Pyrochlore-type oxides (A2B2O7) and glass-ceramic composites have been investigated for nuclear waste sequestration applications due to the remarkable compositional diversity and structural flexibility of these materials. These properties can enhance the incorporation of radioactive waste elements and resistance to radiation induced structural damage. Radiation induced structural damage can be simulated by bombarding materials using high-energy heavy ions. The study has shown how the metal-oxygen bond covalency, cationic radii ratio (rA/rB), and oxygen vacancies of pyrochlore type oxides affect the resistance of these materials to radiation induced damage. RE2Ti2O7 (RE=La–Lu), Yb1.85Ca0.15Ti2O7-δ, Yb2Ti1.85Fe0.15O7-δ, and Gd2Ti2-xSnxO7 were synthesized by the ceramic method and investigated by X-ray absorption near edge spectroscopy (XANES), which allows for the study of the effect of elemental substitution on the electronic structure of materials. Surface sensitive glancing angle and total electron yield XANES (GA/TEY XANES) spectra have been used to study the damaged surface of the materials, as the high energy ions can only implant in the near-surface region (~ 450 nm) of the pellets. These measurements have allowed for an investigation of how the local structure of the materials changed after ion implantation and discussed in terms of coordination number and bonding environment. After investigating the ceramic materials, the glass-ceramic composite materials containing Gd2Ti2O7 pyrochlore type crystallites in a (borosilicate- and Fe-Al-borosilicate) glass were investigated. These glass-ceramic materials were synthesized and analyzed by backscattered electron (BSE) images and XANES spectra. The study has shown how the Gd2Ti2O7 crystallites interact within a glass matrix depending on glass composition, pyrochlore loading, and annealing temperature. Further, the GA-XANES spectra from these materials have shown that the glass ceramic composite materials show a similar response to ion implantation as pure ceramics (i.e., Gd2Ti2O7). All of these studies and techniques could provide a better understanding of how to develop and design materials for nuclear waste sequestration applications.

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.

Orientador: Oswaldo Luiz Alves / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Quimica / Made available in DSpace on 2018-08-07T03:13:18Z (GMT). No. of bitstreams: 1 DeSantis_MarcoAurelioCulhari_M.pdf: 3832539 bytes, checksum: 536117264acf4225d0c02eb3b2150a00 (MD5) Previous issue date: 2006 / Resumo: O presente Trabalho trata da obtenção do óxido HTaWO6.H2O nas formas: bidimensional (trirrutilo lamelar) e tridimensional (pirocloro com canais). Os compostos foram preparados a partir de reações de troca iônica do metal alcalino pelo próton nas respectivas fases (Li ou K)TaWO6. A síntese das fases precursoras envolveu típicas reações de estado sólido. Neste processo, foi estudada a evolução das estruturas em função da variação da temperatura e do tempo de síntese, no intervalo entre 350 e 850°C. As fases cristalinas com as estruturas trirrutilo e pirocloro apresentam propriedades estruturais e eletrônicas que as habilitam a aplicações tecnológicas. Por exemplo, na forma de matrizes hospedeiras para a obtenção de nanocompósitos orgânico/inorgânicos, assim como na forma de eletrólitos sólidos. Ambos os sólidos, obtidos na forma de pó, foram funcionalizados por reações de troca iônica do próton por íons Ag, conduzindo à formação dos compostos do tipo H1-xAgxTaWO6.H2O com estrutura bi- e tridimensional. Adicionalmente, o composto apresentando a estrutura trirrutilo (lamelar) foi utilizado como matriz hospedeira para a reação de intercalação de duas aminas lineares (n-butilamina e n-dodecilamina). As reações conduziram à formação dos compostos de intercalação: HTaWO6.H2O@C4H11N e HTaWO6.H2O@C12H27N. Os materiais preparados foram caracterizados por diferentes técnicas, dentre elas: difratometria de raios X (XRD), fluorescência de raios X (XRF), espectroscopias Raman e infravermelho (FTIR) e termogravimetria (TG). Os resultados dos ensaios revelaram os comportamentos distintos das estruturas frente às reações de troca iônica e intercalação/inserção. No estudo da evolução das estruturas precursoras, os resultados revelaram a possibilidade de obtenção dos compostos (Li ou K)TaWO6, via reação no estado sólido, com reduções substanciais do tempo e da temperatura de síntese em relação aos procedimentos descritos na literatura / Abstract: The present work is about the preparation of the HTaWO6.H2O oxide with bidimensional (trirutile) and tridimensional (pyrochlore) structures. These compounds were prepared by ion-exchange reactions of the alkaline metal by the proton, starting from precursor (Li or K)TaWO6. The synthesis of the precursors compounds involved typical solid state reactions. In the process, the structural evolution of the materials was evaluated related to the temperature (350 ¿ 850 ºC) and time of synthesis. Trirutile and pyrochlore crystalline phases present structural and electronic properties that make technological applications become possible, such as host matrices for the preparation of organic/inorganic nanocomposites and as solid electrolytes. Both solids, obtained as powders, were functionalized by ion exchange reactions of the proton by Ag ions, leading to the formation of compounds H1-xAgxTaWO6.H2O, with bi- and tridimensional structure. In addition, the solid presenting trirutile (lamellar) structure was used as a host matrix for intercalation of two linear amines (n-butylamine and n-dodecylamine). Reactions yielded the following intercalation products: HTaWO6.H2O@C4H11N and HTaWO6.H2O@C12H27N. The ¿as prepared¿ materials were characterized by different techniques, such as X-ray difratometry (XRD), X-ray fluorescence (XRF), Raman spectroscopy, infrared spectroscopy (FTIR) and thermogravimetry (TG). The results revealed different behavior of the structures towards ion exchange and intercalation/insertion reactions. In the study of structural evolution of the precursors materials, results revealed the possibility of preparation of the compounds (Li or K)TaWO6 via solid state reactions, with substantial reduction of time and temperature of synthesis related to the procedures described in the literature / Mestrado / Quimica Inorganica / Mestre em Química

