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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
51

Structure, properties and chemistry of layered oxypnictides

Corkett, Alex J. January 2012 (has links)
This thesis reports the synthesis and characterisation of the layered oxypnictides Sr<sub>2</sub>MO<sub>3</sub>FeAs (M = Sc, V and Cr) and CeOMnAs. In these materials the choice of transition metal cation at the tetrahedral site in the arsenide layer chiefly dictates the physical properties that are observed. The bulk of this work has focussed on the development of a new family of iron arsenide superconductor with the general formula Sr<sub>2</sub>MO<sub>3</sub>FeAs (M = Sc, V, Cr). This structure is comprised of anti-PbO-type [FeAs]<sup>-</sup> layers which alternate with insulating [Sr<sub>2</sub>MO<sub>3</sub>]<sup>+</sup> oxide fragments that resemble a portion of the K<sub>2</sub>NiF<sub>4</sub> structure. In contrast to other FeAs parent materials, no member of the Sr<sub>2</sub>MO<sub>3</sub>FeAs family exhibits any strong evidence for long range Fe order or a tetragonal to orthorhombic distortion upon cooling. Attempts to electron and hole dope Sr<sub>2</sub>ScO<sub>3</sub>FeAs into the superconducting regime have as yet been unsuccessful. Although Sr<sub>2</sub>ScO<sub>3</sub>FeAs shows no evidence for Fe ordering, a checkerboard arrangement of Cr<sup>3+</sup> spins in the ab-plane is observed below 40 K (k = (½, ½, 0)) analogous to that seen in Pr<sub>2</sub>CuO<sub>4</sub>. The partial substitution of Fe<sup>2+</sup> (d<sup>6</sup>) by Co<sup>2+</sup> (d<sup>7</sup>) in Sr<sub>2</sub>CrO<sub>3</sub>Fe<sub>1-x</sub>Co<sub>x</sub>As has been shown to be a fruitful strategy for electron-doping this material into the superconducting regime with T<sub>c</sub> maximised at 18 K in Sr<sub>2</sub>CrO<sub>3</sub>Fe<sub>0.92</sub>Co<sub>0.08</sub>As. It is also established that this substitution influences the ordering on the Cr sub-lattice with a doubling in the size of the magnetic cell along the c axis (k = (½, ½, ½)). Sr<sub>2</sub>VO<sub>3</sub>FeAs, a rare example of an “undoped” superconductor (T<sub>c</sub> = 25 K), is shown to be electron-doped by mixed valence vanadium +3.13(5). Magnetometry measurements also reveal a series of magnetic transitions in Sr<sub>2</sub>VO<sub>3</sub>FeAs, however μSR and powder neutron diffraction studies suggest that this system is some way from commensurate long range order. In contrast to Sr<sub>2</sub>CrO<sub>3</sub>FeAs, electron-doping strategies in Sr<sub>2</sub>VO<sub>3</sub>FeAs have the effect of decreasing T<sub>c</sub> and ultimately suppressing superconductivity entirely as Sr<sub>2</sub>V<sub>1-x</sub>Ti<sub>x</sub>O<sub>3</sub>FeAs and Sr<sub>2</sub>VO<sub>3</sub>Fe<sub>1-x</sub>Co<sub>x</sub>As materials are over electron-doped. Sr<sub>2</sub>V<sub>1-x</sub>Mg<sub>x</sub>O<sub>3</sub>FeAs samples were also prepared, but rather than this strategy hole-doping the FeAs layer it preferentially oxidises vanadium towards V<sup>4+</sup>. This substitution also has a considerable effect on the superconducting critical temperature (T<sub>c</sub>) which is raised as high as 31 K in Sr<sub>2</sub>V<sub>0.775</sub>Mg<sub>0.225</sub>O<sub>3</sub>FeAs. The isovalent substitution of Sr<sup>2+</sup> by Ca<sup>2+</sup> in Sr<sub>2-x</sub>Ca<sub>x</sub>VO<sub>3</sub>FeAs has been shown to strongly influence the superconducting properties of this material and a clear correlation between the evolution of T<sub>c</sub> and the shape of the FeAs<sub>4</sub> tetrahedron has been established. These results demonstrate that superconductivity in iron-based superconductors is extremely sensitive to both electron count and the crystal structure. Finally, investigations into the manganese oxide arsenide CeOMnAs reveal room temperature ordering of Mn<sup>2+</sup> spins and a spin reorientation transition of Mn moments at 36 K. This transition is concomitant with Ce ordering and an apparent weak structural distortion, demonstrating that f electrons are able to dictate the orientation of Mn moments.
52

