Les perovskites hybrides organique-halogénure de plomb représentent une classe de matériaux émergents, proposés en tant qu'absorbeur de lumière dans le cadre d'une nouvelle génération de cellules solaires. La formule chimique de ces composés est APbX3, où A est un cation organique, X représente un anion halogénure (normalement Cl-, Br-, ou l-, ou alors un alliage composé par ces éléments). Les perovskite hybrides combinent d'excellentes propriétés d'absorption avec une grande longueur de diffusion et de longues durées de vie des porteurs de charge, ce qui permet d'atteindre des efficacités de conversion de photons de 22%. Un autre avantage réside dans leur bas coût de fabrication. Par conséquent, avec le développement de cette classe de matériaux, le photovoltaïque basé sur les perovskites sera potentiellement capable de fortement améliorer les performances de la technologie photovoltaïque actuelle, basée sur le silicium. Dans cette thèse, nous utilisons des méthodes optiques afin d'étudier les propriétés électroniques de base et la morphologie de couches minces de plusieurs représentants des perovskites. Nous étudions notamment des composés ayant le methylammonium et le formamidinium en tant que cations organiques ainsi que les iodures et les bromures à large bande interdite et nous montrons de quelle manière la composition chimique influence les paramètres étudiés. Par magnéto-transmission, nous déterminons directement l'énergie de liaison de l'exciton et sa masse réduite. Nous avons trouvé que les énergies de liaison à T = 2K sont comprises de 14 à 25 meV, plus petites ou comparables à l'énergie thermique moyenne à la température ambiante (25meV). De plus, ces valeurs diminuent à T=160K jusqu'à 10-24meV. Suite à ces résultats, nous concluons que les porteurs photocréés dans les perovksites peuvent être considérés ionisés thermiquement à la température ambiante. Les valeurs de masse effective sont comprises entre 0.09-0.13 fois la masse de l'électron libre. Nous montrons également que l'énergie de liaison de l'exciton ainsi que la masse effective dépendent linéairement de la valeur de la bande interdite. Nos résultats permettent donc d'estimer la valeur des paramètres de ces nouveaux composés perovksites. Nous avons étudié la morphologie de couches minces de perovskite par photoluminescence résolue spatialement avec une résolution micrométrique. Cette technique nous a permis d'observer des grains cristallins uniques. Nous démontrons que la transition de la phase tétragonale à orthorhombique à basse température est incomplète dans tous les matériaux étudiés, comme montré par les résidus de phase tétragonale trouvés à T =4K. En étudiant structurellement certaines régions endommagées et photo-recuites, nous montrons que la présence de la phase tétragonale à basse température augmente, causée par une déplétion de l'halogène. / The hybrid organo-lead halide perovskites are an emerging class of materials, proposed for use as light absorbers in a new generation of photovoltaic solar cells. The chemical formula for these materials is APbX3, where A is an organic cation and X represents halide anions (most commonly Br-, Cl- or I-, or alloyed combination of these). The hybrid perovskies combine excellent absorption properties with large diffusion lengths and long lifetime of the carriers, resulting in photon conversion efficiencies as high as 22%. Another advantage is the inexpensiveness of the fabrication process. Therefore, with the rapid development of this class of materials, the perovskite photovoltaics has perspectives to outperform the well-established silicon technology. Here, we use optical methods to investigate the basic electronic properties and morphology in the thin films of several representatives of the hybrid perovskites. We study the compounds based on Methylammonium and Formamidinium organic cations; the iodides and wide band-gap bromides, showing how the chemical composition influences the investigated parameters. Using magneto-transmission, we directly determine the values of exciton binding energy and reduced mass. We find that the exciton binding energies at T = 2 K, varying from 14 to 25 meV, are smaller or comparable to the average thermal energy at room temperature (˜25 meV). Moreover, these values fall further at T = 160 K, to 10-24 meV. Based on that we conclude that the carriers photocreated in a perovskite material can be considered to be thermally ionized at room temperature. The measured reduced masses are in the range of 0.09-0.13 of the electron rest mass. We also show that both exciton binding energy and reduced mass depend linearly on the band gap energy. Therefore, the values of these parameters can be easily estimated for the synthesis of new perovskite compounds. With the spatially resolved photoluminescence, we probe the morphology of perovskite films with micrometer resolution, which enables us to observe single crystalline grains. The resulting maps show that all investigated thin films are composed from the dark and bright crystalline grains. We demonstrate that the low temperature phase transition from tetragonal to orthorhombic phase is incomplete in all studied materials, as the remains of the tetragonal phase are found even at T = 4 K. By investigating structurally damaged and photo annealed regions, where the occurrence of the tetragonal phase at low temperatures is enhanced, we attribute its presence to the depleted halide content.

