Perovskita de LaFeO3 sintetizadas pelo método de Pechini foram avaliadas como catalisadores para a reação de oxidação parcial do metano. Foi impregnado níquel por via úmida como fase ativa em concentrações de 15 e 30 %, sobre as perovskitas, 15NLF e 30NLF, respectivamente, e o seu efeito foi avaliado para a mesma reação. Foi realizado análises termogravimétricas (TGA/DTGA) nos precursores da perovskitas, constatando a sua formação a 650 °C. A análise de microscopia de varredura (MEV) foi realizada nas amostras da perovskita pura, sendo que em uma delas foi realizada um banho de ultrassom para diminuir o tamanho de suas partículas e avaliar este efeito na reação de POM. Análises de difração de raio X (DRX) mostraram que todas as amostras apresentam as mesmas propriedades cristalográficas, sendo que, nas amostras contendo níquel, o metal apresentou-se na forma de NiO. O tamanho dos cristais, cálculado através da equação de Scherer, foi na ordem de 20 nm. Este resultado apontou que o níquel impregnado não participa da estrutura perovskita, mas sim está sobreposto a esta. Através da microscopia eletrônica de transmissão (TEM) foi possível visualizar a dispersão da fase ativa na superfície óxida e tamanhos de partículas na ordem de 20 nm. A redução a temperatura programada (TPR) apresentou as temperaturas de redução de espécies níquel e de ferro, presente na perovskita, e permitiu compreender a atuação das espécies Ni+2 e Fe0 na formação de H2 e CO. Os testes catalíticos foram realizados a 700ºC e 750°C, a pressão atmosférica, para uma vazão de alimentação de 200 cm3.min-1. Os testes cataliticos mostraram que a conversão de H2 dobrou para perovskita Ni/LaFeO3 em relação a LaFeO3. O catalisador 15NLF apresentou melhor estabilidade que o catalisador 30NLF para a reação. / LaFeO3 perovskite synthesized by the Pechini method were evaluated for the partial oxidation reaction of methane. Nickel was impregnated as the active phase in concentrations of 15 and 30%, on perovskites, 15NLF and 30NLF, respectively, and its effect was evaluated for the same reaction. Thermogravimetric analyzes (TGA / DTGA) were carried out on the perovskite precursors, confirming their formation at 650 °C. Scanning microscopy (SEM) was performed on pure perovskite samples, in one of them an ultrasonic bath was performed to reduce the size of its particles and to evaluate this effect in the POM reaction. X-ray diffraction (XRD) analyzes showed that all samples had the same crystallographic properties, and in the samples containing nickel, the metal was present as NiO. The size of the crystals, calculated through the Scherer equation, was in the order of 20 nm. This result pointed out that the impregnated nickel does not participate in the perovskite structure. Through transmission electron microscopy (TEM) it was possible to visualize the dispersion of the active phase on the oxide surface and particle sizes in the order of 20 nm. The programmed temperature reduction (TPR) showed the iron and nickel species reduction temperatures present in the perovskite, and allowed to understand the Ni+ 2 and Fe0 species in the H2 and CO formation. The catalytic tests were performed at 700 °C and 750 °C at atmospheric pressure for a flow rate of 200 cm3.min-1. The catalytic tests showed that the conversion of H2 doubled to perovskite Ni/LaFeO3 in relation to LaFeO3. 15NLF catalyst presented better stability than the 30NLF catalyst for the reaction.