Submitted by Luciana Ferreira (lucgeral@gmail.com) on 2015-11-19T13:11:08Z No. of bitstreams: 2 Dissertação - Marta Maria de Moura Bezerra - 2015.pdf: 3977918 bytes, checksum: ab4ba8605376cc354b49e3469830fa05 (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Approved for entry into archive by Luciana Ferreira (lucgeral@gmail.com) on 2015-11-19T13:13:15Z (GMT) No. of bitstreams: 2 Dissertação - Marta Maria de Moura Bezerra - 2015.pdf: 3977918 bytes, checksum: ab4ba8605376cc354b49e3469830fa05 (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Made available in DSpace on 2015-11-19T13:13:15Z (GMT). No. of bitstreams: 2 Dissertação - Marta Maria de Moura Bezerra - 2015.pdf: 3977918 bytes, checksum: ab4ba8605376cc354b49e3469830fa05 (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Previous issue date: 2015-04-17 / Fundação de Amparo à Pesquisa do Estado de Goiás - FAPEG / In this work, the Praseodymium Stannate system was successfully synthesized by Polymeric Precursor Method. The crystallization temperature was evaluated by ATD and after calcination the material was characterized by XRD, spectroscopy in the infrared regions, spectroscopy in the region of UV-visible, scanning electron microscope(SEM) ,transmission electron microscope (TEM) equipped with an energy dispersive X-ray spectrometer (EDS) and photoluminescent measures. In the interests of possible applicability of semiconductor in catalysis were made trials in the discoloration of the tartrazine. The precursor powders calcined at different temperatures (400 ° C versus time (4, 8 and 12 hours) and 600, 800 and 1000 ° C for 4 hours) are single phase according to a results of X-ray diffraction. The degree of crystallinity increased with the temperature according to the results of the Crystallite Size and FWHM. The energy of the "bandgap" were calculated from the reflectance curves in the UV and showed obtaining a Semiconductor (~ 2.0 eV). By TEM was observed nanoscale particles monophasic with no secondary phase. The structure and morphology assessed by SEM showed an agglomerate and porous material. The optical property showed a possible applicability of the studied material as photoluminescent. According to the value of the "forbidden band" and the porosity of the materials, photocatalytic tests were carried out on discoloration of the dye tartrazine. The results showed that the studied material is promising for catalysis. / Neste trabalho, o sistema de Estanato de Praseodímio foi sintetizado com sucesso pelo Método Precursor Polimérico. A temperatura de cristalização foi avaliada por DTA e depois de calcinação, o material foi caracterizado por DRX, espectroscopia nas regiões do infravermelho, espectroscopia na região de UV-visível, microscopia eletrônica de varredura(MEV), microscopia eletrônica de transmissão (MET) equipado com um raioX de energia dispersiva espectrômetro (EDS) e medidas fotoluminescentes. No interesse de uma eventual aplicabilidade do semicondutor em catálise foram feitos ensaios na descoloração da tartrazina. Os pós precursores calcinados a diferentes temperaturas (400 ° C em função do tempo (4, 8 e 12 horas) e 600, 800 e 1000 ° C durante 4 horas) são monofásicos de acordo com os resultados de difração de raios-X. O grau de cristalinidade aumentou com a temperatura de acordo com os resultados do tamanho de cristalito e FWHM. A energia da "banda proibida" foram calculadas a partir das curvas de reflectância no UV e mostrou obtenção de um semicondutor (~ 2.0 eV). Por MET foi observado partículas em nanoescala monofásicas sem fase secundária. A estrutura e morfologia avaliada por SEM revelou um aglomerado e material poroso. A propriedade óptica mostrou uma possível aplicabilidade do material estudado como fotoluminescente. De acordo com o valor de "banda proibida" e da porosidade dos materiais, testes fotocatalíticos foram realizados na descoloração do corante tartrazina. Os resultados mostraram que o material estudado é promissor para a catálise.