En premier lieu, des couches minces épitaxiales ont été obtenues sur des substrats de silicium grâce à l’utilisation de l’épitaxie par jets moléculaire et de couches tampons de titanate de strontium SrTiO3. Une technique de croissance par co-déposition a été développée de manière à obtenir un rapport Ba:Ti proche de la stoechiométrie, et ce afin d’éviter la formation de défauts cristallins dans la couche de BaTiO3. Le matériau déposé cristallise dans une structure de symétrie quadratique, ce qui est unpré-requis pour l’obtention de propriétés électro-optiques. De plus, selon les conditions de croissance, l’axe c de la maille élémentaire quadratique a pu être ajusté de manière à être aligné parallèlement ou perpendiculairement à la surface du substrat. L’utilisation d’une mince couche tampon de nucléation a également permis de croitre des films mincesBaTiO3 épitaxiées par pulvérisation, technique largement répandue en milieu industriel.Un coefficient de Pockels élevé a par la suite été obtenu sur de tellescouches épitaxiées. La valeur mesurée de 148pmV est clairement supérieure aux valeurs admises dans la littérature pour d’autres matériaux nonlinéairestels que le niobate de lithium, pour lequel un coefficient de31pmV est rapporté. La méthode de caractérisation électro-optique développée à cette occasion révèle également le caractère ferroélectrique des couches de BaTiO3, observé pour la première fois dans de tels matériaux épitaxiés sur silicium.Finalement, ces couches minces électro-optiquement actives ont été intégrées dans des dispositifs photoniques sur silicium. Dans cette optique,une structure de guide d’onde à fente a été utilisée en insérant 50nm deBaTiO3 entre deux couches de silicium. Dans ce type de structure, le confine mentoptique est 5 fois supérieur à celui obtenu pour des guides d’onde en silicium avec une gaine à base de BaTiO3. Des guides d’ondes rectilignesont tout d’abord été fabriqués, pour lesquels des pertes optiques del’ordre de 50−100 dB/cm ont été mesurées. Par la suite, des composants passifs fonctionnels ont été fabriqués, tels que des interféromètres typeMach-Zehnder, des résonateurs circulaires et des coupleurs. Finalement,la fonctionnalité de composants actifs a été démontrée pour la première fois, en se basant notamment sur des résonateurs ayant un facteur de qualité Q d’environ 5000, et pour lequel la résonance varie en fonction du champ électrique transverse. L’origine physique de cette variation n’a cependant pas pu être expliquée sur la seule base de l’effet Pockels. Cette thèse démontre que l’utilisation de nouveaux matériaux électro optiquement actifs au coeur de dispositifs photoniques sur silicium créede nouvelles opportunités pour la conception et l’ingénierie de circuitsphotoniques. L’intégration d’oxydes tels que barium titanate permet d’envisager de nouveaux concepts de dispositifs pour ajuster, moduler ou commuter la lumière au sein de circuits photoniques denses. De nouveaux défis et perspectives s’ouvrent également aux scientifiques pour modifier artificiellement les propriétés électro-optiques de ces matériaux, que ce soit par contrainte, dopage ou par l’ingénierie de multicouches. De telles avancées pourront sans aucun doute fortement améliorer les performances des dispositifs. / A novel concept of utilizing electro-optical active oxides in silicon photonic devices is developed and realized in the frame of this thesis. The integration of such oxides extends the silicon photonics platform by non-linear materials, which can be used for ultra-fast switching or low-power tuning applications. Barium titanate is used as active material as it shows one of the strongest Pockels coefficients among all oxides. Three major goals are achieved throughout this work: First, thin films of BaTiO3 are epitaxially grown on silicon substrates via molecular beam epitaxy (MBE) using thin SrTiO3 buffer layers. A shuttered co-deposition growth technique is developed in order to minimize the formation of defects in the BaTiO3 films by achieving a 1:1 stoichiometry between barium and titanium. The layers show a tetragonal symmetry and are therefore well-suited for electro-optical applications. The orientation of the long c -axis of the BaTiO3 crystal can be tuned to point perpendicular or parallel to the film surface, depending on the growth conditions. In addition, thin MBE-grown seed layers are combined with rf-sputter deposition. With this hybrid growth approach, rather thick ( > 100 nm), epitaxial BaTiO3 layers on silicon substrates are obtained with a commercially available, wide spread deposition