Dès à présent, le monde est face à des enjeux majeurs : augmentation de la production d'énergie, réduction des impacts de la production et de la consommation d'énergie sur l'environnement. La transition vers des énergies durables a déjà commencé. Le photovoltaïque a sa place parmi les énergies renouvelables qui permettront de relever ce défi. Ce travail de thèse porte sur les pérovskites hybrides halogénées et plus particulièrement leur utilisation dans des cellules solaires. En effet très récemment, ces matériaux ont attiré l'attention de la communauté scientifique en raison de leurs propriétés optoélectroniques remarquables : bande interdite directe, forte absorption de la lumière, longueurs importantes de diffusion des porteurs, propriétés optoélectroniques accordables mais aussi une fabrication aisée et à bas coût. En quelques années, le rendement a connu une augmentation spectaculaire de 3,8 % en 2009 à 22,7 % en 2017. Ainsi, ces derniers résultats placent les cellules pérovskites comme des concurrents potentiels face aux cellules solaires à base de silicium cristallin qui représentent aujourd'hui 90 % des cellules en service. Dans la conception des cellules solaires à base de pérovskite, la couche de pérovskite est généralement intercalée entre deux couches de transporteurs de charges : les couches de transporteurs d'électron et de trou (ETM et HTM, respectivement). La qualité de ces couches est essentielle pour obtenir de hauts rendements. Dans ce travail, les propriétés optoélectroniques des pérovskites halogénées sont étudiées ainsi que plusieurs couches de transport de charge. / In the future, the world has to face up to major challenges: increasing the energy production, reducing the environmental impact, moving towards sustainability in energy, etc. Renewable energies such as photovoltaics can meet these challenges. This thesis concerns hybrid halide perovskite materials and their use in solar cells. These materials have recently attracted a lot of attention owing to their direct bandgaps, strong light absorption, large carrier diffusion lengths, tunable optoelectronic properties, and their facile and low-cost fabrication In few years, their energy conversion efficiency has rapidly increased from 3.8 % in 2009 to 22.7 % in 2017, hence approaching efficiencies of crystalline silicon based-devices which represent 90% of commercial photovoltaic cells. In the design of perovskite cells, the perovskite photoabsorber is generally sandwiched by two interfacial layers that yield selective charge collections: the hole and electron transport layers (HTM and ETM). Good quality and adapted interfacial layers are required to obtained high efficiency cells. In this thesis, both the perovskite material and the interfacial layers are investigated.

Perovskita de LaFeO3 sintetizadas pelo método de Pechini foram avaliadas como catalisadores para a reação de oxidação parcial do metano. Foi impregnado níquel por via úmida como fase ativa em concentrações de 15 e 30 %, sobre as perovskitas, 15NLF e 30NLF, respectivamente, e o seu efeito foi avaliado para a mesma reação. Foi realizado análises termogravimétricas (TGA/DTGA) nos precursores da perovskitas, constatando a sua formação a 650 °C. A análise de microscopia de varredura (MEV) foi realizada nas amostras da perovskita pura, sendo que em uma delas foi realizada um banho de ultrassom para diminuir o tamanho de suas partículas e avaliar este efeito na reação de POM. Análises de difração de raio X (DRX) mostraram que todas as amostras apresentam as mesmas propriedades cristalográficas, sendo que, nas amostras contendo níquel, o metal apresentou-se na forma de NiO. O tamanho dos cristais, cálculado através da equação de Scherer, foi na ordem de 20 nm. Este resultado apontou que o níquel impregnado não participa da estrutura perovskita, mas sim está sobreposto a esta. Através da microscopia eletrônica de transmissão (TEM) foi possível visualizar a dispersão da fase ativa na superfície óxida e tamanhos de partículas na ordem de 20 nm. A redução a temperatura programada (TPR) apresentou as temperaturas de redução de espécies níquel e de ferro, presente na perovskita, e permitiu compreender a atuação das espécies Ni+2 e Fe0 na formação de H2 e CO. Os testes catalíticos foram realizados a 700ºC e 