No description available.

The development of more advanced materials forms the basis of technological progress. One group of fascinating compounds with many potential applications in spintronic devices are the mixed-valence perovskite manganites. These have attracted considerable interest during the last decade through their very large magnetoresistance near the Curie Temperature. Although the properties of a material determinie any application, the development of reliable and flexible synthesis methods is crucial, as is the understanding of these methods. Knowledge of how different materials are formed is also of general importance in tailoring new materials. The aim of this project has therefore been not only to develop a new synthesis route, but also to understand the mechanisms involved. This thesis describes the synthesis and characterization of a novel manganese alkoxide and its use in sol–gel processing of magnetoresistive perovskite manganites. In searching for a soluble manganese alkoxide for sol–gel processing, we found that the methoxy-ethoxide, [Mn19O12(moe)14(moeH)10]·moeH, has a high solubility in appropriate organic solvents. Being 1.65 nm across, it is one of the largest alkoxides reported; it is also of interest because of its (for oxo-alkoxides) rare planar structure. After mixing with La, Nd, Ca, Sr, and Ba methoxy-ethoxides, [Mn19O12(moe)14(moeH)10]·moeH was used in the first purely alkoxide based sol–gel processing of perovskites manganites. The phase evolution on heating xerogel powders to 1000°C was studied, and thin films were prepared by spin-coating. It was found that the easily oxidised Mn-alkoxide facilitates the formation of high oxygen-excess modifications of the perovskites. The reactive precursor system yields fully hydrolysed gels almost without organic residues, but the gel absorbs CO2 from the air, leading to carbonate formation. The carbonate decomposition is the limiting step in oxide formation. Transport measurements of La0.67Ca0.33MnO3 films on LaAlO3 substrate show that all-alkoxide sol–gel derived films can compete with PLD films in terms of quality of epitaxy and transport. The somewhat different behaviour of the sol–gel derived films compared to PLD films is attributed to differences in morphology and oxygen stoichiometry.

This work involved the development of new perovskite oxides based on SmFeO3 and testing their performances as sensors for reducing gases (H2, CO & CH4) and as anode materials for dry methane oxidation in solid oxide fuel cells. The new perovskite oxide materials with formula Sm0.95Ce0.05Fe1-xMxO3-δ (M= Co, Ni & Cr) were synthesized by a sol gel method using citric acid as a complexing agent. The resulting materials were characterized by using a battery of techniques including XRD, XRF, XPS, SEM and electrochemical methods. Sensing experiments revealed that both cobalt doped and Cr doped materials can detect H2, CO and CH4 in air at different temperatures including room temperature. The Ni doped materials did not prove good candidates as sensors. However, their reduction treatment studies showed the formation of metallic nanoparticles on the surface which deeply influence their electrical conductivity as well as sensing ability. Consequently, this modification in surface structure and chemical composition enabled them to sense hydrogen gas at 300oC very effectively. The response of sensors based on these reduced materials was measurable and reversible. Some materials were also selected on the basis of their reduction stability and electrical properties, and their electrochemical performances were evaluated as SOFC anodes under dry methane and dry hydrogen fuels separately. The performance tests as SOFC anode revealed that the best anode material for the oxidation of dry hydrogen fuel is Sm0.95Ce0.05FeO3-δ. Furthermore, Sm0.95Ce0.05FeO3-δ proved to be coke resistant anode under dry methane fuel and exhibited reasonably low charge transfer resistance values at temperatures between 600-700oC. The doping of Co and Ni at the B-site of Sm0.95Ce0.05FeO3-δ found to be very effective in further improving its performance as SOFC anode towards oxidation of dry methane fuel at the lower temperatures.

This work involved the development of new perovskite oxides based on SmFeO3 and testing their performances as sensors for reducing gases (H2, CO & CH4) and as anode materials for dry methane oxidation in solid oxide fuel cells. The new perovskite oxide materials with formula Sm0.95Ce0.05Fe1-xMxO3-δ (M= Co, Ni & Cr) were synthesized by a sol gel method using citric acid as a complexing agent. The resulting materials were characterized by using a battery of techniques including XRD, XRF, XPS, SEM and electrochemical methods. Sensing experiments revealed that both cobalt doped and Cr doped materials can detect H2, CO and CH4 in air at different temperatures including room temperature. The Ni doped materials did not prove good candidates as sensors. However, their reduction treatment studies showed the formation of metallic nanoparticles on the surface which deeply influence their electrical conductivity as well as sensing ability. Consequently, this modification in surface structure and chemical composition enabled them to sense hydrogen gas at 300oC very effectively. The response of sensors based on these reduced materials was measurable and reversible. Some materials were also selected on the basis of their reduction stability and electrical properties, and their electrochemical performances were evaluated as SOFC anodes under dry methane and dry hydrogen fuels separately. The performance tests as SOFC anode revealed that the best anode material for the oxidation of dry hydrogen fuel is Sm0.95Ce0.05FeO3-δ. Furthermore, Sm0.95Ce0.05FeO3-δ proved to be coke resistant anode under dry methane fuel and exhibited reasonably low charge transfer resistance values at temperatures between 600-700oC. The doping of Co and Ni at the B-site of Sm0.95Ce0.05FeO3-δ found to be very effective in further improving its performance as SOFC anode towards oxidation of dry methane fuel at the lower temperatures.