technique. As a second goal, a strong Pockels coefficient of reff = 148 pm/V is determined in the epitaxial BaTiO3 films. This first experimental result on the electro-optical activity of BaTiO3 layers on silicon shows a clear enhancement compared to alternative non-linear materials such as lithium niobate with reff = 31 pm/V. By means of the electro-optical characterization method, also the presence of ferroelectricity in the films is demonstrated. Third, the electro-optical active BaTiO3 layers are embedded into silicon photonic devices. For this purpose, a horizontal slot-waveguide structure with a ~50 nm-thick BaTiO3 film sandwiched between two silicon layers is designed. With this design, the optical confinement in the active BaTiO3 layer is enhanced by a factor of 5 compared to Si-waveguide structures with a standard cross section and BaTiO3 as cladding. Straight BaTiO3 slot-waveguides with propagation losses of 50 − 100 dB/cm as well as functional passive devices such as Mach-Zehnder-interferometers, couplers, and ring resonators are experimentally realized. Additionally, first active ring resonators with Q-factors of Q~5000 are fabricated. The physical origin of the observed resonance shift as a function of the applied bias voltage, however, can not be conclusively clarified in the present work. The combination of high-quality, functional BaTiO3 layers with silicon photonic devices as demonstrated in this thesis offers new opportunities by extending the design palette for engineering photonic circuits with the class of electro-opticalactive materials. The integration of oxides such as BaTiO3 enables novel device concepts for tuning, switching, and modulating light in extremely dense photonic circuits. The integration also opens exciting challenges for material scientists to tailor the electro-optical properties of those oxides by strain engineering or fabrication of superlattice structures, which could ultimately lead to another boost of their electro-optical properties.

No description available.

La pile à combustible H+-SOFC (Protonic Conducting Solid Oxide Fuel Cell) basée surl’utilisation d’un électrolyte conducteur protonique peut représenter une alternative intéressanteà la pile SOFC qui présente actuellement le meilleur rendement. Cependant, la surtension à lacathode reste élevée et ce travail est dédié à la compréhension du mécanisme de réductionl'oxygène à cette électrode.Différents matériaux conducteurs mixtes O2- / e- de structures dérivées de la perovskite ABO3,tels que les doubles perovskites LnBaM2O5+d (Ln = Pr, Nd et Gd et M = Co et Fe) ainsi que lesphases de Ruddlesden-Popper A2MO4+d (Ln = Pr et Sr et M = Ni), ont été étudiés. Leur niveaude conductivité électronique ainsi que leur non-stoechiométrie en oxygène ont d’abord étédéterminées. Puis, à l’aide de la détermination des coefficients de diffusion de l’oxygène par laméthode de relaxation de conductivité électrique, leur conductivité ionique O2- a été estimée.Une étude électrochimique et plus spécialement la détermination des étapes limitant la réactionde réduction de l’oxygène à la cathode de pile H+-SOFC a ensuite permis de démontrer le rôledu proton dans le mécanisme de réaction pour les matériaux présentant les meilleuresperformances électrochimiques.Enfin, dans le cadre d’un projet ANR HPAC 2009 « CONDOR », des mono-cellules de piles H+-SOFC ont été mises en forme et des densités de puissance proche de 180 mW/cm² à 0.6 V à600°C ont été obtenues. / The H+-SOFC (Protonic Conducting Solid Oxide Fuel Cell) based on a protonic conductingelectrolyte can represent an interesting alternative to the SOFC fuel cell. Nevertheless, the highcathodic overpotential remains a severe drawback and this thesis is dedicated to the study of theunderstanding of the oxygen reduction at the cathode.Several mixed O2- / e- conductors derived from perovskite ABO3 such as double perovskiteLnBaM2O5+d and Ruddlesden Popper phases A2MO4+d were studied. Their electronic conductivityas well as their oxygen non-stoichiometry were first determined. Then, their oxygen diffusioncoefficients were measured using the electrical conductivity relaxation method and their O2-conductivity estimated. Rate determining steps of the oxygen reduction reaction weredetermined from electrochemical measurements and it was shown that proton is involved in theoxygen reduction for materials showing the best electrochemical performances.Finally, single H+-SOFC cells were developed in the framework of the ANR HPAC 2009 project“CONDOR” and power densities of about 180 mW/cm² at 0.6 V at 600°C were obtained.