750°C, a pressão atmosférica, para uma vazão de alimentação de 200 cm3.min-1. Os testes cataliticos mostraram que a conversão de H2 dobrou para perovskita Ni/LaFeO3 em relação a LaFeO3. O catalisador 15NLF apresentou melhor estabilidade que o catalisador 30NLF para a reação. / LaFeO3 perovskite synthesized by the Pechini method were evaluated for the partial oxidation reaction of methane. Nickel was impregnated as the active phase in concentrations of 15 and 30%, on perovskites, 15NLF and 30NLF, respectively, and its effect was evaluated for the same reaction. Thermogravimetric analyzes (TGA / DTGA) were carried out on the perovskite precursors, confirming their formation at 650 °C. Scanning microscopy (SEM) was performed on pure perovskite samples, in one of them an ultrasonic bath was performed to reduce the size of its particles and to evaluate this effect in the POM reaction. X-ray diffraction (XRD) analyzes showed that all samples had the same crystallographic properties, and in the samples containing nickel, the metal was present as NiO. The size of the crystals, calculated through the Scherer equation, was in the order of 20 nm. This result pointed out that the impregnated nickel does not participate in the perovskite structure. Through transmission electron microscopy (TEM) it was possible to visualize the dispersion of the active phase on the oxide surface and particle sizes in the order of 20 nm. The programmed temperature reduction (TPR) showed the iron and nickel species reduction temperatures present in the perovskite, and allowed to understand the Ni+ 2 and Fe0 species in the H2 and CO formation. The catalytic tests were performed at 700 °C and 750 °C at atmospheric pressure for a flow rate of 200 cm3.min-1. The catalytic tests showed that the conversion of H2 doubled to perovskite Ni/LaFeO3 in relation to LaFeO3. 15NLF catalyst presented better stability than the 30NLF catalyst for the reaction.

L'innovation de la technologie de pointe et l'exigence du marché électronique se concentrent toujours sur l'électronique bon marché, qui présente une fabrication facile, avec des performances sans cesse améliorées. Les pérovskites hybrides organiques-inorganiques, qui combinent les propriétés des semi-conducteurs organiques et inorganiques, sont des candidats prometteurs pour de futurs dispositifs opto-électroniques. L’énergie de liaison des excitons et la force d'oscillateur sont très élevées dans ces systèmes, ce qui rend possible leurs applications à température ambiante. Dans cette thèse, nous avons étudié des couches minces auto-assemblées de molécules de pérovskite (R-NH3)2PbX4. En modifiant la structure R, des pérovskites avec des propriétés optimisées (propriétés optiques d’émission, rugosité de surface et photostabilité) ont été découvertes. Nous avons aussi développé des méthodes pour fabriquer des cristaux massifs et des nanoparticules de pérovskites, et nous avons créé de nouveaux cristaux de pérovskite mixtes: (RNH3)2PbYxX4-x et AB-(NH3)2PbX4. Des cavités verticales en régime de couplage fort ont été réalisées avec ces matériaux, l’émission du polariton de basse énergie a été observée à température ambiante. / The innovation of advanced technology and the requirement of electronic market are always focusing on low cost electronics, presenting an easy processing and having enhanced performance. Organic-inorganic hybrid perovskites, which combine the properties of organic and inorganic semiconductors, are hopeful candidates for future opto-electronic devices. The exciton binding energies and oscillator strengths are very large in these systems making the applications at room temperature possible. In this thesis, we study the flexibility and photostability of self-assembled two-dimensional layered perovskites (R-NH3)2PbX4. By modifying the R structure, perovskites with optimized photoluminescence efficiency, surface roughness and photostability are discovered. We develop also some methodologies to fabricate crystal bulks and nanoparticles of perovskites, and we create new mixed perovskite crystals: (RNH3)2PbYxX4-x and AB-(NH3)2PbX4. Vertical microcavities containing these new materials and working in the strong coupling regime at room temperature have been realized, the emission of the lower energy polariton is observed.