No description available.

Ce mémoire présente une étude de la préparation de couches minces du SmFeO3 (SFO) et du LaNiO3 (LNO) par ablation laser. <br />L'étude s'est d'abord focalisée sur la croissance du SFO sur silice amorphe afin de déterminer les conditions de croissance. Les épaisseurs sont mesurées in situ pendant la croissance par ellipsométrie spectroscopique. La variation thermique des indices optiques ainsi que les paramètres de maille présentent deux transitions qui semblent être corrélées aux températures de Curie (Tc) et de réorientation de spin (TRS). Une croissance épitaxiale du SFO (cube sur cube) sur STO (001) a été aussi obtenue. Les mesures magnétiques à l'ambiante (SQUID) montrent que les moments magnétiques pointent dans la direction perpendiculaire au film, c'est à dire suivant c, axe de facile aimantation à haute température (T> TRS) contrairement aux prévisions (axe a).<br />Une optimisation des températures de substrat et de pression d'oxygène a été ensuite réalisée lors de la croissance épitaxiale (cube sur cube) du LNO sur STO (001). La variation thermique des indices optiques, linéaire, présente un changement de pente dans la gamme [200-300°C]. L'ellipsométrie permet également la détection des transitions métal-isolant associées à la réduction et ré-oxygénation des films. La cinétique de la ré-oxygénation est détaillée ainsi que le contrôle de l'état d'oxydation.<br />Enfin, nous avons étudié les hétérostructures SFO/LNO/STO et SFOII/LNOII/SFOI/LNOI /STO. Nous avons montré que SFO croit de manière épitaxiale (cube sur cube) sur LNO. L'influence des épaisseurs de SFO et de LNOII sur les propriétés de transport à basse température des empilements est finalement démontrée.

Parmi la grande diversité de composés formant la famille des perovskites hexagonales, les systèmes à base de cobalt sont largement étudiés à cause de leurs propriétés électroniques et magnétiques complexes. Ainsi, l'investigation des systèmes Ba-Co-O-X avec X=F, Cl ou Br a permis de synthétiser de nouvelles phases dont les structures sont caractérisées par des groupements trimères Co3O12 (trois octaèdres CoO6 reliés par une face) ou tétramères Co4O15 (quatre octaèdres CoO6 reliés par une face). De fortes relations structurales ont été mises en évidence entre les formes trimères et tétramères des composés oxydes, oxyfluorures, oxychlorures et oxybromures, notamment l'existence de désordres des atomes d'oxygène ou des transformations de phases à haute température (tétramères -> trimères). Dans tous ces matériaux, la couche d'interface entre blocs élémentaires joue un rôle prépondérant sur la dimensionnalité des structures créées. Par ailleurs, des mesures de susceptibilité magnétique combinées à des expériences de diffraction des neutrons ont permis d'établir l'existence, en l'absence de champ magnétique, d'une mise en ordre antiferromagnétique selon l'axe c à basse température dans les matériaux halogéno-cobaltites. L'évolution des courbes d'aimantation en fonction du champ appliqué montre un comportement magnétique plus complexe pour les matériaux bromés, avec notamment la possibilité d'aligner les moments magnétiques dans le plan (ab) par application d'un champ magnétique. Finalement, ce travail de thèse présente les relations entre structure cristalline, dimensionnalité et propriétés magnétiques de ces nouvelles phases halogéno-cobaltites.

Contrairement à leurs homologues LiMPO4 (M : métal de transition) de structure olivine, les vanadates LiMVO4 cristallisent avec des structures type spinelle ou Na2CrO4. De façon générale, peu de composés cristallisent avec ce dernier type structural car de fortes contraintes stériques existent au sein de cette structure. Ainsi l'étude de la substitution du cadmium au lithium dans le composé LiCdVO4 a mis en évidence l'existence de structures modulées pour des compositions (Li1 xCdx/2)CdVO4 (0,4 < x ≤ 2/3). Les structures de ces composés ont été résolues dans le formalisme de super-espace. <br />L'étude du système homologue au sodium (Na1xCdx/2)CdVO4 (0 ≤ x ≤ 1) n'a pas permis d'isoler des phases modulées. La substitution du sodium au cadmium engendre de fortes contraintes stériques qui sont à l'origine d'une transition structurale de la structure type Na2CrO4 à la structure maricite. Cette transition a permis d'étendre le système jusqu'au composé limite (Cd1/2)CdVO4 (x = 1).<br />Afin d'étendre notre compréhension des différences structurales entre les phosphates et les vanadates de formule générale AA'BO4 (A : alcalin et cation d10, A' : métal de transition, B : P et V), les séries AMnPO4 et AMnVO4 ont été étudiées et ceci a permis de synthétiser et caractériser les nouveaux composés AMnVO4 avec A : Cu, Na, Ag, K, Rb et AMnPO4 avec A : Rb et Ag. Un résultat surprenant a été obtenu avec le nouveau composé KMnVO4 qui cristallise avec une structure de type pérovskite déficitaire en oxygène.