Tesis por compendio / [ES] Las perovskitas orgánico-inorgánicas de halogenuros son una familia de materiales estudiados en profundidad por su potencial para construir células solares de alta eficiencia y de bajo coste. Han experimentado un crecimiento exponencial de la eficiencia, pasando del 3% del PCE en 2009 a más del 25% en 2021. Pero todavía quedan retos por superar, como la estabilidad. El objetivo del trabajo es estudiar diferentes tipos de HOIP para aplicaciones fotovoltaicas, incluyendo la optimización del proceso de fabricación y la caracterización completa mediante el análisis de difracción de rayos X, microscopía electrónica de escaneo de emisión de campo, microscopio de fuerza atómica, fotoluminiscencia, UV-absorción visible, cálculo de band-gap, microscopía electrónica de transmitancia, simulación teórica y estudio de degradación de las películas. El objetivo del análisis es obtener la cristalinidad, morfología, topografía, propiedades ópticas, PCE y estabilidad de las capas. Se utilizaron varias estrategias para cumplir el objetivo del trabajo, incluyendo el dopaje con distintos compuestos, la ingeniería de extinción antisolvente y el cambio del catión "B" en la fórmula de las perovskitas; ABX3. La presente disertación se centra en el yoduro de plomo de metilamonio III (CH3NH3I3), que es conocido por presentar una absorción directa de bandgap desde la parte superior de la banda de valencia hasta la parte inferior de la banda de conducción. El bandgap puede modificarse fácilmente variante A, B y X y modularse mediante la selección adecuada de cationes mixtos. Entre las combinaciones posibles, el catión MA y el metal Pb2+ han mostrado excelentes propiedades optoelectrónicas, capas procesables en disolución a baja temperatura y potencial para una estabilidad adecuada, debido a la movilidad del portador de carga muy alta, la gran longitud de difusión de electrones y agujeros, grandes coeficientes de absorción, y bajas tasas de recombinación no radiativa. Concretamente, este doctorado se compone de cuatro artículos: - Stability Improvement of Methylammonium Lead Iodide Perovskite Thin Films by Bismuth Doping - Tetrabutylammonium (TBA)-Doped Methylammonium Lead Iodide: High Quality and Stable Perovskite Thin Films - Manufacture of High-Efficiency and Stable Lead-Free Solar Cells through Antisolvent Quenching Engineering - Investigation on the Stability and Efficiency of MAPbI3 and MASnI3 Thin Films for Solar Cells En resumen, el dopaje de bismuto mejoró la estructura cristalina de la capa absorbente de MAPbI3, mejorando las propiedades optoelectrónicas, la morfología de la superficie de las capas y la estabilidad de los dispositivos. Se estudió el dopaje con bismuto introduciendo cantidades variables de bismuto entre el 1 y el 8% en la solución inicial. Los mejores resultados se obtuvieron con un 2% de bismuto. También se analizó el dopaje con TBA introduciendo diferentes proporciones en la mezcla inicial para la síntesis de capas de MAPbI3. Observamos que un 5% de TBA reduce la densidad de agujeros en las capas y mejora la cristalinidad, mejorando la estabilidad de las capas MAPbI3. Con la proporción óptima de TBA, aumenta el tamaño del grano y la intensidad de la fotoluminiscencia, debido a la disminución de los centros de recombinación. Dado que el plomo es un elemento contaminante, sustituimos a Pb por Sn para obtener capas de MASnI3, lo que provocó un aumento del tamaño del grano y una mejora del coeficiente de absorción de la luz. Sin embargo, el MASnI3 es más inestable que el MAPbI3. Por eso, se utilizaron diferentes antisolventes en la síntesis de MASnI3. Conocida como ingeniería antisolvente, esta técnica estudia varios antisolventes. En este trabajo, el tolueno ha logrado mejorar la estabilidad de las capas de MASnI3. Utilizando un enfoque numérico mediante SCAPS-1D, se calculó que la eficiencia de los dispositivos fotovoltaicos de MASnI3 aumenta un 9,5% en comparación con los dispositivos de MAPbI3. / [CAT] Les perovskites orgàniques-inorgàniques d'halogenurs són una família de materials estudiats en profunditat pel seu potencial