Submitted by Rosivalda Pereira (mrs.pereira@ufma.br) on 2017-06-06T17:35:07Z No. of bitstreams: 1 EdsonFirmino.pdf: 1970355 bytes, checksum: d3777c3685602b1f299f41ead9f30f1f (MD5) / Made available in DSpace on 2017-06-06T17:35:07Z (GMT). No. of bitstreams: 1 EdsonFirmino.pdf: 1970355 bytes, checksum: d3777c3685602b1f299f41ead9f30f1f (MD5) Previous issue date: 2007-02-16 / Fundação de Amparo à Pesquisa e ao Desenvolvimento Científico e Tecnológico do Maranhão (FAPEMA) / In this work we have performed static atomistic simulations to investigate the structural and bulk properties of the complex double perovskites with rock-salt ordering with the Ba2BiM+5O6 stoichiometry, where M+5 = Ta5+, Sb5+ and Nb5+, under pressure changes. For this we have adopted a Buckingham potential model for the ionic interactions and the shell model in order to model the atomic polarizations. In the case of Ba2BiTaO6 and Ba2BiNbO6 compounds, we assumed potential parameters listed in the literature and in the case of Ba2BiSbO6 compound, new potential parameters for the Sb5+¡O interaction were founded. The results observed with theses potentials for the cases of Ba2BiTaO6 and Ba2BiSbO6 materials, shown that the potential sets are sensitive to the structural phase transition observed experimentally between the phases R¹3 and I2=m at high pressures. Thus it was possible to de ne the critical pressure for Ba2BiSbO6. Some elastic and mechanical properties of these materials had been calculated and had shown these transitions clearly, where it was observed the Young's modulus collapse at the critical pressure and discontinuities in the shear modulus and in the longitudinal and transversal sound propagation speeds. The elastic constants had not presented collapse and the volume practically did not change at the transition, what as expected from the experimental results. For the case of Ba2BiNbO6 material, once that there are not experimental results, the aim was to predict their structural characteristics. There were tested four potential sets presented in the literature using as input initial cell parameters data generated by the SPUDS software. Two of these potential sets exhibited the same phase transition that Ba2BiTaO6 and Ba2BiSbO6 present. / Neste trabalho foram empregadas simulações atomísticas estáticas para investigar as propriedades estruturais e de bulk das perovskitas complexas duplas com ordenamento do tipo NaCl com a estequiometria Ba2BiM5+O6, onde M5+ = Ta5+, Sb5+ e Nb5+ sob varia ção de pressão. Para tanto assumimos um modelo de potencial de Buckingham para as interações entre os íons e o modelo de casca esférica para tratar as polarizações dos átomos. No caso dos compostos Ba2BiTaO6 e Ba2BiNbO6 potenciais adotados da literatura foram empregados e para o Ba2BiSbO6, o potencial para a interação Sb5+¡O foi encontrado. Os resultados obtidos com os potenciais para os casos do Ba2BiTaO6 e Ba2BiSbO6, mostraram que ambos são sensíveis à transição de fase estrutural observada experimentalmente entre as fases R¹3 e I2=m que ocorre para altas pressões, de nindo o ponto de transição para o Ba2BiSbO6. Algumas propriedades elásticas e mecânicas destes compostos foram calculadas e mostraram claramente esta transição, onde foi observado o colapso do módulo de Young na pressão crítica e descontinuidades no módulo de cisalhamento e nas velocidades, longitudinal e transversal, de propagação do som dos compostos. As componentes das constantes elásticas não apresentaram colapsos, e o volume praticamente não variou na transição, o que era esperado dos resultados experimentais, implicando em um módulo de bulk praticamente contínuo na transição. Já no caso do Ba2BiNbO6, como não se dispunha de resultados experimentais, o principal objetivo foi predizer suas características estruturais. Testou-se quatro conjuntos de potenciais encontrados na literatura utilizando-se como dados iniciais os parâmetros de rede e posições atômicas gerados pelo software SPUDS. Dois destes conjuntos de potenciais mostraram a mesma transição de fase que o Ba2BiTaO6 e Ba2BiSbO6.

