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Disseler, Steven Michael Thomas. "Magnetic Order in the Pyrochlore Iridates." Thesis, Boston College, 2013. http://hdl.handle.net/2345/3148.
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Thesis advisor: Michael J. GrafThis 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 Nd3+ 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
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Wilde, Peter Joseph. "The defect chemistry of pyrochlore structured oxides." Thesis, University College London (University of London), 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.309471.
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Mostaed, Ali. "Atomic structure study of pyrochlore ytterbium titanate." Thesis, University of Warwick, 2017. http://wrap.warwick.ac.uk/96357/.
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There has been great interest in the magnetic behaviour of pyrochlore oxides with the general formula A2B2O7, in which rare-earth (A), and transition metal (B) cations are ordered on separate interpenetrating lattices of corner-sharing tetrahedra. Such materials exhibit behaviours including quantum spin-ice, (quantum) spin-liquid, and ordered magnetic ground states. Yb2Ti2O7 lies on the boundary between a number of competing magnetic ground states. Features in the low-temperature specific heat capacity that vary in sharpness and temperature from sample to sample suggest that in some cases the magnetic moments order, while in others the moments remain dynamic down to temperatures as low as ~16 mK. In this work, three different Yb2Ti2O7 single crystal samples, all grown by the optical floating zone technique but exhibiting quite different heat capacity behaviour, are studied by aberration-corrected scanning transmission microscopy (STEM). Atomic-scale energy-dispersive X-ray (EDX) analysis shows that a crystal with no specific heat anomaly has substitution of Yb atoms on Ti sites (“stuffing"). In fact, EDX analysis shows for the first time that “stuffing” of Yb (A) cations onto Ti (B) sites in the lattice can be observed directly in the pyrochlore structure. Moreover, I show that the detailed intensity distribution around the visible atom columns in annular dark field STEM images is sensitive to the presence of nearby atoms of low atomic number (in this case oxygen) and find significant differences between the samples that correlates both with their magnetic behaviour and measurements of Ti oxidation state using electron energy loss spectroscopy. These measurements support the view that the magnetic ground state of Yb2Ti2O7 is extremely sensitive to disorder. On the other hand, structural modification of ytterbium titanate by deviation from stoichiometry as well as electron irradiation is studied in this work. I show that Ti excess is accommodated by Yb cation vacancies while Yb excess is compensated by Yb interstitials. Furthermore, two MATLAB programs, Detect Columns and BurgersVectors, to analyse STEM images and calculate the dislocation density tensor of STEM images as well as their Burgers vector(s) are respectively introduced in this work.
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Simon, Charles Francis. "The synthesis and characterisation of pyrochlore frameworks." Thesis, University of Southampton, 2010. https://eprints.soton.ac.uk/203757/.
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Structural studies have been undertaken on a wide range of pyrochlore phases, with a focus on garnering a better understanding of the factors determining the formation and behavior of these materials. Extensive structural characterisation has been undertaken primarily using powder X-ray and powder neutron diffraction techniques to explore the subtle structural changes that occur as a result of changes in composition on the two interlocking networks that make up the pyrochlore structure. Particular attention has been paid to hydrated beta-pyrochlore materials, using high resolution TOF and constant wavelength PND to accurately determine the location and orientation of the water molecules within the structure. The location of these molecules has been shown to be directly correlated to the overall lattice dimensions of the pyrochlore framework, with the subsequent molecular orientation and internal bond angles changing to provide an optimum stabilization effect on the A-site cation. Investigation into the effects of hydration upon the superconducting beta-pyrochlore series AOs2O6.nH2O (where A = K, Rb and Cs) has also been investigated; with a correlation between the structural changes, the degree of hydration and the onset of the superconducting state being discussed. Also presented is work seeking to determine the scope and effects of ion-exchange on the A2O’ network of the pyrochlore framework, with several new phases synthesised and other known phases extensively characterized along with a discussion regarding the limits of what may be included in the pyrochlore framework.
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Champion, John Dickon Mathison. "Theoretical and experimental investigations of frustrated pyrochlore magnets." Thesis, University College London (University of London), 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.249647.
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Eberman, Kevin W. (Kevin Wilmot) 1971. "Crystallographic origins of fast-ion conduction in pyrochlore." Thesis, Massachusetts Institute of Technology, 1998. http://hdl.handle.net/1721.1/50456.
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Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1998.Includes bibliographical references (p. 129-132).
We have examined the crystallographic origins of fast-ion conduction in oxides with the pyrochlore structure-type, ideally A2B2O7, a superstructure of (A,B)20 3.5 defect fluorite-like array. These materials have technological promise for use in electrochemical devices such as oxygen sensors and solid-oxide fuel-cells. The pathway for ion conduction proposed by several authors has been a jump between equivalent nearest-neighbor oxygen sites through the tetrahedral edges of the relatively-immobile cation array. Substitution of a third cation species in slid-solution in the B site, A2(B-yB'y)2O7, results in a marked change in the structure and properties of the materials which is not fully understood. As the average radius of the cations occupying the B-site increases (with changing composition) towards that of the larger average radius of the cations occupying the Asite, a tendency for increasing disorder has been observed, where complete disorder corresponds to the fluorite structure. A decrease in Frenkel-defect formation-energy and an increase in the migration enthalpy accompanies the disordering. We have executed several studies of pyrochlore structures employing neutron and x-ray powder diffraction. In particular, we have focused on high-temperature in-situ experiments that should better correspond to the structure for which the conductivity measurements were made at elevated temperature ...
by Kevin W. Eberman.
Ph.D.
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Pirzada, Mohsin Tanveer-Ul-Haq Farouqi. "Atomic scale computer simulations of pyrochlore and spinel oxides." Thesis, Imperial College London, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.406556.
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Galati, Rosa. "Synthesis, structure and properties of heavy metal pyrochlore oxides." Thesis, University of Southampton, 2010. https://eprints.soton.ac.uk/173963/.
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Many interesting and often exotic magnetic phenomena have been observed over the past twenty years in the family of the heavy metal pyrochlore oxides, A2B2O6O’. In these systems, competing interactions are common and tend to emerge mainly at low temperatures. Magnetic properties are related to the contribution of d orbitals that transition metal (TM) offer and their interplay with the positive contribution coming from the cation sitting on the A site. The work described in this thesis explores the effects of chemical substitution of the A site cation on the symmetry and the properties of a number of pyrochlores containing TM of the second and third transition row, such as Ir, Ru, Mo and Os. Depending if this site is fully occupied and on the radius size the behaviour of the resulting material can change significantly. The defect pyrochlore family AOs2O6 (A = Cs, Rb, K) was studied in detail following the attention that its members attracted for having been the first materials, belonging to this subgroup, to exhibit superconducting properties [1, 2, 3] and for the diversity in their properties. In these compounds, the smaller the ionic radius of an element, the weaker is the bond to the structure. As a result, the temperature dependence of their energy becomes quite different from normal lattice vibrations and gives interesting low temperature behaviours. The effect of replacing the weak bonded potassium ions on the structures and superconducting properties of the AOs2O6 phases has also been studied. The ability to exchange the A-type in pyrochlores is well known and leads to applications for example in nuclear clean-up. Thus the ion exchange of K+ by smaller cations such as Li+ or Na+ has been investigated with the aim of synthesising compounds which cannot be obtained by direct synthesis. New defect pyrochlore materials, AA’Os2O6 (A’ = Li+, Na+) have been synthesised together with hydrated derivatives, AA’OsO6.nH2O. Parent materials were obtained by ion exchange in non-aqueous solvents under strictly dry conditions. A Ba-based osmium oxide pyrochlore, Ba(Os2O6)2.H2O was also synthesised, for the first time, from the K-osmate starting material by ion-exchange methods in aqueous medium. In this work, diffraction techniques, X- ray and neutron powder diffraction, were employed and found suitable for detecting the change in cation and anion location and general structure investigation.These structural descriptions are correlated with the measured superconducting properties and theoretical descriptions of variations in the Tc in these systems as a function of cation size and rattling.