per construir cèl·lules solars d'alta eficiència i de baix cost. Han experimentat un creixement exponencial de l'eficiència, passant del 3% del PCE el 2009 a més del 25% el 2021. Però encara queden reptes per superar, com l'estabilitat. L'objectiu d'aquest treball és estudiar diferents tipus de HOIP per a aplicacions de cèl·lules solars, incloent l'optimització del procés de fabricació i la caracterització completa mitjançant l'anàlisi de difracció de raigs X, microscòpia electrònica d'escaneig d'emissió de camp, microscopi de força atòmica, fotoluminescència, UV- absorció visible, càlcul de band-gap, microscòpia electrònica de transmitància, simulació teòrica amb SCAPS-1D i estudi de degradació de les pel·lícules. L'objectiu de l'anàlisi és obtenir la cristalinitat, morfologia, topografia, propietats òptiques, PCE i estabilitat de les capes. Es van utilitzar diferents estratègies per complir l'objectiu del treball, incloent el dopatge amb diferents compostos, l'enginyeria d'extinció antisolvent i el canvi del catió "B" en la fórmula general de les perovskites; ABX3. La present dissertació se centra en el iodur de plom de metilamoni III (CH3NH3I3), que és conegut per presentar una absorció directa de bandgap des de la part superior de la banda de valència fins a la part inferior de la banda de conducció. El bandgap es pot modificar fàcilment variant A, B i X i modular-se mitjançant la selecció adequada de cations mixts. Entre les combinacions possibles, el catió MA i el metall Pb2+ han mostrat excel·lents propietats optoelectròniques, capes processables en dissolució a baixa temperatura i potencial per a una estabilitat adequada, a causa de la mobilitat del portador de càrrega molt alta, la gran longitud de difusió d'electrons i forats, grans coeficients d'absorció, i baixes taxes de recombinació no radiativa. Concretament, aquest doctorat es compon de quatre articles: -Stability Improvement of Methylammonium Lead Iodide Perovskite Thin Films by Bismuth Doping -Tetrabutylammonium (TBA)-Doped Methylammonium Lead Iodide: High Quality and Stable Perovskite Thin Films -Manufacture of High-Efficiency and Stable Lead-Free Solar Cells through Antisolvent Quenching Engineering -Investigation on the Stability and Efficiency of MAPbI3 and MASnI3 Thin Films for Solar Cells En resum, el dopatge de bismut ha millorat l'estructura cristal·lina de la capa absorbent de MAPbI3, millorant les propietats optoelectròniques, la morfologia de la superfície de les capes i l'estabilitat dels dispositius. Es va estudiar el dopatge amb bismut introduint quantitats variables de bismut entre l'1 i el 8% en la solució inicial. Els millors resultats es van obtenir amb un 2% de bismut. També es va analitzar el dopatge amb tetrabutilamoni introduint diferents proporcions de TBA a la mescla inicial per a la síntesi de capes de MAPbI3. Observem que afegir un 5% de TBA a la mescla inicial redueix la densitat de forats a les capes i millora la cristal·linitat, millorant en l'estabilitat de les capes MAPbI3. Amb la proporció òptima de TBA, augmenta la mida del gra i la intensitat de la fotoluminescència, a causa de la disminució dels centres de recombinació. Com que el plom és un element contaminant, vam substituir Pb per Sn per obtenir capes de MASnI3, cosa que va provocar un augment de la mida del gra i una millora del coeficient d'absorció de la llum. Tanmateix, el MASnI3 és més inestable que el MAPbI3. Per això, es van utilitzar diferents antisolvents en la síntesi de MASnI3. Coneguda com a enginyeria antisolvent, aquesta tècnica estudia diversos antisolvents. En este treball, el toluè ha aconseguit millorar l'estabilitat de les capes de MASnI3. Utilitzant un enfocament numèric mitjançant SCAPS-1D, hem calculat que l'eficiència dels dispositius fotovoltaics basats en MASnI3 augmenta un 9,5% en comparació amb els dispositius de MAPbI3. / [EN] Halide Organic-Inorganic Perovskites are a family of materials that have been studied in depth due to their potential for building high-efficiency and low-cost solar cells. In recent years, they have experienced exponential growth in