Metal-halide perovskites show great promise as solution-processable semiconductors for efficient solar cells and LEDs. In particular, the diffusion range of photogenerated carriers is unexpectedly long and the luminescence yield is remarkably high. While much effort has been made to improve device performance, the barriers to improving charge transport and recombination properties remain unidentified. I first explore charge transport by investigating a back-contact architecture for measurement. In collaboration with the Snaith group at Oxford, we develop a new architecture to isolate charge carriers. We prepare thin films of perovskite semiconductors over laterally-separated electron- and hole-selective materials of SnOₓ and NiOₓ, respectively. Upon illumination, electrons (holes) generated over SnOₓ (NiOₓ) rapidly transfer to the buried collection electrode, leaving holes (electrons) to diffuse laterally as majority carriers in the perovskite layer. We characterise charge transport parameters of electrons and holes, separately, and demonstrate that grain boundaries do not prevent charge transport. Our results show that the low mobilities found in applied-field techniques do not reflect charge diffusivity in perovskite solar cells at operating conditions. We then use the back-contact architecture to investigate recombination under large excess of one charge carrier type. Recombination velocities under these conditions are found to be below 2 cm s⁻¹, approaching values of high quality silicon and an order of magnitude lower than under common bipolar conditions. Similarly, diffusion lengths of electrons and holes exceed 12 μm, an order of magnitude higher than reported in perovskite devices to date. We report back-contact solar cells with short-circuit currents as high as 18.4 mA cm⁻², giving 70% external charge-collection efficiency. We then explore the behaviour of charge carriers in continuously illuminated metal-halide perovskite devices. We show that continuous illumination of perovskite devices gives rise to a segregated charge carrier population, and we find that the distance photo-induced charges travel increases significantly under these conditions. Finally, we examine intermittancy in the photoluminescence intensity of metal-halide perovskite films.

Ce projet est dédié à la recherche industrielle pour le développement de systèmes catalytiques innovants tels que le contrôle des émissions de véhicules. L'Europe connait actuellement une forte dépendance au niveau de l'importation de certains éléments utilisés comme support de catalyseur (oxyde de Cerium), nous souhaitons nous concentrer sur des éléments plus facilement disponibles tels que Ca, Fe, Mn, Sr, Cu... tout en essayant de garder le mécanisme catalytique bien connu de l'oxyde de cerium. Pour ce faire, nous avons sélectionné des conducteurs en oxygène de la famille des brownmillerites comme matériaux supports. Ceux ci présente des lacunes en oxygènes ayant un impact bénéfique sur leur activité catalytique pour les réactions d'oxydations. Il est aussi prévu de regarder les interactions entre métaux nobles et support conducteurs en oxygène pour une application de dépollution des gaz. Les réactions modèles étudiées au début de ce projet seront l'oxydation du CO ainsi que le stockage et la réduction des NOx. Les brownmillerites peuvent être vues comme des oxydes de type pérovskite avec un défaut en oxygène. Les brownmillerites ont une structure anisotropique avec un enchainement de lacines d'oxygènes-1D apportant une augmentation de l'activité catalytique. Ces browmillerites sont bien connues pour présenter une mobilité de l'oxygène à basse température. La présence de défauts tels que des liaisons anti-phase peut significativement diminuer la diffusion de l'oxygène. CaFeO2.5 riche en défauts, connu pour être une phase stœchiométrique peut être oxyder dans de "douces" conditions en CaFeO3 alors que l'oxydation d'un CaFeO2.5 ordinaire requiert des conditions extrêmes (1100°C et plusieurs GPa de pression en oxygène). Ainsi, introduire un nombre élevé de défauts dans la structure cristalline semble être une manière prometteuse de transformer des phases stoechiométriques en réservoir à oxygène. Les matériaux obtenus alors ayant des capacité de stockage et d'amélioration des réactions d'oxydations à température très modérée. Le mécanisme mis en jeu est comparable à celui de la capacité de stockage en oxygène des cérines dopées et offre donc un vrai potentiel catalytique. Au cours de ce projet CaFeO2.5 sera premièrement étudié mais nous étendrons l'étude avec des dopages (Cu, Mn, W) et une autre composition sera aussi étudiée : SrFeO2.5; Concernant le support