Hydroxycalciomicrolite, Ca1.5Ta2O6(OH) is a new microlite-group mineral found in the Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil. It occurs as isolated octahedral and as a combination of octahedral and rhombic dodecahedral crystals, up to 1.5 mm in size. The crystals are yellow and translucent, with a white streak and vitreous to resinous lustre. The mineral is brittle, with a Mohs hardness of 56. Cleavage is not observed and fracture is conchoidal. The calculated density is 6.176 g cm(3). Hydroxycalciomicrolite is isotropic, n(calc). = 2.010. The infrared and Raman spectra exhibit bands due to OH stretching vibrations. The chemical composition determined from electron microprobe analysis (n = 13) is (wt.%): Na2O 0.36(8), CaO 15.64(13), SnO2 0.26(3), Nb2O5 2.82(30), Ta2O5 78.39(22), MnO 0.12(2), F 0.72(12) and H2O 1.30 (from the crystal structure data), O = F -0.30, total 99.31(32), yielding an empirical formula, (Ca1.48Na0.06Mn0.01)(Sigma 1.55)(Ta1.88Nb0.11Sn0.01)S2.00O6.00[(OH)(0.76)F0.20O0.04]. Hydroxycalciomicrolite is cubic, with unit-cell parameters a = 10.4205(1) angstrom, V = 1131.53(2) angstrom(3) and Z = 8. It represents a pyrochlore supergroup, microlite-group mineral exhibiting P4(3)32 symmetry, instead of Fd (3) over barm. The reduction in symmetry is due to long-range ordering of Ca and vacancies on the A sites. This is the first example of such ordering in a natural pyrochlore, although it is known from synthetic compounds. This result is promising because it suggests that other species with P4(3)32 or lower-symmetry space group can be discovered and characterized.

The thermodynamics of disorder have been studied for hundreds of years, with physicists using entropy to quantitatively connect the macroscopic properties of a system to its microscopic multiplicity (disorder). Here, we consider the effect of disorder in magnetic materials. The pyrochlore oxides (A2B2O7), comprised of a bipartite lattice of corner-sharing tetrahedra, have been central to the study of geometric frustration for the past several decades. Pyrochlores, in which the A-site is occupied by the magnetic cation dysprosium, tend to exhibit spin ice ordering down to low temperatures, in spite of chemical perturbations to the B-site lattice. With the motivation of this study being the investigation of how adding B-site disorder to the traditional Dy2ScNbO7 form of Dy-pyrochlores, a stoichiometric mixture of Sc-3+ and Nb-5+ was used to synthesize Dy2ScNbO7, the pyrochlore material that is central to this thesis work. We show using magnetometry, heat capacity, muon spin relaxation, and inelastic neutron scattering that the mixed B-site pyrochlore Dy2ScNbO7, does not adopt the spin ice ground state. The low temperature spin dynamics are much faster than other analogous dysprosium pyrochlores, the residual entropy is significantly smaller than that predicted for a spin ice and there are low-lying crystal field excitations. These results all indicate that the B-site disorder appears to destroy the predicted Ising anisotropy of dysprosium. / Thesis / Master of Science (MSc)

No description available.

