L' intérêt des phases oxydes à base de terres rares est certes multiple et leurs propriétésont été explorées depuis longtemps: mais nous nous intéressons ici aux comportements de cesphases en tant que phases thermiques, catalytiques ou conductrices ioniques à hautetempérature. Il s'agit en particulier de développer des systèmes innovants de matériauxpouvant intervenir dans la conception de dispositifs pour la microélectronique, pour capteursde gaz ou membranes sélectives ou pour systèmes dépolluants.Les phases de structure pyrochlore ou fluorine de type TR2Ce207 où TR désigne unélément de Terre Rare présentent divers potentiels d' applications: elles ont été considéréescomme des phases pouvant résister à de hautes températures. Les oxydes de structurespérovskites, de formule générale ABO), présentent de multiples applications potentielles,notamment en tant que phases diélectriques pour condensateurs, ou phases conductricesioniques (en ions oxygène ou en protons) pour électrolytes solides, du fait même de leur hautestabilité chimique à haute température.Ce travail a été divisé en deux parties. La première a consisté à élaborer la phase« thermique)} de type pyrochlore TR2Ce20 7 (TR = La, Ce, ... ) en utilisant un minéralcomplexe à base d'allanite-monazite et de silico-aluminates issus de déchets industriels, doncà bas coût.La deuxième partie a consisté à élaborer BaCe03 et à étudier ses propriétéscatalytiques et conductimétriques en fonction de la température. Une nouvelle méthode desynthèse reposant sur l' utilisation du mélange EDTA-citrate a été utilisée afin d'élaborer unprécurseur, qui, traité thermiquement à 950°C, a permis d'élaborer des poudressubmicroniques de la phase BaCe03. L'activité catalytique du composé BaCe03 démarre à450°C pour atteindre la conversion totale à 675°C : dans cette gamme de température,l'efficacité catalytique de la phase BaCe03 est maximale. L'évolution de la conductivité enfonction de la température de pastilles compactées de BaCe03 a révélé l'existence d'une sériede modifications électriques fortement corrélées aux transitions structurales connues pourBaCe03 dans la littérature. À basse température (300 à 450°C), la faible conductivité de laphase orthorhombique, associée à la faible énergie d'activation, peut être liée à la migrationdes défauts extrinsèques (gaz adsorbés). Cependant, au-dessus de 500°C, la conductivité de laseconde phase orthorhombique augmente: ceci pourrait être attribué à une mobilité croissantedes atomes d'oxygène. / Rare earth elements have recently been involved in a range of advanced technologies like microelectronics, membranes for catalytic conversion and applications in gas sensors. In the family of rare earth elements like cerium can play a key role in such industrial applications. However, the high cost of these materials and the control and efficiencies associated processes required for its use in advanced technologies, are a permanent obstacle to its industrial development. In present study was proposed the creation of phases based on rare earth elements that can be used because of its thermal behavior, ionic conduction and catalytic properties. This way were studied two types of structure (ABO3 and A2B2O7), the basis of rare earths, observing their transport properties of ionic and electronic, as well as their catalytic applications in the treatment of methane. For the process of obtaining the first structure, a new synthesis method based on the use of EDTA citrate mixture was used to develop a precursor, which undergone heat treatment at 950 ° C resulted in the development of submicron phase BaCeO3 powders. The catalytic activity of perovskite begins at 450 ° C to achieve complete conversion at 675 ° C, where at this temperature, the catalytic efficiency of the phase is maximum. The evolution of conductivity with temperature for the perovskite phase revealed a series of electrical changes strongly correlated with structural transitions known in the literature. Finally, we can establish a real correlation between the high catalytic activity observed around the temperature of 650 ° C and increasing the oxygen ionic conductivity. For the second structure, showed clearly that it is possible, through chemical processes optimized to separate the rare earth elements and synthesize a pyrochlore phase TR2Ce2O7 particular formula. This "extracted phase" can