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Porter, Spencer H. "Perovskite and Pyrochlore Tantalum Oxide Nitrides: Synthesis and Characterization." The Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1332499280.
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Benton, John Owen. "Classical and quantum spin liquids on the pyrochlore lattice." Thesis, University of Bristol, 2015. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.681486.
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The study of frustrated magnetism sits at the frontier of modern condensed matter physics, offering the possibility to discover new states of matter with exotic excitations. A textbook example of this is seen in the rare earth pyrochlore oxides H02Ti20 7 and DY2Ti207, better known as "spin ice". These magnets exhibit a classical spin liquid state with emergent magnetic monopole excitations. Inspired by this, we explore in this thesis a wide landscape of possibilities for novel magnetic behaviour on the pyrochlore lattice. Starting from the most general model for anisotropic nearest neighbour exchange interactions we show that the full zero temperature classical phase diagram can be obtained simply by considering the symmetries of a tetralledron. In the process of doing this we obtain an exact rewriting of the model in terms of a set of local order parameter fields defined on tetrahedra. After classifying and describing the ordered phases of this model we turn to give consideration to the novel physics which may be found near the boundaries where phases with different symmetry meet. We find that interesting properties of several materials can be traced back to the "accidental" degeneracies which arise on these boundaries. These include the "order by disorder" in Er2Ti207, the "dimensional-reduction" observed in Yb2Ti207, and the suppression of The observation that the physics of the phase boundaries manifests itself throughout the nearby regions of the phase diagram leads us to a systematic analysis of all of the phase boundaries of the model. In several cases dipole order persists at the phase boundary, stabilised by fluctuations. By constructing the ground state manifolds in each case we identify those configurations which are favoured by fluctuations. We also identify two limits of the phase diagram in which dipolar order vanishes to be replaced by spin nematic order. At certain phase boundaries we find that magnetic order vanishes entirely. This leads to the discovery of a new classical spin liquid on the pyrochlore lattice- the anisotropic Coulomb liquid. This spin liquid, like the Coulomb phase observed in spin ice, possesses algebraic correlations and conserved fluxes but also presents several new features including an anisotropic Coulomb interaction between its excitations. A long-wavelength theory of this spin liquid is derived from the microscopic model by coarse graining the fluctuations of the local order-parameter fields. Homing in on a different limit of the phase diagram, we move on to consider the effects of quantum fluctuations on a spin ice material and in particular how these effects would manifest themselves in neutron scattering experiments. We explore the correlations of the U(I) spin liquid which arises in this limit by explicitly constructing a basis of photon operators on the pyrochlore lattice and calculating the associated dispersion relation and correlation functions. This enables us to quantitatively describe how these unusual excitations would manifest themselves in neutron scattering experiments. We find that "pinch points," which are the signal feature of a classical spin ice, fade away as a quantum ice is cooled to its quantum spin liquid ground state.
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Fukazawa, Hideto. "Magnetic ground state of "S=1/2" pyrochlore oxides." 京都大学 (Kyoto University), 2003. http://hdl.handle.net/2433/149045.
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Whitaker, Mariana J. "Synthesis and characterisation of chemically modified schafarzikite and pyrochlore minerals." Thesis, University of Birmingham, 2013. http://etheses.bham.ac.uk//id/eprint/3944/.
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Schafarzikite, FeSb\(_2\)O\(_4\), is a tetragonal mineral (space group P4\(_2\)/mbc) belonging to a family of compounds with the general formula MX\(_2\)O\(_4\). Schafarzikite has successfully been synthesised and chemically modified in various different ways. Lead has been substituted onto the pyramidal X sites to form compounds of formulation FeSb\(_{2-x}\)Pb\(_x\)O\(_4\) (x = 0.2-0.75), excess oxygen ions have been incorporated into the structure to form FeSb\(_{2-x}\)Pb\(_x\)O\(_{4+y}\) and compounds with mixed octahedral M ions, Fe\(_{1-x}\)Mg\(_x\)Sb\(_2\)O\(_4\), have been prepared. The modifications give rise to changes in the magnetic ordering and magnetic structures and the conductivity of FeSb\(_{2-x}\)Pb\(_x\)O\(_4\) is increased with respect to that of schafarzikite. Pyrochlores (space group Fd-3m) have the general formula A\(_2\)B\(_2\)O\(_7\). The structures of Bi\(_{2-x}\)Fe\(_x\)(FeSb)O\(_7\) (x = 0.1, 0.2, 0.3) and Nd\(_{1.8}\)Fe\(_{0.2}\)(FeSb)O\(_7\) have been studied and found to undergo static displacive disorder in the A\(_2\)O’ chains. Pyrochlore materials are magnetically frustrated and the samples Bi\(_{2-x}\)Fex(FeSb)O\(_7\) (x = 0.1, 0.2, 0.3) display spin-glass behaviour. The magnetic properties of RE\(_2\)CrSbO\(_7\) pyrochlores have also been investigated because, despite the inherent magnetic frustration in the pyrochlore structure, Ho\(_2\)CrSbO\(_7\) is known to order magnetically at 10 K. Neutron diffraction showed that the Ho\(^{3+}\) moments form an ordered spin-ice arrangement while the Cr\(^{3+}\) moments order ferromagnetically.
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Rivière, Martine. "Synthèse, structure et propriétés de conducteurs protoniques de type pyrochlore." Grenoble 2 : ANRT, 1986. http://catalogue.bnf.fr/ark:/12148/cb37600815m.
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Duarte, William. "Composés de type La2Zr2O7 élaborés par projection plasma de solution et par chimie douce : application aux moteurs spatiaux." Thesis, Limoges, 2015. http://www.theses.fr/2015LIMO0083/document.
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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 poudresThese 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
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Bezerra, Lopes Francisco Wendell. "ESTUDO DOS ÓXIDOS A2B2O7 E ABO3 A BASE DE TERRAS RARAS, PARA APLICAÇÕES TÉRMICAS E CATALÍTICAS A ALTAS TEMPERATURAS." Phd thesis, Toulon, 2011. http://tel.archives-ouvertes.fr/tel-00714810.
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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.