The rare earth pyrochlore oxides, with formula R2B2O7, are a chemically versatile family of materials that exhibit a diverse array of magnetic phenomena. In this structure the R and B site cations each form a corner-sharing tetrahedral network, a motif that is prone to intense geometric magnetic frustration. As a consequence of their magnetic frustration, rare earth pyrochlores are observed to host a number of remarkable states such as spin ice and spin liquid states. In this thesis we endeavor to explore the phase diagrams of the rare earth pyrochlores through the lens of chemical pressure. Chemical pressure is applied by varying the ionic radius of the non-magnetic B site cation, which either expands or contracts the lattice, in analogy to externally applied pressure. We apply positive chemical pressure by substituting germanium at the B site and negative chemical pressure by substituting lead at the B site. We also consider the effect of platinum substitution, which has nominally negligible chemical pressure effects. In the ytterbium pyrochlores, we find that positive chemical pressure tunes the magnetic ground state from ferromagnetic to antiferromagnetic. Remarkably, we also find that the ytterbium pyrochlores share a ubiquitous form to their low temperature spin dynamics despite their disparate ordered states. In the terbium pyrochlores, we find that positive chemical pressure promotes ferromagnetic correlations - the opposite effect of externally applied pressure. Our studies of platinum pyrochlores reveal that platinum, while non-magnetic, is able to facilitate superexchange pathways. Thus, the magnetic ground states of the platinum pyrochlores are significantly altered from their titanate analogs. The work in this thesis highlights the delicate balance of interactions inherent to rare earth pyrochlore magnetism and shows that chemical pressure is a powerful tool for navigating their phase spaces. / Thesis / Doctor of Philosophy (PhD) / Rare earth pyrochlores have the chemical formula R2B2O7, where R is a magnetic rare earth element and B is a non-magnetic element. Materials of this type are widely studied because they have a propensity to exhibit exotic magnetic properties. In this thesis, we study the effect of varying the size of the non-magnetic B site atom, which is termed chemical pressure. As B is made larger or smaller, the crystal lattice expands or contracts, mimicking the effect of externally applied pressure. High-pressure synthesis techniques were used to prepare R2B2O7 compounds with B site cations that are typically too small (germanium), too large (lead), or too unstable (platinum) under ambient pressure conditions. Our characterizations of these high-pressure materials have revealed that their magnetism is remarkably sensitive to the application of chemical pressure.

A way to obtain materials that show novel phenomena is to explore the interplay between geometry and interactions. When it is not geometrically possible to satisfy all the interactions by a given configuration, then to find the ground state becomes very complicated. This interplay between geometry and interactions defines geometrical frustration. One of the most popular examples of geometrical frustration in magnetism is spin ice. In this system, nearest neighbour ferromagnetic interactions between Ising spins in a pyrochlore structure emulate water ice by showing the same degree of frustration. This is manifested by the same ground state residual entropy. Although the clearest example of spin ice among magnets is shown by Dy₂Ti₂O₇, the behaviour of this material is richer than that of pure spin ice. The large magnetic moments of the rare earth Dy form a spin ice that also interacts via dipolar interactions. These long range interactions give rise to monopolar excitations which dramatically affect the dynamics of the system with respect to the pure spin ice case. In this thesis magnetisation experiments and numerical methods are used to explore the properties of the magnetic insulator Dy₂Ti₂O₇. We study its excitations at low temperature and describe the out-of-equilibrium characteristics of the magnetisation processes, below a temperature where the system freezes out. For temperatures above the freezing temperature, we describe and measure a 3D Kasteleyn transition and the concomitant Dirac strings associated to it, for the field in the [100] crystallographic direction. For temperatures below the freezing temperature, we find new out-of-equilibrium phenomena. Magnetic jumps are measured and their sweep rate dependence analysed. A deflagration theory is proposed and supported by simultaneous magnetisation and sample temperature measurements obtained by a new design of a Faraday magnetometer.