be obtained directly at low cost, based on complex systems made of natural minerals and tailings, such as monazite. Moreover, this method is applied to matters of "no cost", which is the case of waste, making a preparation method of phases useful for high technology applications. / Elementos terras raras têm sido recentemente envolvido em uma ampla gama de tecnologias avançadas, como a microeletrônica, membranas para a conversão catalítica e aplicações em sensores de gás. Na família de terras raras, elementos como o cério pode desempenhar um papel chave em tais aplicações industriais. No entanto, o alto custo desses materiais e do controle e eficiências dos processos associados necessários para sua utilização em tecnologias avançadas, são um obstáculo permanente ao seu desenvolvimento industrial. No presente trabalho, foi proposto a obtenção de duas fases baseadas em elementos de terras raras que podem ser utilizados devido o seu comportamento térmico, condução iônica e propriedades catalíticas. Desta maneira, foram estudados dois tipos de estrutura (ABO3 e A2B2O7), a base de terras raras, observando suas propriedades de transporte iônico e eletrônico, bem como suas aplicações catalíticas no tratamento do metano. Durante o processo de obtenção da primeira estrutura, um novo método de síntese baseado no uso da mistura EDTA citrato foi usado para desenvolver um precursor, que submetidos a tratamento térmico a 950°C, resultou no desenvolvimento de pós submicrométricos da fase BaCeO3. A atividade catalítica desta perovskita começa a 450°C para alcançar a conversão completa em 675°C, onde nesta temperatura, a eficiência catalítica da fase é máxima. A evolução da condutividade em função da temperatura para a fase perovskita revelou uma série de mudanças elétricas fortemente correlacionada com transições estruturais conhecida na literatura. Finalmente, pode-se estabelecer uma correlação real entre a alta atividade catalítica observada em torno da temperatura de 650°C e o aumento da condutividade iônica de oxigênio. Para a segunda estrutura, mostrou-se claramente que é possível, através de processos químicos otimizados, separar os elementos terras raras e sintetizar uma fase pirocloro específica de fórmula TR2Ce2O7. Esta “fase extraída" pode ser obtida diretamente, de baixo custo, baseado em sistemas complexos feitos de minerais naturais e resíduos, como a monazita. Além disso, este método é aplicado a matérias de "custo zero", que é o caso dos resíduos, tornando um método de preparação de fases útil para aplicações de alta tecnologia.

Ces travaux ont portés sur l’élaboration de dépôts de La2Zr2O7 par projection plasma de solution (SPPS) pour une application barrière thermique pour un moteur spatial. Différentes solutions de précurseurs de solvants divers ont été formulées pour la synthèse des poudres et des différents dépôts. Les précurseurs et les solvants ont un impact important sur les interactions en solution entrainant des modifications des propriétés des poudres, particulièrement les phases cristallines ou la densification. De même, la variété des solutions étudiées et l’optimisation des paramètres de projections ont permis l’élaboration de différentes microstructures de dépôts (dense, homogène poreux et colonnaire). La microstructure colonnaire présente la meilleure résistance aux essais de cycles thermiques par rapport aux autres dépôts développés. Il a également était établie une corrélation entre les dépôts réalisés par SPPS et les données structurales des poudres. / These work deal with the conception of La2Zr2O7 coatings by Solution Precursor Plasma Spraying (SPPS) for thermal barrier application in space engine. Different precursors’ solutions of various solvents were prepared for powder synthesis and the realization of different coatings. Precursors and solvents have important effect on interactions in solution leading to the modification of powder properties, especially crystalline phases or densification. Similarly the diversity of studied solutions and the optimisation of thermal projection parameters allowed the elaboration of various coatings’ microstructures (dense, homogeneous porous and columnar). Columnar microstructure show better resistance to thermal cycling shock experiment than others coatings. It was also established a correlation between the SPPS coatings and powders structural data.