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Pinto, Henrique Senna Diniz. "Pyrochlore der Pegmatit-Provinz Nazareno, Brasilien Modellvorstellungen zu Mineralisation - Alteration - Kristallchemie /." [S.l. : s.n.], 2000. http://ArchiMeD.uni-mainz.de/pub/2000/0034/diss.pdf.
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Espinosa, Gómez Rodolfo. "Recovery of pyrochlore from slimes discarded at Niobec by column flotation." Thesis, McGill University, 1987. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=75946.
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The Niobec concentrator recovers niobium minerals (principally pyrochlore) from carbonate and silicate gangue. In the process some 15% of the Nb is rejected in a slimes ($-$10 $ mu$m) product. Recovery of pyrochlore from the slimes was studied. The study was performed using plant water which contained about 200 ppm Ca, 100 ppm Mg and variable quantities of organics as the main contaminants. Selective pyrochlore collectors and selective silicate depressants were evaluated. Four possible routes to float pyrochlore selectively were found. All routes required prior carbonate flotation. Carbonate flotation was attempted using fatty acids and sodium silicate dispersant (as conducted at Niobec) in a flotation column. However, pyrochlore also floated as a result of inadequate dispersion of the slimes. Therefore, new dispersants were tested, which proved effective only at high concentration, because reaction with Ca and Mg cations in the water. High dispersant concentration caused carbonate depression.An additional problem in the flotation column was bubble coalescence and collapse of the froth. This was caused by the fatty acids, enhanced by the high dispersant concentration.
A novel method of quantifying coalescence using the change in gas holdup in the column was developed. A stable froth could be made at high fatty acid emulsifier concentration, but his promoted unselective flotation. Only by working at high percent solids (10% vs 1%) could sufficient emulsifier be introduced into solution without exceeding the dosage (per unit mass of solids) above which flotation is unselective. For such high percent solids work must be conducted at plant site. Preliminary plant column work was encouraging.
Extensive testwork on very fine feeds (80% $-$20 $ mu$m) at Niobec, and at Mt Isa Mines (Australia) confirmed that column flotation is ideal for selective flotation of fines principally due to reduced gangue entrainment.
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Kramer, Steve Andrew. "Mixed ionic-electronic conduction in rare earth titanate/zirconate pyrochlore compounds." Thesis, Massachusetts Institute of Technology, 1994. http://hdl.handle.net/1721.1/26863.
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Ikoma, Daisaku, Hiroki Tsuchiura, and Jun-ichiro Inoue. "Magnetic phase diagram of metallic pyrochlore lattice in the double-exchange model." The American Physical Society, 2003. http://hdl.handle.net/2237/7114.
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Merka, Oliver [Verfasser]. "Yttrium bismuth titanate pyrochlore mixed oxides for photocatalytic hydrogen production / Oliver Merka." Hannover : Technische Informationsbibliothek und Universitätsbibliothek Hannover (TIB), 2012. http://d-nb.info/1029589895/34.
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Walker, Robert. "The surface chemistry and bulk electronic structure of bismuth based pyrochlore oxides." Thesis, Imperial College London, 2016. http://hdl.handle.net/10044/1/48468.
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Metal oxides of bismuth show applications from photocatalysts to dielectric materials often due to the influence of the so called “lone pair” electrons that give rise to structural distortions and modified electronic structure. These effects upon which fundamental properties are derived are still not yet fully understood. Polycrystalline Bi 2 Ti 2 O 7 , Bi 2 Zr 2 O 7 , and Bi 2 Hf 2 O 7 materials - the latter in both pyrochlore (p) and monoclinic (m) phases - were prepared via co-precipitation and, except for Bi 2 Hf 2 O 7 (m), thin films. The bulk, surface, and electronic properties were characterised using a combination of X-ray diffraction and spectroscopic techniques. The structural determination, completed in Chapter 4, confirmed the formation of a directional lone pair by O 2p-assisted Bi 6s-6p hybridisation in all except Bi 2 Zr 2 O 7 . X-ray photoelectron spectroscopy quantified a Bi surface excess that in Chapter 5 was probed further by low-energy ion spectroscopy achieving greater surface sensitivity. The top atomic layer was determined to be almost exclusively BiO x with surface relaxation of the directional lone pair suggested as the driving force behind this surface reconstruction. By in-situ diffuse reflectance infrared Fourier-transformed spectroscopy, CO 2 was found to strongly chemisorb onto the surface of Bi 2 Ti 2 O 7 , Bi 2 Zr 2 O 7 , and Bi 2 Hf 2 O 7 (p) indicating a basic surface. Both the surface reconstruction and surface basicity were not observed in Y 2 Ti 2 O 7 demonstrating the clear influence of the Bi cation. In Chapter 6, the electronic structure of the films was investigated by XPS and polarisation-dependent HAXPES and indicated a strong Bi contribution to the valence band either by O 2p-assisted Bi 6s-6p hybridisation or, as in Bi 2 Zr 2 O 7 , anti-bonding interactions between O 2p and Bi 6s. The Bi 6s was shown to shift the valence band towards the Fermi edge, in addition, a valence band with a high metal character is beneficial for the formation of holes and mobility while Bi 6p contributions to the conduction band improve the stability of excited electrons from the valence band with high O 2p character.
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Baroudi, Kristen. "Synthesis and characterization of novel magnetically frustrated oxides with honeycomb and pyrochlore structures." Thesis, Princeton University, 2015. http://pqdtopen.proquest.com/#viewpdf?dispub=3712079.
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In this thesis I present the synthesis and characterization of materials exhibiting frustrated magnetism. In Chapter 1 I describe magnetic frustration and some of the magnetic states that can arise from it followed by the background on iridates and platinates with honeycomb structures and rare earth pyrochlores.
In Chapter 3 I discuss my work on the synthesis and properties of ternary sodium iridates with formulas NaxM2/3Ir1/3O 2 and NaxM1/3Ir2/3O2 (M = Mn, Fe, Co, Ni, Cu, Zn). The ternary iridates are based on the honeycomb compound Na2IrO3 but show more disorder in the honeycomb layer than the parent. The six new compounds are all spin glasses but show distinct magnetic properties from one another.
In Chapter 4 I continue my work on honeycombs by exploring new ternary sodium platinates. These three new compounds with formulas Na3MPt 2O6+x (M = Mg, Cu, Zn) are structurally very similar to the iridates discussed in Chapter 2 but have non-magnetic Pt4+ in place of magnetic Ir4+. The Mg and Zn variants are non-magnetic while the Cu variant is paramagnetic at 2 K.
Chapter 5 is a synchrotron X-ray diffraction study of the magnetically frustrated rare earth pyrochlores Ho2Ti2O7, Er2Ti2O7 and Yb2Ti2O 7. Previous neutron scattering studies have shown reflections that are forbidden by the assigned space group Fd-3m, therefore high intensity, high resolution X-ray diffraction data was collected to determine if the reflections are present. Slight variations in sample stoichiometry were studied to account for possible sample variation. The forbidden reflections are absent from the X-ray diffraction patterns, providing strong evidence that the extra reflections in neutron scattering experiments are not structural in origin.
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Sprague, John Jason 1971. "Mixed conduction and defect chemistry of manganese and molybdenum substituted gadolinium titanate pyrochlore." Thesis, Massachusetts Institute of Technology, 1999. http://hdl.handle.net/1721.1/85250.