nous souhaitons obtenir : -une grande dispersion du métal noble dans la matrice -une grande mobilité de l'oxygène à température modérée -une grande surface spécifique Obtenir ces trois caractéristiques simultanément est actuellement un challenge pour les brownmillérites. Pour ce faire nous souhaitons étudier différentes voies de synthèse. Une grande partie du projet sera dédiée aux caractérisations des matériaux avec des analyses structurales et spectroscopiques incluant de l'échange isotopique pour l'étude de la mobilité en oxygène. Ces études permettront une meilleure compréhension des propriétés des matériaux en relation avec leur activité catalytique. Les matériaux les plus prometteurs à l'issue de cette étude seront synthétisés à l'échelle du pilote par un processus d'électro-fusion. / The present project is dedicated to industrial research for the development of innovative catalytic systems for air purification, such as those used for the control of road vehicle emission (three way converter, TWC). In the context of Europe’s dependency on imports of some critical elements currently used as catalyst support (e.g. cerium oxide), we focus on more available elements such as Ca, Fe, Mn, Sr, Cu… by keeping the well-understood mechanisms governing the catalytic activity of cerium oxide in mind. As such, we choose oxygen ion conductors of the Brownmillerite family as support material, because it has been reported that lattice oxygen atoms have a beneficial impact on the catalytic activity of oxidation reactions. Next to the pure support material, also the interaction of a noble metal with the oxygen ion conductive support for the efficient removal of gas phase pollutants will be studied. In terms of catalytic reactions, the oxidation of CO, and the storage and reduction of NOx will be the primary metrics. In this project, oxygen ion conductors of the Brownmillerite family are chosen as support material. Brownmillerites can be regarded as oxygen-deficient perovskite type oxides. The Brownmillerite type structure is anisotropic with 1D-oxygen vacancy channels providing a catalytically enhanced surface/interface structure. Brownmillerites are known to reveal oxygen ion mobility down to ambient temperature. The presence of extended defects as anti-phase boundaries can significantly decrease the activation energy for oxygen diffusion. Defect-rich CaFeO2.5, which is traditionally known to be a stoichiometric line-phase, can be oxidized under mild conditions to CaFeO3, while the oxidation of ordinary CaFeO2.5 usually requires extreme reaction conditions, i.e. 1100°C and several GPa oxygen partial pressure. Thus, introducing a high concentration of defects seems to be a promising concept to transform even traditionally known stoichiometric line-phases to become a kind of oxygen sponge and behave as oxygen storage/buffer compound at very moderate temperatures. This mechanism is thus comparable to the oxygen storage capacity of doped cerium oxide, and offers a true potential for application in catalysis. Consequently, the Brownmillerite CaFeO2.5 will be a first candidate to study due to its known oxygen ion conductivity properties, however, also doping with other elements (e.g. Cu, Mn, W) and other compositions (e.g. SrFeO2.5) will be investigated. For the support material, we will attempt to achieve (i)- a high degree of dispersion of the noble metal into the matrix, (ii)- a high oxygen mobility at moderate temperatures (e.g. by introducing defects) and (iii)- a high surface area, which we anticipate to be key aspects for achieving high catalytic activity. To date, it is still a challenge to achieve these goals simultaneously for Brownmillerites. As a result, in this project, several synthesis routes are foreseen. More straightforward synthesis routes, such as citrate- EDTA gel methods and spray pyrolysis, will be investigated alongside with more advanced synthetic approaches such and hard-templating routes. This multitude of possibilities allows for an easy adaption of a synthesis route to the material under study. A major part of the project will be dedicated to the detailed characterization of the materials involving large scale facilities for structure analysis and spectroscopy (in-situ studies), including oxygen isotope exchange reactions to trace the oxygen ion mobility. These studies will allow for a detailed understanding of the materials properties in relation to its catalytic activity. The most promising materials will be synthesized on a pilot-scale using electrofusion. This technique is well-established by the industrial partner and is extremely suitable for the synthesis of reduced powders, such as CaFeO2.5.