Thesis advisor: Michael J. Graf / This thesis is concerned with experimentally determining the magnetic and electronic states in a unique class of transition metal oxides known as the pyrochlore iridates, A₂Ir₂O₇ (A = Y or Rare earth). The extended nature of the 5d Ir orbitals in the iridates places these materials in a regime of intermediate electron correlation and large spin-orbit interaction such that this system may host several novel or topological states of matter which may be perturbed by incorporating different A-species. Additionally, the pyrochlore structure is geometrically frustrated and has been long been studied as a potential host of a number of exotic magnetic phenomenon. However, even after years of intense theoretical and experimental interest many fundamental questions still remain about the nature of the magnetic ground sates in this series which are of vital importance in understanding the roles of various interactions and potential of such novel phenomenon. The primary aim of this thesis is therefore to determine how magnetic order develops on the Ir sublattice in this series, particularly how it is perturbed through variation of the crystalline structure, magnetism of the A-site ions, and presence of mobile charges. This thesis is the first comprehensive experimental study of these effects which has utilized several complementary experimental probes of both bulk and local magnetism in a number of compounds. The techniques presented in this work include magnetotransport, bulk magnetization, elastic neutron scattering, and muon spin relaxation (µSR) measurements. All of the three compounds studied in this work (A = Y, Yb, and Nd) are shown to definitively exhibit long-range magnetic order on the Ir sublattice, which has previously only been inferred based on studies of other compounds. The compounds Y₂Ir₂O₇ and Yb₂Ir₂O₇ are correlated insulators at low temperature and are found to have identical configuration of the Ir moments, despite the presence of the large localized Yb³⁺. Numerical investigations presented here have provided the first conclusive evidence that this order is of the `all-in/all-out' type, consistent with recent resonant x-ray studies; additionally, we have shown that this order exists for all insulating compounds regardless of structural parameters or properties of the A-ion. On the other hand, Nd₂Ir₂O₇ is weakly metallic with Kondo-like behavior at low temperature, with long-range order only on the Ir site, in disagreement with previous results from neutron scattering. Measurements of the field dependent magnetization and Hall effect reveal a large anomalous Hall component, suggesting that the Nd<super>3+</super> may exhibit a spin-ice state with very short correlation length, while the Ir sublattice is likely in the `all-in/all-out' state. From this, it is determined that Nd₂Ir₂O₇ lies at an important cross-over point in the series in which correlation energy and conduction bandwidth yield chiral order with features akin to both the metallic unordered Pr₂Ir₂O₇ and those of the magnetically ordered insulators. These results are discussed with regard to recent theoretical models exploring the role of electron correlation, frustration and various exchange interactions in these materials. / Thesis (PhD) — Boston College, 2013. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

The nuclear industry faces important technical challenges for HLW storage and designing new nuclear fuels. Such materials have to perform under severe conditions and accommodate defects safely. Few materials are suitable for those purposes but certain types of ceramics are promising candidates; among them oxide compounds such as pyrochlores and zirconolites. The aim of this PhD project was to synthesise novel actinide oxide particles, in which two different approaches were taken: controlled hydrolysis and solid state chemistry. Some minerals were naturally doped with actinides and rare earth elements and inspired synthetic formulations of new targeted waste forms for High Level Waste (HLW) instead of other disposal routes. The choice of synthetic pyrochlore and zirconolites as actinide hosts was inspired by the existence of naturally occurring actinide rich minerals. Substitutions within the cubic Y2Ti2O7 were investigated via the respective Y2Ti2-2xFexNbxO7 and Y2-xCexTixFexO7 solid solutions.Controlled hydrolysis was based on previous work with Lewis acidic transition metal cations. It was shown that it was possible to control the hydrolysis products in the presence of chelating organic ligands by carefully controlling the experimental conditions of the system. The same principles should apply to the chemistry of the actinide ions as they too are Lewis acidic. Such chemistry has implications for understanding the behaviour of actinides in nuclear processes within aqueous and organic systems. This project was a collaboration between Sheffield University (Immobilisation Science Laboratory, ISL) and The University of Manchester (Centre for Radiochemistry Research, CRR) through the DIAMOND (Decommissioning Immobilisation and Management of Nuclear waste for Disposal) University research consortium.