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Spears, Marlene Ann. "Defect chemistry and electrical properties of ruthenium- and bismuth-substituted gadolinium titanate pyrochlore." Thesis, Massachusetts Institute of Technology, 1995. http://hdl.handle.net/1721.1/11443.
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Injac, Sean Dusan Alexander. "Structural and Magnetic Characterisation of 4d and 5d Mixed Metal Oxides." Thesis, The University of Sydney, 2019. https://hdl.handle.net/2123/21640.
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Advanced functional materials showing magnetic properties are a cornerstone of modern information technologies, and are keenly studied in order to search for useful materials within emerging technologies, such as spintronics, applied superconductivity and quantum computing. Magnetic materials containing the heavy 4d and 5d metals are relatively understudied compared to their 3d counterparts and are a promising avenue of study in this search for novel functional materials. This work presents the synthesis, structural, and magnetic characterisation of a number of novel 4d and 5d metal oxides within the BaB1-xB’xO3 perovskite, Ba3BB’2O9 6H perovskite, where B = Ti, Zn, Y, and B’ = Ru, Os. Sr2BB’O6 double perovskite, where B = Y, Mn, Fe, and B’ = Re, Os. A2-αB2O7-γ pyrochlore and defect pyrochlore, where A = Pb, Bi, Cd, Tl, and B = Mn, Ru, Ir, Os. ABO¬4 scheelite and baryte structural families, where A = K, Rb, Cs, and B = Ru, Os. These structure classes represent a wide variety of atomic connectivity and metal coordination environments, which in turn drive various magnetic coupling and exchange interactions, such as geometric frustration. The flexibility of these structural classes to composition, cation size and oxidation state, coupled with the electronic properties of the 4d and 5d metal cations gives rise to fascinating and varied magnetic behaviours, including spin ice / glass states, quantum spin liquids, quantum magnetism, antiferromagnetism, and high temperature ferrimagnetism.
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Chin, Earle J. "Carbothermic reduction of pyrochlore and niobium pentoxide in a transferred arc plasma : a thesis." Thesis, McGill University, 1989. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=75989.
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The carbothermic reduction of pyrochlore and niobium pentoxide was studied, with and without the addition of iron as a solvent, in an argon transferred-arc plasma with a single stationary pellet reactor system. A conversion-time model, based on experimental and theoretical studies, was proposed to describe the carbothermic reduction reaction systems. The experimental results confirmed the applicability of the model.The carbothermic reduction of pyrochlore and niobium pentoxide were studied in the temperature range 1530-2440 and 1625-2855 K, respectively. The rate was independent of particle size but was influenced by the separation between the oxide and carbon particles. The rate was lower than the model predictions above a certain conversion, the level of which increased with temperature and carbon concentration.
The carbothermic reduction of pyrochlore and niobium pentoxide with iron addition were studied in the temperature range 1430-2125 and 1575-2440 K, respectively. The rates were independent of particle size but was enhanced by the presence of iron. Deviation from the model was observed to be less at high conversion levels.
Preliminary investigation of the conversion rate for the carbothermic reduction of a pyrochlore and ferric oxide mixture was done to simulate the type of reactants that would be used in a commercial process. The rate was improved by the addition of iron oxide.
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Zietlow, Peter Verfasser], and Ulrich [Akademischer Betreuer] [Bismayer. "Properties and recrystallization of radiation damaged pyrochlore and titanite / Peter Zietlow ; Betreuer: Ulrich Bismayer." Hamburg : Staats- und Universitätsbibliothek Hamburg, 2016. http://d-nb.info/1119319943/34.
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Zakharchuk, Kiryl. "Defect chemistry of pyrochlore-type (Gd,Ca)₂(Ti,Mo)₂O₇ for prospective electrocatalytic applications." Master's thesis, Universidade de Aveiro, 2017. http://hdl.handle.net/10773/21877.
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Mestrado em Engenharia de MateriaisO principal objetivo deste trabalho foi a caracterização de soluções sólidas derivadas do pirocloro Gd2Ti2O7 com co-substituições de Gd e Ti por Ca e Mo, respetivamente, para eventuais aplicações electrocatalíticas. O trabalho foi motivado por desvantagens dos ânodos cermetos de Ni-YSZ propostos para pilhas de combustível, designadamente degradação microestrutural, intolerância a alterações redox, contaminação com enxofre e deposição de carvão. Os pirocloros à base de Gd2Ti2O7 foram considerados adequados para o desenvolvimento de componentes para ânodos alternativos, por cosubstituições em ambas as posições catiónicas, sem degradação da estrutura do pirocloro, conferindo condução mista e atividade electrocatalítica,. Em condições oxidantes, a gama de soluções sólidas de (Gd1-xCax)2(Ti1-yMoy)2O7±δ com estrutura de pirocloro situa-se entre 0.10 < x < 0.15 e 0.05 < y < 0.10, sendo as amostras cerâmicas da composição (Gd1-xCax)2(Ti0.95Mo0.05)2O7±δ estáveis numa vasta gama de pressão parcial de oxigénio, sem degradação detetável por difração de Raios-X, termogravimetria ou comportamento elétrico. Materiais à base de Gd2Ti2O7 com co-substituições possuem coeficiente de expansão térmica linear de (10.5-10.7)×10-6 K-1 a 25-1100°C, em ar, e estabilidade dimensional com variações redox, sendo quase nula a expansão química até p(O2) ~ 10-12 atm, a 950°C. Os mecanismos de compensação e transportadores de carga predominantes em condições oxidantes dependem da razão Ca:Mo. Co-substituição por molibdénio suprime a condutividade iónica e origina mudança de condução predominantemente iónica para eletrónica ou mista. Observou-se aumento de condutividades iónica e eletrónica com decréscimo na pressão parcial de oxigénio, conferindo condutividade mista a cerâmicos de (Gd,Ca)2(Ti,Mo)2O7-δ em condições típicas de operação dos ânodos. Foi desenvolvido um modelo de química de defeitos que descreve as variações de propriedades elétricas em função das condições redox e co-substituições. Além disso, relacionouse a condutividade protónica a mais baixas temperaturas, em atmosferas húmidas, com a concentração de lacunas de oxigénio na rede do pirocloro, tendo a humidade um forte impacto na resistividade das fronteiras de grão.