Atomic Layer Deposition Synthesis and Photoelectrochemical Charge Behavior in Tungsten, Iron, and Titanium Oxide Heterostructures

Sheehan, Stafford Wheeler January 2011 (has links)
Thesis advisor: Dunwei Wang / This thesis explores new approaches to synthesizing and understanding photoanodes for water splitting. By tuning materials' mophology on the nanoscale, their ability to absorb light energy and efficiently convert it in to chemical energy is improved. This is evident by an increase in photocatalytic efficiency and can be demonstrated with visible light sensitive catalysts. Production of these materials involved the development of alternative synthesis routes for traditional water splitting catalysts. Our hypothesis is further supported by probing charge dynamics using microwave reflectivity measurements, which show that the lifetime of charges in these new nanostructures is optimized. / Thesis (BS) — Boston College, 2011. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Chemistry Honors Program. / Discipline: Chemistry.
53

Organic fluorine in solid-state [2+2] photocycloadditions

Sinnwell, Michael Alan 01 January 2017 (has links)
The use of principles of supramolecular chemistry to direct reactions in the crystalline state has emerged as a reliable means to facilitate highly selective reactions in a solvent-free environment. In this context, we have utilized the fixed spatial arrangements of molecules in a crystal lattice to promote intermolecular [2+2] photodimerizations. We have shown how principles of supramolecular chemistry can be used to preorganize olefins to undergo intermolecular [2+2] photodimerizations in the solid state. Explorations into fluorinated and partially fluorinated olefinic substrates has allowed for development of underutilized supramolecular interactions to direct such chemical reactions. We reported interactions between fluorinated molecules in inorganic complexes and organic solids that supported intermolecular [2+2] photodimerizations as well as cross-reactions between olefinic species. Analogous to the hydrogen bond templates commonly utilized by our group to engineer reactive solids, we expanded the methodology by developing templates that direct reactions via halogen bonding. We report on the use of a halogen-bond-acceptor template to direct the formation of a small molecule lined with I- and F-atoms. We demonstrate the olefins in the solid undergo a [2+2] photodimerization with UV radiation in a stereoselective and quantitative manner. Our work provides the first use of halogen bonds to direct the formation of a small-molecule lined with halogen atoms in a solid. We reveal the halogenated product provides access to a unique family of host-guest materials that exhibit an extremely rare form of self-inclusion. We describe the ability of halogen bonds to undergo bending deformations. There are currently great interests in halogen bonds as related to applications to organic synthesis, supramolecular chemistry, and materials science. While halogen bonds are often compared to hydrogen bonds, the wealth of knowledge concerning the fundamentals and behavior of halogen bonds has admittedly experienced less time to develop. In our work on single-crystal reactions and transformations, we have discovered the ability of halogen bonds based on rigid interacting partners to bend and deform. We provide a quantitative comparison of the bending to the literature to demonstrate how the geometric landscape of the bending compares to known data. We report a single-crystal-to-single-crystal transformation of a cocrystalline material involving a fluorinated, olefinic compound. The olefins adopt a well-defined, reactive geometry mediated by halogen-bonding templates. We exploit metal-organic complexes to fix spatial arrangements of molecules in crystal lattices to promote intermolecular [2+2] photodimerizations. Perfluorophenyl-perfluorophenyl (C6F5···C6F5) interactions have recently emerged as a means to control supramolecular architectures and frameworks. However, it was not clear whether perfluorophenyl-perfluorophenyl interactions can be integrated in [2+2] photodimerizations in solids. We describe the significant effect fluorine exercised on the structure and photoreactivity of an octafluoro stilbene derivative in the solid state. Specifically, we report the olefin, trans-1,2-bis(2,3,5,6-tetrafluorophenyl)ethylene, self-assembles via C-H···F and perfluorophenyl-perfluorophenyl interactions for a stereoselective and quantitative topochemical photodimerization. Lastly, principles of crystal engineering and supramolecular chemistry are exploited to design and achieve a supramolecular protecting group strategy for the solid-state synthesis of a cyclobutane—a cyclobutane that supports a rare hydrogen-bonded self-catenating framework. The application of a hydrogen bonding coformer to separate a trans-isomer of a pyrimidyl olefin directly from a Wittig reaction is demonstrated. The coformer supports a head-to-head [2+2] photodimerization in the solid state. The work demonstrated here describes our explorations into the supramolecular interactions of organic fluorine, and how such interactions can be used to facilitate the synthesis of small molecules via [2+2] photodimerizations in solids—alone and in accompaniment of other crystal engineering strategies.
54