The use of solid-state NMR and DFT calculations to study Y₂Sn[subscript(x)]Ti[subscript(2-x)]O₇, Y₂Sn[subscript(x)]Zr[subscript(2-x)]O₇ and Y₂Ti[subscript(x)]Zr[subscript(2-x)]O₇, materials with applications for the safe encapsulation of radioactive actinides is investigated. As a result of cation or anion disorder in these materials, NMR spectra are often complex and difficult to interpret. Therefore, an investigation using a range of NMR active nuclei and measurement of a variety of NMR parameters (isotropic chemical shift, δ[subscript(iso)]; span, Ω and quadrupolar coupling, C[subscript(Q)]) were used to provide a full and detailed picture of each material. The measurement of Ω in these disordered compounds with multiple resonances in the NMR spectra, required the use of 2D CSA-amplified PASS (CAPASS) experiments to enable the separation of each of the spinning sideband manifolds. An experimental assessment of the CAPASS experiment showed that although low ν₁/Ω[subscript(Hz)] ratios (as found in ⁸⁹Y NMR) resulted in distortions in the spectra obtained, a modified fitting procedure could be utilised to compensate for this fact, which allowed the accurate measurement of Ω. Despite the difficulties in acquiring the ⁸⁹Y NMR spectra, they were found to be the most informative of the NMR-active nuclei available. ¹¹⁹Sn NMR spectra, although much easier to acquire than ⁸⁹Y, were more complex and harder to analyse, owing to the overlapping resonances. Therefore, ¹¹⁹Sn NMR could only be used to confirm or support the results obtained using ⁸⁹Y NMR. Although ¹⁷O NMR was found to be useful, a full study could not be implemented due to the lack of ¹⁷O enriched samples; an area where future investigation may prove fruitful. Finally, [superscript(47/49)]Ti and ⁹¹Zr NMR spectra were found to be the most difficult to acquire due to their low receptivities and the quadrupolar broadened lineshapes, and as a result, little additional information was obtained. As a result of this analysis, for the Y₂Sn[subscript(x)]Ti[subscript(2-x)]O₇ pyrochlore solid solution, using primarily ⁸⁹Y δ[subscript(iso)] and Ω, and additionally confirmed with ¹¹⁹Sn δ[subscript(iso)], it was found that the Sn and Ti cations were randomly ordered throughout the B-sites. Additionally, ⁸⁹Y Ω could be used to obtain approximate Y-O[subscript(48f)] and Y-O[subscript(8b)] bond lengths for each type of Y environment. The study of Y₂Sn[subscript(x)]Zr[subscript(2-x)]O₇ using ⁸⁹Y NMR showed that although the end members were single phase, pyrochlore (Y₂Sn₂O₇) or defect fluorite (Y₂Zr₂O₇), the intermediate compositions were mostly two phase mixtures, consisting of an ordered pyrochlore (with an average formula of Y₂Sn₁.₈Zr₀.₂O₇) and a disordered phase, where the proportions of the pyrochlore and disordered phases decreased and increased, respectively, with the Zr content. Additionally, although the coordination states of the Y and Sn cations were easily determined using ⁸⁹Y and ¹¹⁹Sn NMR, respectively, the coordination states of the Zr cations could not be confirmed directly by ⁹¹Zr NMR. However, using indirect analysis from results obtained with ⁸⁹Y and ¹¹⁹Sn NMR, it was determined that 6 coordinate Zr was present in each composition, and it was always present in a greater proportion than 8 coordinate Zr. Finally, although ⁸⁹Y NMR spectra of Y₂Ti[subscript(x)]Zr[subscript(2-x)]O₇ were extremely difficult to analyse, it was tentatively proposed that they could be similar to Y₂Sn[subscript(x)]Zr[subscript(2-x)]O₇ due to some similarities observed between the spectra.