The main objective of the work was design and comprehensive characterization of solid solutions derived from pyrochlore-type Gd2Ti2O7 by co-substitutions of Gd and Ti by Ca and Mo, respectively, for prospective electrocatalytic applications. The work was motivated by the disadvantages of commonly used cermet Ni-YSZ cermet anodes including long-term microstructural degradation and intolerance to redox changes, sulfur poisoning and carbon deposition. Pyrochlore-type Gd2Ti2O7 was considered as a suitable parent material for the development of alternative mixed-conducting ceramic components for SOFC anodes, while simultaneous co-substitutions of both host cations are likely to allow flexible tuning of transport and redox properties of Mocontaining pyrochlores, with impact on electrocatalytic activity, while preserving phase stability provided by pyrochlore-type titanate lattice. The range of (Gd1-xCax)2(Ti1-yMoy)2O7±δ solid solutions with cubic pyrochlore-type structure was found to be limited to 0.10 < x < 0.15 and 0.05 < y < 0.10 under oxidizing conditions. (Gd1-xCax)2(Ti0.95Mo0.05)2O7±δ ceramics demonstrate good phase stability in a wide range of oxygen partial pressures with no degradation or phase decomposition evidenced in the course of electrical and thermogravimetric studies and by subsequent XRD. Co-substituted Gd2Ti2O7-based ceramics possess moderate thermal expansion coefficients compatible with solid electrolytes, (10.5-10.7)×10-6 K-1 at 25-1100°C in air, and demonstrate remarkable dimensional stability with nearly zero chemical expansion down to p(O2) ~ 10-12 atm at 950°C. Dopant compensation mechanism and type of dominating charge carriers under oxidizing conditions is demonstrated to be strongly dependent on calcium/molybdenum ratio. Co-substitution by molybdenum suppresses ionic conduction and results in a change from dominating ionic to prevailing electronic or mixed ionic-electronic transport. Reducing oxygen partial pressure increases both ionic and ntype electronic conductivities, and (Gd,Ca)2(Ti,Mo)2O7-δ ceramics exhibit mixed conductivity under SOFC anode operation conditions. Defect chemistry model is developed to describe variations of electrical properties in co-substituted materials. Low-temperature protonic transport in Gd2Ti2O7-based titanates in wet atmospheres was found to correlate with concentration of oxygen vacancies in the pyrochlore lattice, with stronger impact of humidity on the grain boundary resistivity.
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Pors, Françoise. "Préparation et caractérisation d'oxynitrures de type pérovskite, KNiF et pyrochlore étude structurale et diélectrique /." Grenoble 2 : ANRT, 1987. http://catalogue.bnf.fr/ark:/12148/cb37608978q.
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Zietlow, Peter [Verfasser], and Ulrich [Akademischer Betreuer] Bismayer. "Properties and recrystallization of radiation damaged pyrochlore and titanite / Peter Zietlow ; Betreuer: Ulrich Bismayer." Hamburg : Staats- und Universitätsbibliothek Hamburg, 2016. http://d-nb.info/1119319943/34.
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De, La Fontaine Carlos. "Controlled hydrolysis and solid state chemistry : two approaches to the synthesis of actinide oxide materials." Thesis, University of Manchester, 2013. https://www.research.manchester.ac.uk/portal/en/theses/controlled-hydrolysis-and-solid-state-chemistry-two-approaches-to-the-synthesis-of-actinide-oxide-materials(ccc8d554-a083-4cc4-815b-6ec59a1f0665).html.
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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.
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Zinkin, Martin Pen. "Neutron and X-ray scattering studies of Rbâ†2ZnClâ†4, frustrated pyrochlore antiferromagnets, and Nâ†2." Thesis, University of Oxford, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.320160.
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Tolla, Bruno. "Phénomènes d'échange d'oxygène au sein de nouveaux stannates de cérium de structure pyrochlore : défauts et mécanismes." Bordeaux 1, 1999. http://www.theses.fr/1999BOR10583.
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Ce travail concerne l'etude des proprietes d'echange d'oxygene de la solution solide pyrochlore ce 2sn 2o 7-ce 2sn 2o 8. La synthese d'un nouveau stannate ce 2sn 2o 7, ou le cerium est au degre d'oxydation iii, a ete mise au point. Sa structure a ete determinee par diffraction de neutrons. Ce compose est susceptible d'intercaler des 400\c un atome d'oxygene par unite formulaire. Une experience de diffraction de neutrons, completee par des mesures de spectroscopie a l'echelle locale (ir/raman, exafs, rmn 1 1 9sn/ 1 7o), permet de caracteriser deux types de defauts au sein du nouveau reseau qui conserve la symetrie pyrochlore. Ces especes sont a l'origine de plusieurs mecanismes d'echange d'oxygene qui ont ete caracterises par diffraction de neutrons in-situ.
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Abate, Chiara. "Novel ruthenium pyrochlore materials for cathode application in intermediate temperature solid oxide fuel cells (IT-SOFCs)." [Gainesville, Fla.] : University of Florida, 2008. http://purl.fcla.edu/fcla/etd/UFE0022800.
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Sellami, Neila. "Oxydes pyrochlores : de l’élaboration aux propriétés sous irradiation." Thesis, Paris 11, 2015. http://www.theses.fr/2015PA112003.
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En raison de la résistance élevée à l’irradiation de certaines compositions, les oxydes de structure pyrochlore A₂B₂O ₇ sont considérés comme des matrices potentielles d’immobilisation des actinides produits en réacteur nucléaire. Dans ce contexte, l’étude de la stabilité structurale sous irradiation de ces oxydes est particulièrement importante. Ce travail concerne donc l’étude de l’influence de la composition des pyrochlores et de leur microstructure sur leur comportement vis-à-vis de l’irradiation.Dans ce but, quatre compositions d’oxydes pyrochlores de formule générale A₂B₂O ₇ avec A = Gd, Nd et B = Zr, Ti ont tout d’abord été élaborées. Différentes voies de synthèse (réaction à l’état solide, chimie douce par la méthode Pechini) de poudres pyrochlores ont été explorées afin de maîtriser in fine la taille de grains des céramiques densifiées par frittage classique ou par frittage SPS. Les résultats obtenus par chimie douce sont prometteurs : des poudres fines (<100 nm) de structure pyrochlore ont été élaborées. Enfin, selon les conditions de frittage SPS choisies, des céramiques denses ont été obtenues, avec une taille de grains de 700 nm pour Y₂Ti₂O ₇ et de 300 nm pour Nd₂Zr₂O ₇ , ce qui, à notre connaissance, correspond aux plus petites tailles de grains pour ce type de composé.Afin d’étudier le comportement sous irradiation, les oxydes élaborés de compositions Gd₂Ti₂O ₇ , Y₂Ti₂O ₇ , Gd₂Zr₂O ₇ et Nd₂Zr₂O ₇ , ont été irradiés avec des ions lourds de basse énergie (Au 4MeV délivrés par l’installation JANNuS à Orsay) pour déterminer les modifications structurales produites par les effets nucléaires et par des ions lourds d’une centaine de MeV (ligne IRRSUD) ou de l’ordre du GeV (ligne SME) du GANIL à Caen afin d’étudier les effets induits par les excitations électroniques. Les transformations induites par irradiation ont été caractérisées par Diffraction des Rayons X (parfois in situ avec le dispositif ALIX du GANIL), Microscopie Electronique à Balayage et en Transmission (MEB, MET) et spectroscopie Raman. La spectroscopie d'absorption (XANES et EXAFS effectuées au synchrotron SOLEIL) a été utilisée pour caractériser l'ordre local dans la phase amorphe induite par irradiation dans les titanates. Dans le domaine du ralentissement nucléaire, les résultats principaux sont que les titanates s’amorphisent sous irradiation alors que seule une transition pyrochlore-fluorine est observée pour les zirconates. Dans le domaine du ralentissement électronique, les titanates s’amorphisent par un mécanisme d’impact direct et Gd₂Zr₂O ₇ subit uniquement une transition