-Ernzerhof (HSE) functional used for Ag2O and Cu2O does predict the bandgaps very accurately. The core-hole effect is estimated and proven to be negligible in these systems. Charge transfer and on-site Coulomb repulsion energies, important quantities in the electronic behaviour of TMOs, are determined and compared to previously reported values.

There is no apparent, dominant interaction in heavy transition metal oxides (TMO), especially in 5d-TMO, where all relevant interactions are of comparable energy scales, and therefore strongly compete. In particular, the spin-orbit interaction (SOI) strongly competes with the electron-lattice and on-site Coulomb interaction (U). Therefore, any tool that allows one to tune the relative strengths of SOI and U is expected to offer an opportunity for the discovery and study of novel materials. BaIrO3 is a magnetic insulator driven by SOI whereas the isostructural BaRuO3 is a paramagnetic metal. The contrasting ground states have been shown to result from the critical role of the strong SOI in the iridate. This dissertation thoroughly examines a wide array of newly observed novel phenomena induced by adjusting the relative strengths of SOI and U via a systematic chemical substitution of the Ru4+(4d4) ions for Ir4+(5d5) ions in BaIrO3, i.e., in high quality single crystals of BaIr1-xRuxO3(0.0 < x < 1.0) . Our investigation of structural, magnetic, transport and thermal properties reveals that Ru substitution directly rebalances the competing energies so profoundly that it generates a rich phase diagram for BaIr1-xRuxO3 featuring two major effects: (1) Light Ru doping (0 < x < 0.15) prompts a simultaneous and precipitous drop in both the magnetic ordering temperature TC and the electrical resistivity, which exhibits metal-insulator transition at around TC. (2) Heavier Ru doping (0.41 < x < 0.82) induces a robust metallic and spin frustration state. For comparison and contrast, we also substituted Rh4+(4d5) ions for Ir4+(5d5) ions in BaIrO3, i.e. BaIr1-xRhxO3(0.0 < x < 0.10), where Rh only reduces the SOI, but without altering the band filling. Hence, this system remains tuned at the Mott instability and is very susceptible to disorder scattering which gives rise to Anderson localization.