Doctor of Philosophy(PhD) / This project focuses on the examination of the structures of lanthanide containing double perovskites of the type Ba2LnB'O6-d (Ln = lanthanide or Y3+ and B' = Nb5+, Ta5+, Sb5+ and/or Sn4+) using synchrotron X-ray and neutron powder diffraction. The first part of this project examined the relative stability of R3 rhombohedral and I4/m tetragonal structures as the intermediate phase adopted by the series Ba2LnB'O6 (Ln = lanthanide (III) or Y3+ and B' = Nb5+, Ta5+ or Sb5+). It was found that I4/m tetragonal symmetry was favoured when B' was a transition metal with a small number of d electrons, such as Nb5+ or Ta5+. This is due to the presence of p-bonding in these compounds. In the Ba2LnNbO6 and Ba2LnTaO6 series R3 rhombohedral symmetry was, however, favoured over I4/m tetragonal symmetry when Ln = La3+ or Pr3+ due to the larger ionic radius of these cations. The incompatibility of the d0 and d10 B'-site cations in this family of compounds was indicated by significant regions of phase segregation in the two series Ba2Eu1-xPrxNb1-xSbxO6 and Ba2NdNb1-xSbxO6. In the second part of this project the compounds in the series Ba2LnSnxB'1-xO6-d (Ln = Pr, Nd or Tb and B' = Nb5+ or Sb5+) were examined to understand the relative stability of oxygen vacancies in these materials compared to the oxidation of the lanthanide cations and to determine if any oxygen vacancy ordering occurred. It was found, using a combination of structural characterisation, X ray Absorption Near Edge Structure and Ultra-Violet, Visible and Near Infrared spectroscopies, that with Ln = Pr or Tb increased Sn4+ doping results in a change in the oxidation state of the Ln3+ cations to Ln4+. This leads to those series containing little or no oxygen vacancies. A loss of B site cation ordering was found to accompany this oxidation state change and phase segregation was found to occur in the Ba2PrSnxSb1-xO6-d series most likely due to the Pr3+ and Pr4+ cations segregating into different phases. The Nd3+ cations in the series Ba2NdSnxSb1-xO6-d, however, can not oxidise to the tetravalent state so the number of oxygen vacancies rises with increasing x. It was found that oxygen vacancies concentrate onto the axial site of the compounds with x = 0.6 and 0.8 at ambient temperature. In Ba2Sn0.6Sb0.4O5.7 the oxygen vacancies were found to change to concentrating on the equatorial site at higher temperatures and it is suggested that this oxygen vacancy ordering plays a role in the adoption of I2/m monoclinic symmetry.

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.

Les matériaux oxydes possèdent une physique très riche et une large gamme de propriétés qui en font des matériaux très attractifs les applications. Notre but est d'utiliser ces matériaux oxydes pour améliorer les performances de capteurs magnétiques ultrasensibles appelés capteurs mixtes à 77K. Ces capteurs combineraient un transformateur flux champ supraconducteur en YBaCuO fonctionnant à 77K et la magnétorésistance d'une jonction tunnel dont les électrodes sont composées du manganite demi-métallique La0.7Sr0.3MnO3 (LSMO). Durant cette thèse, nous avons développé l'élément clé de ce capteur qu'est la jonction tunnel. Toutes les hétérostructures oxydes ont été déposées par ablation laser. Après avoir vérifié les propriétés de films minces de LSMO et obtenu une couche de référence avec des bicouches LSMO dopé Ru / LSMO, nous avons étudié des jonctions tunnel de type LSMO dopé Ru / LSMO/SrTiO3/LSMO. Sur cet empilement inédit, des magnétorésistances tunnel (TMR) allant jusqu'à 300% ont été mesurés. Le bruit a été caractérisé pour la première fois dans des jonctions tunnel oxydes. Nous nous sommes aussi intéressés au couplage antiferromagnétique à l'interface du LSMO et du SrRuO3 afin de piéger un film de LSMO. Des cycles asymétriques inhabituels ont été observés sur ces bicouches. Des mesures magnétométriques, de réflectivité de neutrons polarisés et des simulations ont alors été combinées pour comprendre le comportement magnétique des bicouches. Cette étude a montré que le couplage antiferromagnétique n'est pas homogène à l'interface entre le LSMO et le SrRuO3