pyrochlore-fluorine. Par contre, contrairement à son comportement observé sous irradiation avec des ions de basse énergie, Nd₂Zr₂O ₇ s’amorphise avec des ions lourds de haute énergie, avec une cinétique d’endommagement complexe qui met en jeu des transitions pyrochlore-amorphe, pyrochlore-fluorine et fluorine-amorphe. La structure interne des traces induites par excitations électroniques a été étudiée par MET. Les analyses par spectroscopie Raman en coupe transverse des échantillons irradiés ont permis la détermination de l’épaisseur de la phase amorphe ainsi que du pouvoir d’arrêt électronique seuil pour la formation des traces. Les conductivités thermiques des échantillons irradiés sont plus faibles que celles obtenues dans les composés non irradiés. Enfin, l’étude de la restauration thermique des composés irradiés avec des ions lourds de haute énergie montre que les transformations de phases ainsi que les températures auxquelles se produisent ces transformations dépendent de la compositionDue to the high resistance to irradiation of some compositions, pyrochlore-type oxides with A₂B₂O ₇ structure are considered as potential matrices for the immobilization of actinides produced in nuclear reactor. In this context, the study of the structural stability after irradiation of these oxides is particularly important and should be investigated. This work aims at studying the effects of both the composition and the microstructure on the behavior of pyrochlores upon irradiation. For this purpose, four compositions of pyrochlore oxides of general formula A₂B₂O ₇ (with A = Gd, Nd and B = Ti, Zr) were first elaborated. Powders were prepared using different routes (solid state reaction, soft chemistry with Pechini process) in order to control the grain size of the ceramics densified either by conventional sintering or by SPS. The results obtained by soft chemistry are promising: fine powders (<100 nm) with the pyrochlore structure were prepared. Finally, according to the selected SPS conditions, dense ceramics were obtained with a grain size of 700 nm for Y₂Ti₂O ₇ and 300 nm for Nd₂Zr₂O ₇ , which, to our knowledge, corresponds to the smaller grain size for these compositions. Pyrochlore oxides with the compositions Gd₂Ti₂O ₇ , Y₂Ti₂O ₇ , Gd₂Zr₂O ₇ and Nd₂Zr₂O ₇ were irradiated with low energy heavy ions (4 MeV Au ions delivered by the JANNuS platform in Orsay) to determine the structural modifications produced by nuclear collisions. The same ceramics were also irradiated with swift heavy ions (hundreds of MeV on the IRRSUD beamline or of the order of GeV on the SME beamline) at the GANIL accelerator in Caen to study the effects induced by electronic excitations. The transformations induced by irradiation were characterized by XRD (in situ with the ALIX set up of the GANIL or ex situ after irradiation), scanning and transmission electron microscopy (SEM, TEM) and Raman spectroscopy. Fine absorption spectroscopy (XANES and EXAFS performed at the synchrotron Soleil) was implemented in order to characterize the local order in the amorphous phase induced by irradiation in the titanates. In the ballistic process regime, the main results are the amorphization of the titanates, while only an anion-deficient fluorite phase is formed for zirconates. The phase transformations induced by electronic excitation show that titanates become amorphous by a direct impact mechanism whereas a pyrochlore-fluorite transition occurs for Gd₂Zr₂O ₇ . However, in contrast to the behavior observed upon irradiation with low energy ions, Nd₂Zr₂O ₇ becomes amorphous upon high energy ion irradiations, with a complex damage kinetics involving pyrochlore-amorphous, pyrochlore-fluorite and fluorite-amorphous phase transitions. The internal structure of tracks induced by electronic excitation was studied by TEM. Raman spectroscopy analyses performed on cross-sectioned irradiated samples allowed the determination of the amorphous phase thickness and the electronic stopping power threshold for the formation of tracks.The thermal conductivities of the irradiated samples are lower than those obtained for unirradiated compounds. Finally, a specific study concerning the thermal recovery of irradiated pyrochlores with swift heavy ions shows that the phase transitions and the temperature at which these transformations occur depend on the composition
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Finkeldei, Sarah Charlotte [Verfasser], Dirk [Akademischer Betreuer] Bosbach, and Georg [Akademischer Betreuer] Roth. "Pyrochlore as nuclear waste form : actinide uptake and chemical stability / Sarah Charlotte Finkeldei ; Dirk Bosbach, Georg Roth." Aachen : Universitätsbibliothek der RWTH Aachen, 2015. http://d-nb.info/1129261654/34.
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Feng, Erxi Verfasser], Thomas [Akademischer Betreuer] [Brückel, and Carsten [Akademischer Betreuer] Honerkamp. "Magnetic order and excitation in frustrated pyrochlore 5d-transition metal oxides / Erxi Feng ; Thomas Brückel, Carsten Honerkamp." Aachen : Universitätsbibliothek der RWTH Aachen, 2018. http://d-nb.info/1181193109/34.
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Finkeldei, Sarah [Verfasser], Dirk [Akademischer Betreuer] Bosbach, and Georg [Akademischer Betreuer] Roth. "Pyrochlore as nuclear waste form : actinide uptake and chemical stability / Sarah Charlotte Finkeldei ; Dirk Bosbach, Georg Roth." Aachen : Universitätsbibliothek der RWTH Aachen, 2015. http://nbn-resolving.de/urn:nbn:de:hbz:82-rwth-2015-040350.
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Feng, Erxi [Verfasser], Thomas [Akademischer Betreuer] Brückel, and Carsten [Akademischer Betreuer] Honerkamp. "Magnetic order and excitation in frustrated pyrochlore 5d-transition metal oxides / Erxi Feng ; Thomas Brückel, Carsten Honerkamp." Aachen : Universitätsbibliothek der RWTH Aachen, 2018. http://nbn-resolving.de/urn:nbn:de:101:1-2019032005565476512003.
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Thuries, Fabienne. "Préparation et étude cristallographique de niobates et tantalates de plomb à structure dérivée de la phase pyrochlore." Aix-Marseille 3, 1995. http://www.theses.fr/1995AIX30016.
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Au cours de ce travail, nous avons identifie les oxydes obtenus avec les cristaux de pb#2sctao#6 (pst) et pb#2sc(nb#0#,#5ta#0#,#5)o#6 qui sont etudies pour leurs proprietes dielectriques. Ce sont des tantalates de plomb contenant une proportion en scandium tres inferieure a celle de pst et presentant une structure cubique de type pyrochlore. Dans un deuxieme temps, nous avons elabore des cristaux de niobate et tantalate de plomb, a partir des compositions: 2pbo-1nb#2o#5 et 2pbo-1ta#2o#5. Il s'avere que les phases obtenues ne sont pas stchiometriques et presentent un exces de plomb par rapport a la composition initiale. Nous avons montre qu'il n'existe pas de solution solide entre les composes pb#2m#2o#7 et pb#2#,#5m#2o#7#,#5, avec m=nb ou ta. Dans ce domaine de concentration, apparaissent en fait des phases polysomes dont la structure est constituee des deux modules: les couches octaedriques a, b et c, riches en plomb et les couches a, b et c, riches en niobium, caracteristiques de la structure cubique pyrochlore qui correspond a l'empilement aabbcc, le long de la direction 111 du cube, et a la formule chimique calculee pb#2m#2o#7. Par contre, les phases derivees ont une structure rhomboedrique, qui peut etre decrite comme un empilement de ces modules en nombre impair et variable, le long de la direction 00. 1 d'une maille hexagonale, ce qui entraine l'existence de plusieurs phases de compositions chimiques differentes, mais de rapport pb/m toujours superieur a 1. L'etude comparee des niobates et tantalates de plomb, pb#2#+#xm#2o#7#+#x avec 0
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Pors, Françoise. "Preparation et caracterisation d'oxynitrures de type perovskite, k::(2)nif::(4) et pyrochlore : etude structurale et dielectrique." Rennes 1, 1987. http://www.theses.fr/1987REN10071.