efficiency, going from 3% of PCE in 2009 to over 25% in 2021. But still, there are numerous challenges to overcome, such as stability. The purpose of this work is to study different kinds of HOIPs for solar cell applications, including the optimization of the manufacturing process and the complete characterization by the analysis of Xray-diffraction, Field Emission Scanning Electron Microscopy, Atomic Force Microscope, photoluminescence, UV-visible absorption, band-gap calculation, Transmittance electron microscopy, theoretical simulation with SCAPS-1D, and degradation study of the films. The objective of the analysis is to obtain crystallinity, morphology, topography, optical properties, PCE, and stability of the layers. Different strategies were used to fulfil the goal of the work, including doping with different compounds, antisolvent quenching engineering, and changing the cation "B" in the general formula of perovskites; ABX3. The present dissertation is focused in methylammonium lead iodide III (CH3NH3I3), which is known for exhibiting direct bandgap absorption from the top of the valence band to the bottom of the conduction band. The bandgap can be easily modified by varying A, B, and X and modulated by the suitable selection of mixed cations. Among the possible combinations, MA cation and metal Pb2+ have exhibited excellent optoelectronic properties, low-temperature solution-processable films, and potential for appropriate stability, due to very high charge-carrier mobility, large electron and hole diffusion length, large absorption coefficients, and low nonradiative recombination rates. Specifically, this PhD is composed of four papers: - Stability Improvement of Methylammonium Lead Iodide Perovskite Thin Films by Bismuth Doping - Tetrabutylammonium (TBA)-Doped Methylammonium Lead Iodide: High Quality and Stable Perovskite Thin Films - Manufacture of High-Efficiency and Stable Lead-Free Solar Cells through Antisolvent Quenching Engineering - Investigation on the Stability and Efficiency of MAPbI3 and MASnI3 Thin Films for Solar Cells To sum up, bismuth doping has improved the crystalline structure of the absorbent layer of MAPbI3, which leads to a significant improvement in the optoelectronic properties, the morphology of the surface of the layers, and even improves the stability of the devices. Bismuth doping was studied introducing variable amounts of bismuth between 1 and 8% in the initial solution. Best results were obtained with 2% bismuth. Doping with tetrabutylammonium (TBA) was also analysed by introducing different proportions of TBA in the initial mixture for the synthesis of MAPbI3 layers. We observed that adding 5% TBA to the initial solution reduces the density of pinholes in the layers and improves crystallinity, which leads to a considerable improvement in the stability of the MAPbI3 layers. With the optimal proportion of TBA, it is possible to increase the grain size and the intensity of the photoluminescence, due to the decrease in recombination centres. Since lead is a polluting element, we substituted Pb for Sn to obtain MASnI3 layers, which led to increasing grain size and enhancing light absorption coefficient. However, MASnI3 shells are more unstable than MAPbI3 shells. To overcome this, different anti-solvents were used in the synthesis of MASnI3 shells. Known as antisolvent engineering, this technique studies several antisolvents. In our work, toluene has managed to improve the stability of the MASnI3 layers. Using a numerical approach using SCAPS-1D, we have calculated that the efficiency of photovoltaic devices (PCE) based on MASnI3 increases by 9.5% when compared to devices based on MAPbI3. / Marí Guaita, J. (2022). Different Approaches for Improving the Stability of Hybrid Perovskites [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/191430 / Compendio