Materials with potential for high temperature inorganic pigment applications (e.g. ceramics) have been synthesised using solid state methods and characterised using powder X-ray and neutron diffraction, magic angle spinning NMR, Mossbauer spectroscopy, UVNis spectroscopy and colour measurement techniques. A number of compounds containing tin (H) and niobium (V) have been modified and doped with various constituents to tailor and widen the colour properties of the bright yellow parent phases. The dark yellow (L*(91.28), a*(13.74), b*(54.76)) tin niobium oxide pyrochlores have been re-characterised using neutron diffraction techniques. These materials exhibit both cation and anion disorder caused by a stereoactive lone pair on the tin (H) metal centres. Solid state NMR and Mossbauer studies have shown the presence of tin (IV) on a symmetric site within the primary phase of the pure oxide pyrochlore and methods have been investigated to minimise the tin (H) disproportion method from where the tin (IV) is derived.

Recentemente, observamos na comunidade científica o despertar de um interesse pelo estudo de lantanídeos com base em titânio; neste ínterim, as amostras Sm2Ti2O7 e Nd2Ti2O7 se destacam por terem sido pouco exploradas, especialmente a baixas temperaturas. Análises cristalográficas de monocristais de Sm2Ti2O7 e Nd2Ti2O7 nos permitiram confirmar inicialmente características de rede como a geometria cúbica e monoclínica, respectivamente, enquanto as análises magnéticas e calorimétricas forneceram a temperatura de transição, até então inédita no caso do Nd2Ti2O7 (aproximadamente T=0,62K), além de ordenamento do tipo antiferromagnético e presença de anisotropia a altas temperaturas, na amostra de Sm2Ti2O7, e por toda a extensão de temperaturas trabalhadas, na amostra de Nd2Ti2O7. Por fim, identificamos também a entropia esperada para spins do tipo Ising em ambas as amostras. / Recently the scientific community has shown interest for titanium lanthanides, with the titanates Sm2Ti2O7 and Nd2Ti2O7 being poorly approached, specially at low temperatures. Crystallographic analysis were performed on the single crystal samples of Sm2Ti2O7 and Nd2Ti2O7, which allowed us to confirm its expected cubic and monoclinic structures, respectively; Magnetic and calorimetric data displayed the transition temperature, not previously seen in case of Nd2Ti2O7, of T=0.35K and T=0.62K, respectively, and the antiferromagnetic behaviour of both samples; Anisotropy was observed at high temperatures on the Sm2Ti2O7 data and along all the temperature range studied for the Nd2Ti2O7 sample. Finally, we have identified the entropy of Ising spins on both samples.

Recentemente, observamos na comunidade científica o despertar de um interesse pelo estudo de lantanídeos com base em titânio; neste ínterim, as amostras Sm2Ti2O7 e Nd2Ti2O7 se destacam por terem sido pouco exploradas, especialmente a baixas temperaturas. Análises cristalográficas de monocristais de Sm2Ti2O7 e Nd2Ti2O7 nos permitiram confirmar inicialmente características de rede como a geometria cúbica e monoclínica, respectivamente, enquanto as análises magnéticas e calorimétricas forneceram a temperatura de transição, até então inédita no caso do Nd2Ti2O7 (aproximadamente T=0,62K), além de ordenamento do tipo antiferromagnético e presença de anisotropia a altas temperaturas, na amostra de Sm2Ti2O7, e por toda a extensão de temperaturas trabalhadas, na amostra de Nd2Ti2O7. Por fim, identificamos também a entropia esperada para spins do tipo Ising em ambas as amostras. / Recently the scientific community has shown interest for titanium lanthanides, with the titanates Sm2Ti2O7 and Nd2Ti2O7 being poorly approached, specially at low temperatures. Crystallographic analysis were performed on the single crystal samples of Sm2Ti2O7 and Nd2Ti2O7, which allowed us to confirm its expected cubic and monoclinic structures, respectively; Magnetic and calorimetric data displayed the transition temperature, not previously seen in case of Nd2Ti2O7, of T=0.35K and T=0.62K, respectively, and the antiferromagnetic behaviour of both samples; Anisotropy was observed at high temperatures on the Sm2Ti2O7 data and along all the temperature range studied for the Nd2Ti2O7 sample. Finally, we have identified the entropy of Ising spins on both samples.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第23898号 / 工博第4985号 / 新制||工||1778(附属図書館) / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 田中 功, 教授 邑瀬 邦明, 教授 宇田 哲也 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