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Ce travail entre dans le cadre d'une etude systematique de la substitution de o par n dans le reseau anionique de differents types d'oxydes. Reparation des perovskites abo::(2)n ca: alcalinoterreux ou lanthanide, b: ta, nb ou ti) et abon::(2) (a: lanthanide, b: ta ou nb). Etude structurale de certains composes par diffraction de neutrons. Le type structural k::(2)nif::(4) est illustre par lu::(2)alo::(3)n (ln: la, nd, sm) et a::(2)tao::(3)n (a: sr, ba). Enfin, les phases de stoechiometrie ln::(2)ta::(2)o::(5)n::(2) constituent les premiers exemples d'oxynitrures a structure pyrochlore. Etude dielectrique
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Cheatum, Jeremiah J. "A validation of a molecular dynamics simulation in determining the thermal conductivity of a La-Zr pyrochlore." Thesis, Monterey, Calif. : Naval Postgraduate School, 2008. http://edocs.nps.edu/npspubs/scholarly/theses/2008/Dec/08Dec%5FCheatum.pdf.
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Thesis (M.S. in Mechanical Engineering)--Naval Postgraduate School, December 2008.Thesis Advisor(s): Lloyd, John R. "December 2008." Description based on title screen as viewed on January 30, 2009. Includes bibliographical references (p. 77-79). Also available in print.
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Kirk, Christopher J. "Pigments for the 21st century." Thesis, Loughborough University, 2008. https://dspace.lboro.ac.uk/2134/12528.
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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.
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Maryasin, Vladimir. "Ordre par le désordre structural et les effets du champ magnétique dans les systèmes frustrés." Thesis, Université Grenoble Alpes (ComUE), 2015. http://www.theses.fr/2015GREAY051/document.
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La compétition des interactions est une caractéristique essentielle des systèmes frustrés, elle est à l'origine d'une large dégénérescence des états fondamentaux classiques ou, obtenus par une théorie de champ moyen.Fréquemment la dégénérescence peut être levée par des fluctuations thermiques ou quantiques, ce qui constitue la base du mécanisme appelé textit{ordre par le désordre}.Les systèmes magnétiques étudiés expérimentalement contiennent une quantité inévitable de désordre structural.Dans cette thèse de doctorat, l'influence des défauts, créé par des sites inoccupés ou par un désordre des liens sur l'espace dégénéré des états fondamentaux est étudiée pour des systèmes frustrés divers.Nous avons trouvé qu'un désordre structural est, lui aussi, capable de lever systématiquement la dégénérescence; par ailleurs, la tendance est inverse par rapport au le mécanisme d'ordre par le désordre produit par les fluctuations.Pour chacun des modèles considérés, les corrections à l'énergie ont été calculées sous la forme de termes anisotropes effectifs qui agissent sur l'espace dégénéré des états fondamentaux.Ces arguments analytiques ont été confirmés par des calculs numériques que nous avons effectués par minimisation de l'énergie, ainsi que par simulation de type Monte-Carlo classique. La séquence des états ordonnés que nous avons détectée est attribuée à la compétition entre l'effet d'ordre induit par les fluctuations et celui induit par les défauts structuraux. L'effet observé peut ouvrir des possibilités supplémentaires de contrôler la structure magnétique des systèmes.Enfin, les effets d'un champ magnétique externe ont été étudiés pour le système antiferromagnétique pyrochlore pur avec anisotropie de plan facile.Nous avons observé des transitions de phases qui dépendent de l'orientation du champ et qui n'existent pas dans la description de type champ moyen du système. Elles constituent une généralisation des transitions de type spin-flop pour le cas de la symétrie discrète $mathbb{Z}_k$ brisée avec $k > 2$Competing interactions is an essential feature of frustrated systems, they stand behind the large degeneracy of classical or mean-field ground states.%produce degeneracy of classical mean-field ground states.In many cases the degeneracy can be lifted by thermal and quantum fluctuations, such mechanism is commonly called textit{order from disorder}.Experimentally studied magnetic systems inevitably contain a finite amount of structural disorder.In this work the influence of defects, namely vacancies and bond disorder, on a degenerate ground state manifold is studied for various frustrated systems.We find that quenched disorder is also capable of consistently lifting the degeneracy, moreover, it has%in a wide range of frustrated systems.%Moreover, the effect of quenched disorder leads toan opposite tendency, compared to the order by disorder mechanism, produced by fluctuations.For every considered model, analytic energy corrections are derived in the form of effective anisotropic terms, which act on the manifold of degenerate ground states.Analytical arguments are confirmed by numerical calculations, which include energy minimization and classical Monte Carlo simulations.The detected sequences of ordered states is attributed to competition of fluctuations and structural disorder.The observed effect can open additional possibilities in tuning magnetic structure of the system.Finally, the effect of external magnetic field is investigated for the pure $XY$ pyrochlore antiferromagnet.Depending on the field orientation we observe phase transitions, which do not exist within the mean-field description of the system.They are generalizations of the spin-flop transition for the case of broken discrete $mathbb{Z}_k$ symmetry with $k > 2$
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Ishida, Lina. "Antiferromagnetismo nos titanatos geometricamente frustrados Sm2Ti2O7 e Nd2Ti2O7." Universidade de São Paulo, 2019. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-01022019-104912/.
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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.
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Bespalko, Nicolas. "Préparation de nouveaux catalyseurs à base d'oxydes mixtes de lanthane et zirconium et application au réformage d'éthanol pour la production d'hydrogène." Strasbourg, 2011. http://www.theses.fr/2011STRA6129.