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A new family of perovskite titanates with formulae La4+nSr8-nTi12-nMnnO38 and La4Sr8Ti12-nMnnO38-δ have been investigated as potential fuel electrode materials for SOFCs. The series La4+nSr8-nTi12-nMnnO38 present layered domains within their structure. As such layers appear to have a large negative effect over the electrochemical properties only a few compounds have been characterised. The series La4Sr8Ti12-nMnnO38-δ present a rhombohedral (R-3c) unit cell at room temperature which becomes cubic when increasing the temperature up to 900°C both in air and in reducing conditions. The primitive volume correlates with the oxygen content for the reduced samples. TGA and magnetic studies have revealed that the Mn present is mainly as Mn⁺³. Preliminary HRTEM investigations have revealed that some crystallographic shears distributed randomly within a perovskite matrix remain in the structure, which implies that the oxygen overstoichiometry is compatible with rhombohedral distortions in the oxygen sublattice. Mn substitution does not have a large impact on the bulk conductivity of the phases studied, which remains close to the values observed in other related titanates, although the grain boundary contributions are largely improved. Relatively low polarisation resistances were observed under both hydrogen and methane conditions for the lowest n compounds of the series. The anodic overpotential for n=1 was fairly low to those reported in the literature for other materials and especially for titanate-based anodes, i.e. a value of 55mV at 0.5A/cm2, at 950°C, under wet hydrogen was obtained. Additionally, a value 72mV was obtained in the same conditions under methane. These values indicate that the use of Mn as dopant for perovskite-related titanates enhanced electrochemical performance of these anodes, especially at high temperatures.

Topochemical strategies, techniques that allow one to effectively manipulate the structures of nonmolecular solids once a crystal lattice is established, are effective in the low temperature (< 500 °C) modification of solid state structures, allowing the preparation of nonmolecular compounds not accessible by standard synthetic routes. Some of the techniques, ion exchange, intercalation/deintercalation, have proven to be excellent synthetic methods for preserving specific frameworks. The combination of these techniques can allow one to create a multistep approach that can be used to design new compounds with interesting properties. As an expansion to the field of topotactic reactions, a multistep approach was developed towards the synthesis of the new compounds (A xM0.5Cly)LaNb2O7 (where A = Rb, Cs; M = Fe, Ni; x ≈ 1.5;y ≈ 1) at temperatures below 400oC. The first reaction step involved the ion exchange of the host materials (ALaNb2O7, A = Rb, Cs) to form the products M0.5LaNb2O7 (where M = Fe, Ni), a structure open to further chemistry. The next step involved reductive intercalation with Rb or Cs metal to form the air sensitive mixed-valence products with the nominal compositions, A1.5M0.5LaNb2O7. The last step involved the oxidative intercalation of chlorine using chlorine gas to obtain the final compounds. This multistep approach is a design to form mix-metal halide layers, specifically those with divalent cations, within layered perovskites, opening the doors to compounds that can have interesting properties. This reaction series was also applied to the tantalate layered oxides, leading to the formation of the new compound Ni 0.5LaTa2O7 through ion exchange. The further multistep topochemical manipulation of this new compound was not successful and was indicative of the difference in chemical behavior of the tantalates versus the niobates. We have also investigated the oxidative intercalation of halogens into a series of Ruddlesden-Popper (R-P) ruthenate oxides with the formula Ae n+1RunO3n+1 (Ae = Ca, Sr; n = 1, 2, 3) using several sources of fluorine, chlorine, and bromine. A new method was developed to intercalate chlorine into layered systems; this new approach avoids the use of chlorine gas which is highly toxic. The new phase Sr3Ru2O7Cl0.7 was synthesized by the new method and further topotactic manipulations were explored. The chemistry was not limited to the n = 2 phase but was also applied to the n = 3 phase, Sr4Ru3O10.