Three cobalt-based oxides operating at the Co(III)/Co(II) redox couple have been investigated as potential cathode materials for the intermediate-temperature solid oxide fuel cells (IT-SOFCs). X-ray absorption spectroscopy measurements confirmed that both the oxygen-deficient perovskite Sr[subscript 0.7]Y[subscript 0.3]CoO[subscript 2.65-delta] (SYCO) and the double-perovskite Ba₂[Co][Bi[subscript x]Sc[subscript 0.2]Co[subscript 1.8-x]][subscript O6-delta] (x = 0.1 and 0.2) (BBSC) contain high-spin Co(III) in the bulk at room temperature and thus avoid the thermally driven spin-state crossover of the Co(III) ions usually observed in other cobalt-containing perovskite oxides. Electrochemical characterizations demonstrated that both cobalt oxides operating on the Co(III)/Co(II) redox couple are equally catalytically active for the oxygen reduction reaction as those operating on the Co(IV)/Co(III) redox couple. With an LSGM electrolyte-supported single test cell and NiO+GDC as anode, the maximum power densities Pmax at 800 ºC reach 927 and 1180 mW·cm⁻² for SYCO and BBSC cathodes, respectively. The oxygen-deficient perovskites Sr[subscript 1-x]R[subscript x]CoO[subscript 3-delta] (R = Eu-Ho, Y, x [approximately equal] 0.3) are identified as a new class of cathode materials for IT-SOFCs in this dissertation. On the other hand, the layered Ba2Co9O14 (BCO) containing the low-spin Co(III) at room temperature undergoes a thermally driven spin-state crossover, which has prevented it from being evaluated as the cathode of IT-SOFCs. This problem was overcome by fabrication of a 50-50 wt.% BCO + SDC (Sm[subscript 0.2]Ce[subscript 0.8]O[subscript 1.9]) composite cathode. The addition of SDC not only improved the adhesion to the electrolyte, but also enhanced the electrocatalytic activity for the oxygen reduction reaction. The composite cathode delivers a nearly stable P[subscript max] of ~450 mW·cm-2 at 800 °C in an LSGM electrolyte-supported single test cell. In addition, the electrochemical lithium intercalation process in the monoclinic Nb12O29 was studied with a Li/Nb₁₂O₂₉ half-cell, and the results showed that it can reversibly incorporate a relatively large amount of Li-ions in the voltage window of 2.5-1.0 V at a slow discharge/charge rate while retaining structural integrity. Compared with that of the bare Nb₁₂O₂₉, samples with carbon coating show an improved rate capability. The lithium insertion mechanism into Nb₁₂O₂₉ has also been discussed in terms of sites available to the lithium ions / text

The perovskite structure has an incredible versatility that results in myriad compounds with varied and eccentric behaviors. Perovskite oxides have been extensively studied and used for over 60 years. In order to expand on our already thorough knowledge of these compounds, it is necessary to use modern and creative experimental techniques. High-pressure synthesis and high oxygen-gas pressure annealing techniques are used to synthesize oxygen stoichiometric RNiO₃ (R = lanthanide). The particularly rich phase diagram of this compound allows for the study of the crossover from localized to itinerant electronic behavior and from an enhanced Pauli to a Curie-Weiss law paramagnetism. Single crystals of RFeO₃ are grown in order to analyze the spin canting in these antiferromagnetic samples. The size of the rare earth-cation is used to tune the magnitude of octahedral-tilt distortions. This tuning allows distinguishing between the two possible drivers for spin canting and weak ferromagnetism in these compounds, the octahedral-tilt-dependent single-ion anisotropy and the octahedral-tilt-independent Dzyaloshinskii-Moriya interaction. Although it is a fluoride compound, KCuF₃ has been used as an analogue to transition-metal oxide perovskites such as LaMnO₃ because of the similarity of their orbital ordering. Through the use of high-temperature neutron diffraction, it is shown that the orbital ordering and Jahn-Teller distortion in this compound are not lifted at the predicted temperature. Another mechanism for orbital ordering is identified. La₂[subscript-x] Sr [subscript x] CuO₄ has long been of interest as the progenitor system of the highTc superconductors. Despite having an exceedingly well-studied phase diagram in the over-doped region of its superconducting dome, little is known about this system in the region x > 0.3 because of the difficulty of synthesizing fully oxygen-stoichiometric samples. With high-oxygen-gas-pressure annealing and high-pressure synthesis, the completion of the phase diagram up to x = 1.0 is attempted. Finally, like many iridates, post-Perovskite CaIrO₃ exhibits a very strong spinorbit coupling of its 5d electrons. Because its magnetism is very weak, traditional methods to measure the magnitude of its orbital moment and spin-orbit coupling, such as neutron powder diffraction, are not viable. In order to address this issue, direct measurement of the orbital moments was conducted by using x-ray absorption spectroscopy and x-ray magnetic circular dichroism techniques. / text