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Cette thèse se base sur le vaporéformage catalytique de l'éthanol pour la production d'hydrogène. Les catalyseurs employés sont des systèmes à base de métaux nobles ou métaux de transition, en présences de cations dopants et de supports oxydes dont on peut modifier les propriétés redox et acido-basiques. Dans une première étape, divers oxydes mixtes Ni-La-Zr et Co-la-Zr ont été synthétisés par différentes techniques de préparation et caractérisés par diverses techniques physico-chimiques de volume et de surface. Dans un second temps, des catalyseurs dopés à 1 % de rhodium (Rh-Ni-La-Zr et Rh-Co-La-Zr) ont été préparés et étudiés afin de déterminer l'effet catalytique de Rh dans la réaction de réformage d'éthanol au niveau de l'activité, la sélectivité en hydrogène et la désacti vati on. Pour l'évaluation de ces oxydes mixtes en réformage d'éthanol, et en accord avec l'analyse thermodynamique et la connaissance des propriétés physico-chimiques des catalyseurs, les conditions opératoires suivantes ont été choisies : température de réaction entre 500 et 650 oC, rapports molaires eau-éthanol 9 : 1 ou 6 : 1. Des tests complémentaires avec injections ponctuelles de COz ont été menés, afin d'expliciter les différences de comportement entre Ni et Co dans la distribution des produits de réaction. Les catalyseurs Ni-La-Zr et Co-la-Zr ont été caractérisés après réaction, au moyen de diverses techniques, de façon à comprendre les causes de désactivation. Globalement, les résultats de la caractérisation physico-chimique avant et après test couplés aux résultats catalytiques, ont permis de comprendre l'acte catalytique dans son ensemble: activité, désactivation et mécanisme de réactionThis thesis is based on the catalytic steam reforming of ethanol for hydrogen production. The catalysts used are systems based on noble metals or transition metal cations in the presence of dopants and oxide supports which may alter the redox properties and acid-base. As a first step, various mixed oxides Ni-La-Zr and Co-la-Zr were synthesized by different preparation techniques and characterized by various physicochemical techniques of volume and surface. In a second step catalysts doped with 1 % rhodium (Rh-Ni-La-Zr and Rh-Co-La-Zr) were prepared and studied to determine the catalytic effect of Rh in the reaction of reforming ethanol at the level of activity, selectivity and deactivation in hydrogen. For the evaluation of these mixed oxides in reforming of ethanol, and in agreement with the thermodynamic analysis and knowledge of the physicochemical properties of catalysts, the following operating conditions were chosen: reaction temperature between 500 and 650 ° C , molar ratios ofwater-ethanoI9: 1 or 6: 1. Additional tests with occasional injections OfC02 were carried out in order to explain the differences in behavior between Ni and Co in the distribution ofreaction products. The catalysts Ni-La-Zr and Co-Zr-La after reaction were characterized by means ofvarious techniques in order to understand the causes of deactivation. Overall, the results of the physico-chemical characterization before and after test-coupled catalytic results have allowed us to understand the catalytic act as a whole: activity, deactivation and reaction mechanism
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Peçanha, Antonio Viviane [Verfasser], Joachim [Akademischer Betreuer] Wuttke, Peter [Gutachter] Müller-Buschbaum, and Joachim [Gutachter] Wuttke. "Neutron Scattering Studies of Pyrochlore Quantum Magnets / Viviane Peçanha Antonio ; Gutachter: Peter Müller-Buschbaum, Joachim Wuttke ; Betreuer: Joachim Wuttke." München : Universitätsbibliothek der TU München, 2019. http://d-nb.info/119189729X/34.
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Ku, Esther M. (Esther Mei-Hua) 1973. "Synthesis, cation distribution, and disorder of fast-ion conducting pyrochlore oxides : a combined neutron and X-ray Rietveld analysis." Thesis, Massachusetts Institute of Technology, 1999. http://hdl.handle.net/1721.1/85268.
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Lutique, Stéphanie. "Etude de zirconates de structure pyrochlore en tant que matrice pour la transmutation ou le conditionnement des actinides mineurs." Paris 11, 2003. http://www.theses.fr/2003PA112080.
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La loi "Bataille" définit le cadre des études pour une gestion optimisée des déchets nucléaires, incluant la séparation des éléments les plus radiotoxiques pour les transmuter en élément moins radiotoxiques ou les stocker à long terme. Ces deux applications nécessitent l'utilisation d'une matrice répondant à plusieurs critères spécifiques. Ce travail effectué au cours de la thèse a pour but d'étudier une matrice potentielle pour la transmutation ou le stockage des actinides après leur séparation: le zirconate de type pyrochlore A2Zr2O7 (A est un lanthanide ou un actinide). Le développement d'un procédé de synthèse a conduit à la production de pastilles de zirconate de néodyme d'une densité de plus de 95 % de la densité théorique. La méthode d'infiltration de solution active dans un précurseur inactif a permis l'incorporation de plutonium et d'uranium et la production de pastilles denses de structure pyrochlore de type Nd(1,57)[Pu/U](0,43)Zr2O(7+y). A l'aide de trois techniques de calorimétrie, la capacité calorifique de zirconate de lanthanide à été déterminée dans l'intervalle de température 0,4 à 1400 K. La diffusivité thermique à été mesurée pour le zirconate de néodyme entre 400 et 1400 K et la conductivité thermique a été déduite. Une simulation numérique du comportement thermique sous irradiation d'un combustible à base de zirconate a été réalisée. La résistance à l'irradiation a été testée par implantation d'ions par accélérateur simulant les produits de fission et les noyaux de recul alpha. Enfin, après implantation d'ions d'hélium dans la matrice, le relâchement du gaz hélium a été contrôlé. Au vu de l'ensemble de ces résultats, il est apparu que les zirconates de type pyrochlore sont utilisables comme matrice pour la transmutation uniquement sous forme d'inclusions dans un composite. Une étude complémentaire du comportement de ce type de composé sous lixiviation est nécessaire pour apporter une conclusion quant à son usage comme matrice de conditionnementThe "Bataille" law gives orientations for nuclear waste management optimisation, which includes the partitioning of the most radiotoxic elements (the actinides and some fission products) in order to transmute them into less radiotoxic elements or to store for long term. For both applications a matrix with specific criteria is needed to incorporate the radionuclides. The aim of this work was the study of a potential matrix for the transmutation or the storage of actinides after their partitioning: pyrochlore zirconate, which general formula is A2Zr2O7, where A is a lanthanide or a actinide. A fabrication process was developed leading to the production of neodymium zirconate with a density higher than 95 % of the theoretical density. The method of infiltration of active solution in an inactive precursor permitted to incorporate plutonium and uranium and to produce highly dense pellet with pyrochlore structure and general formula Nd(1,57)[Pu/U](0,43)Zr2O(7+y). Using three calorimetric techniques, the lanthanide zirconate heat capacity was measured in the temperature range [0. 4 -1400 K]. The thermal diffusivity of the neodymium zirconate was determined between 400 and 1400 K and its thermal conductivity was deduced in the same temperature range, yielding to the constant value of 1,33 W. M^(-1). S^(-l). Using this value, the thermal behaviour of a zirconate based fuel was numerically simulated. The radiation resistance of the neodymium zirconate was tested using ion implanted with several energy by accelerator in order to simulate fission products and alpha recoil atoms. Finally He ions were implanted in the matrix and their release was monitored using a Knudsen cell. Basis on all those results, it appears that pyrochlore zirconates could be used as inert matrix for the transmutation only as inclusions in composites. However, a complementary study of the compound behaviour against leaching is needed before to be able to conclude concerning it use as waste form for actinides storage
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Gallagher, James C. "Synthesis and Investigation of High Quality Materials for Spintronics Applications." The Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1469141590.
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