Layered perovskites can be classified in three major groups: Dion-Jacobson AA′n-1BnO3n+1, Ruddlesden-Popper A2A′n-1BnO3n+1,and Aurivillius phase (Bi2O2)A′n-1BnO3n+1. (A: Alkali metal, Alkali-earth metal; A′: Lanthanides and Bi; B: Ti, Nb, Ta; n: thickness of slabs). For more than two decades researchers have shown much interest in this series because of their magnetic and electrical properties. Tuning synthesis parameters such as temperature, time, and host structure can be used to direct such properties. Low temperature synthetic methods (topochemical methods) allow the preparation of compounds not accessible by traditional high temperature reactions. This dissertation mainly considers the topochemical methods of ion exchange and reductive and oxidative intercalation to build new low temperature or metastable layered perovskites. The two-dimensional Dion-Jacobson ALaNb2O7 layered perovskites were intercalated reductively to produce A2LaNb2O7 andthen oxidized with water or hydro-chalcogenides (H2Ch, Ch: S, Se) to produce the novel alkali metal hydroxide, (A2OH)LaNb2O7, and alkali metal hydro-chalcogenides, (A2ChH)LaNb2O7, respectively. The synthesis and characterization of these compounds are presented in Chapters 2 and 3. In another set of studies, high temperature ceramic methods lead to the new host APrNb2O7. When this reaction is followed by ion exchange, (CuCl)PrNb2O7 can be prepared. The structural refinement, magnetic properties, and thermal stability of new phases have been studied in Chapter 4. The utility of praseodymium niobates for the formation of other metal oxyhalides was also developed; the series (MX)PrNb2O7 (M: Mn, Fe, Co, Cu and X: F, Cl) were prepared by the ion exchange of LiPrNb2O7 and the obtained phases characterized (Chapter 5). Further, to expand the library of materials and because of interesting properties of lanthanides (Ln: La, Pr, Nd, Sm), lanthanide tantalates have been explored for the preparation of oxyhalides and resulted in the compounds (CuCl)LnTa2O7 (Ln: Pr, Nd) (Chapter 6). Manipulation of Dion-Jacobson layered perovskites are not limited to lanthanides, other hosts with interesting properties have been examined (e.g. ABiNb2O7) (A: alkali metal, CuCl) and their crystal structures characterized along with thermal stability and magnetic response.

Um dos maiores problemas que o homem tem encontrado em função de seu desenvolvimento é a poluição. Os principais responsáveis pela poluição atmosférica são os veículos automotores e para minimizar a poluição gerada por estes fez-se necessário o uso de catalisadores. Estes catalisadores, chamados de catalisadores de \"três vias\", atualmente têm como sítios ativos metais nobres, o que eleva muito o seu custo e leva motoristas a dispensarem este equipamento. O objetivo deste estudo foi preparar, caracterizar e estudar materiais tipo perovskitas (La2CuO4, La(2-x)CexCuO4, La2Mo2O9, LaCoO3, LaNiO3, La2CuO4/LaNiO3) como catalisadores para a reação de redução de NO com CO, com a finalidade de encontrar uma alternativa para substituir os metais nobres nos conversores catalíticos comerciais. Os catalisadores foram preparados pelo método do citrato, calcinados em 800 ou 900oC, caracterizados por difração de raios-X, redução a temperatura programada, área específica e análise química, sendo observada a formação de perovskita em todas as caracterizações. Durante os ensaios catalíticos frente a reação de redução de NO com CO, os catalisadores calcinados em 800oC foram mais ativos em relação aos calcinados em 900oC e em relação aos metais componentes da perovskita, quanto mais preenchida a banda de valência do metal na estrutura perovskita, maior a sua atividade. / One of the biggest problems that man has found because of his development is the pollution. The main responsible for atmospheric pollution are the automotive vehicles and to minimize the pollution produced by them it has been necessary to use catalysts. These catalysts, called \"three way catalysts\", actually have noble metals like active sites, what takes up very much their cost and let drivers dispense this equipment. The objective oh this study was to prepare, characterize and study materials of perovskites kind (La2CuO4, La(2-x)CexCuO4, La2Mo2O9, LaCoO3, LaNiO3, La2CuO4/LaNiO3) like catalysts to the reaction of reduction of NO with CO, with the end of find a alternative to substitute the noble metal in the commercial catalytic conversers. The catalysts were prepared by citrate method, calcined at 800 or 900oC, characterized by X-ray diffraction, temperature programmed reduction, specific area and chemical analysis, being observed the formation of perovskite in all characterizations. During the catalytic research to the reaction of NO with CO, the catalysts calcined at 800oC were more active compared to those calcined at 900o and, in relation to the component metal of the perovskite, how more filled the valence band of the metal in the structure of perovskite, better its activity.