The (BMN) bulk materials were sintered at 1050°C, 1100°C, 1150°C, 1200°C by the conventional ceramic process, and their microstructure and dielectric properties were investigated by Scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, Transmission electron microscopy (TEM) (including the X-ray energy dispersive spectrometry EDS and high resolution transmission electron microscopy HRTEM) and dielectric impedance analyzer. We systematically investigated the structure, dielectric properties and voltage tunable property of the ceramics prepared at different sintering temperatures. The XRD patterns demonstrated that the synthesized BMN solid solutions had cubic phase pyrochlore-type structure when sintered at 1050°C or higher, and the lattice parameter (a) of the unit cell in BMN solid solution was calculated to be about 10.56Å. The vibrational peaks observed in the Raman spectra of BMN solid solutions also confirmed the cubic phase pyrochlore-type structure of the synthesized BMN. According to the Scanning Electron Microscope (SEM) images, the grain size increased with increasing sintering temperature. Additionally, it was shown that the densities of the BMN ceramic tablets vary with sintering temperature. The calculated theoretical density for the BMN ceramic tablets sintered at different temperatures is about 6.7521 . The density of the respective measured tablets is usually amounting more than 91% and 5 approaching a maximum value of 96.5% for sintering temperature of 1150°C. The microstructure was investigated by using Scanning Transmission Electron Microscope (STEM), X-ray diffraction (XRD). Combined with the results obtained from the STEM and XRD, the impact of sintering temperature on the macroscopic and microscopic structure was discussed. The relative dielectric constant ( ) and dielectric loss ( ) of the BMN solid solutions were measured to be 161-200 and (at room temperature and 100Hz-1MHz), respectively. The BMN solid solutions have relative high dielectric constant and low dielectric loss. With increasing sintering temperature, the dielectric constant showed the maximum at 1150°C. The leakage current of BMN ceramic material is extraordinary small. When the voltage and thickness of the BMN capacitor are 4000V and 300um, the leakage current amounts only about 0.13-0.65 . The excellent physical and electrical properties make BMN thin films promising for potential tunable capacitor applications.

This work is the result of two separate lines of study into the family of frustrated rare-earth pyrochlores with ferromagnetic interactions. The first is an examination of Yb2Ti2O7 and Ho2Ti2O7 through small angle neutron scattering (SANS) techniques. The sensitivity to anisotropic ferromagnetic correlations of SANS makes it an ideal tool to investigate the anomalous scattering of Yb2Ti2O7 , and to take a closer look into the low Q region of the spin ice Ho2Ti2O7 , where long range dipolar effects modify magnetic scattering. We show that in Yb2Ti2O7 the ferromagnetic order observed by other researchers coexists with short range HHH correlations to 0.03 K. We identify a new feature in Yb2Ti2O7 , a medium range (on the order of 100 ̊A) ferromagnetic correlation which appears to correlate well with the systems heat capacity anomaly. In Ho2Ti2O7 , we observe isotropic magnetic scattering in the low Q region that correlates to the system heat capacity anomaly. The second research project involves the system Ho2Sn2−xTixO7 with x=0,0.5,1,1.5,2. We use SQUID magnetometry and inelastic neutron scattering to examine the effects of B-site disorder on the spin ice system. We find some discrepancies with previous generation instrument results for the crystal electric field (CEF) Hamiltonian, and find results consistent with our previously hypothesized picture of the effects of B-site disorder on the CEF levels. We also observe increased spin dynamics in the disordered compounds, which is consistent with recent theory work predicting a transition to a quantum spin liquid phase in disordered non-kramers spin ice compounds. / Thesis / Master of Science (MSc)