A study of plastic crystals as novel solid state electrolytes

Huang, Junhua, 1973- January 2003 (has links)
Abstract not available
55

5,7,12,14-tetramethyldibenzo[b,i]-1,4,8,11-tetraazacyclotetradecine-nickel(II)ester derivatives and supramolecular complexes with ionic substrates /

Malic, Nino,1974- January 2002 (has links)
Abstract not available
56

Photophysical properties of europium and terbium benzoate complexes in the solid state : interrelation between composition, structure and optophysical properties

Hilder, Matthias January 2004 (has links)
Abstract not available
57

5,7,12,14-tetramethyldibenzo[b,i]-1,4,8,11-tetraazacyclotetradecine-nickel(II) : ester derivatives and supramolecular complexes with ionic substrates

Malic, Nino, 1974- January 2002 (has links)
Abstract not available
58

Structural Studies of Lanthanide Double Perovskites

Saines, Paul James January 2008 (has links)
Doctor of Philosophy(PhD) / This project focuses on the examination of the structures of lanthanide containing double perovskites of the type Ba2LnB'O6-d (Ln = lanthanide or Y3+ and B' = Nb5+, Ta5+, Sb5+ and/or Sn4+) using synchrotron X-ray and neutron powder diffraction. The first part of this project examined the relative stability of R3 rhombohedral and I4/m tetragonal structures as the intermediate phase adopted by the series Ba2LnB'O6 (Ln = lanthanide (III) or Y3+ and B' = Nb5+, Ta5+ or Sb5+). It was found that I4/m tetragonal symmetry was favoured when B' was a transition metal with a small number of d electrons, such as Nb5+ or Ta5+. This is due to the presence of p-bonding in these compounds. In the Ba2LnNbO6 and Ba2LnTaO6 series R3 rhombohedral symmetry was, however, favoured over I4/m tetragonal symmetry when Ln = La3+ or Pr3+ due to the larger ionic radius of these cations. The incompatibility of the d0 and d10 B'-site cations in this family of compounds was indicated by significant regions of phase segregation in the two series Ba2Eu1-xPrxNb1-xSbxO6 and Ba2NdNb1-xSbxO6. In the second part of this project the compounds in the series Ba2LnSnxB'1-xO6-d (Ln = Pr, Nd or Tb and B' = Nb5+ or Sb5+) were examined to understand the relative stability of oxygen vacancies in these materials compared to the oxidation of the lanthanide cations and to determine if any oxygen vacancy ordering occurred. It was found, using a combination of structural characterisation, X ray Absorption Near Edge Structure and Ultra-Violet, Visible and Near Infrared spectroscopies, that with Ln = Pr or Tb increased Sn4+ doping results in a change in the oxidation state of the Ln3+ cations to Ln4+. This leads to those series containing little or no oxygen vacancies. A loss of B site cation ordering was found to accompany this oxidation state change and phase segregation was found to occur in the Ba2PrSnxSb1-xO6-d series most likely due to the Pr3+ and Pr4+ cations segregating into different phases. The Nd3+ cations in the series Ba2NdSnxSb1-xO6-d, however, can not oxidise to the tetravalent state so the number of oxygen vacancies rises with increasing x. It was found that oxygen vacancies concentrate onto the axial site of the compounds with x = 0.6 and 0.8 at ambient temperature. In Ba2Sn0.6Sb0.4O5.7 the oxygen vacancies were found to change to concentrating on the equatorial site at higher temperatures and it is suggested that this oxygen vacancy ordering plays a role in the adoption of I2/m monoclinic symmetry.
59

Through-bond correlation methods for assigning protein resonances with solid-state NMR spectroscopy

Chen, Lingling. January 2008 (has links)
Thesis (Ph. D.)--University of California, Riverside, 2008. / Includes abstract. Available via ProQuest Digital Dissertations. Title from first page of PDF file (viewed March 10, 2010). Includes bibliographical references. Also issued in print.
60

Replacing hydrogen bonds with coordinate covalent bonds in coordination networks

Rodger, Colin S. January 1900 (has links)
Title from title page of PDF (University of Missouri--St. Louis, viewed Mar. 3, 2010). Includes bibliographical references.

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