This thesis presents a study on several series of unconventional Fe-based superconductors; namely, derivatives of NaFeAs and LiFeAs, as well as molecular-intercalated FeSe. Primarily using muon spin rotation (SR), but also x-ray diffraction (XRD) and magnetic susceptibility measurements, the nature of both magnetic and superconducting phases within these systems is studied. Particular attention is focussed on how these states compete or coexist with one another. The aforementioned experimental techniques are first used to explore the phase diagram of NaFe1xCoxAs. This phase diagram includes regions of long-range antiferromagnetism and short-range order, that both coexist with superconductivity. Magnetism is gradually destroyed, primarily through a diminishment of the size of the ordered moment, as superconductivity is enhanced by Co substitution. This interplay is explored in detail. By contrast, superconductivity in LiFeAs cannot be enhanced by transition metal substitution, suggesting that it is intrinsically optimally-doped. I investigate this conclusion by studying the evolution of the penetration depth in superconducting compositions of LiFe1xCoxAs and LiFe1xNixAs, and comparing these data to those from other electron-doped systems. I also study an unusual and emergent magnetic phase in Li1yFe1+yAs. This work suggests that LiFeAs supports a superconducting phase that resembles those in other Fe-pnictides, but is uniquely close to an additional magnetic instability. I then move on to the study of a recently discovered series, based on FeSe intercalated with ammonia and various metals. I study both the penetration depth and the intrinsic magnetic phases in these systems using SR and compare them with other compounds based on FeSe. I find that these intercalated systems support spacially separated regions of dynamic magnetism and superconductivity and I discuss how much these phases depend on the precise chemical details of the intercalated layer. Finally, I return to the experimental study of NaFe1xCoxAs, extending the range of techniques employed by using high-field magnetometry and high-pressure SR. These studies reveal new features of this system that were not accessible using low-field and ambient-pressure methods. In particular, I show that the magnetic moment size in NaFeAs unexpectedly increases with pressure, suggesting that the electronic structure of this compound may be unique amongst known Fe-based superconductors.

The use of in situ X-ray diffraction (XRD) to investigate reactions involving crystalline materials is the focus of the work described in this thesis. The development of procedures for probing chemical reactions in situ, and the application of this technique for studying in detail the mechanisms and kinetics of solid-state processes, is reported. The information in <strong>Chapter One</strong> provides a background to the in situ study of chemical reactions, with specific emphasis on the application of X-ray diffraction. Three distinct families of inorganic materials are introduced, including layered double hydroxides, Aurivillius phases, and metal-organic frameworks, and the relevance of each in contemporary technologies, is discussed. <strong>Chapter Two</strong> incorporates an account of the design, construction, and development of a chemical reaction furnace, the Oxford-Diamond In Situ Cell (ODISC), for the in situ study of solid-state reactions. The capabilities of this apparatus are discussed, including the efficient and controlled heating of samples to temperatures in excess of 1000 °C, optional sample stirring, and successful incorporation of a range of different sample vessels. Details of the implementation and optimisation of this equipment for use at Beamline I12 at the Diamond Light Source are provided. The synthesis and characterisation of a new series of ternary layered double hydroxides (LDHs) with general formula [M_xM’_2–xAl₈(OH)₂₄](NO₃)4•yH₂O (M, M’ = Zn, Ni or Co), is detailed in <strong>Chapter 3</strong>. It was discovered that these materials exhibit finely tuneable cation ratios in the intralayer regions. A study of the intercalation chemistry of this family is reported, including in situ energy-dispersive and angular-dispersive X-ray diffraction experiments. The chapter concludes with details of an in situ XRD investigation into the synthesis of these materials via direct reaction of metal salts with Al(OH)₃, which was observed to proceed in four stages. <strong>Chapter Four</strong> is concerned with the molten salt synthesis and characterisation of novel cation-doped compounds with the Aurivillius structure. The limited extent of substitution on the B-sites of the parent Bi5Ti3FeO15 material was observed to be highly dependent on the nature of the di- or tri-valent substituent. The impact of varying reaction conditions, such as dwell duration and nature of the molten salt, upon pure product formation is described. A comprehensive in situ XRD investigation into the molten salt synthesis of a novel doped Aurivillius phase is detailed in <strong>Chapter Five</strong>. A discussion of the synthesis mechanism, in addition to a description of the role of the molten salt in product formation, is provided. A brief in situ XRD study of the mechanism and kinetics of crystallisation of metal-organic frameworks (MOFs) is detailed in <strong>Chapter Six</strong>. The use of ion-exchanged polymer resin beads to direct the synthesis of MOFs is probed in real time, and the route to formation is compared to that for the conventional solvothermal technique. Experimental procedures pertaining to all of the above chapters are provided in <strong>Chapter Seven</strong>. Supplementary data are included in the <strong>Appendices</strong>.

This thesis reports the synthesis and characterisation of the layered oxypnictides Sr₂MO₃FeAs (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 Sr₂MO₃FeAs (M = Sc, V, Cr). This structure is comprised of anti-PbO-type [FeAs]^- layers which alternate with insulating [Sr₂MO₃]⁺ oxide fragments that resemble a portion of the K₂NiF₄ structure. In contrast to other FeAs parent materials, no member of the Sr₂MO₃FeAs family exhibits any strong evidence for long range Fe order or a tetragonal to orthorhombic distortion upon cooling. Attempts to electron and hole dope Sr₂ScO₃FeAs into the superconducting regime have as yet been unsuccessful. Although Sr₂ScO₃FeAs shows no evidence for Fe ordering, a checkerboard arrangement of Cr³⁺ spins in the ab-plane is observed below 40 K (k = (½, ½, 0)) analogous to that seen in Pr₂CuO₄. The partial substitution of Fe²⁺ (d⁶) by Co²⁺ (d⁷) in Sr₂CrO₃Fe_1-xCo_xAs has been shown to be a fruitful strategy for electron-doping this material into the superconducting regime with T_c maximised at 18 K in Sr₂CrO₃Fe_0.92Co_0.08As. 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)). Sr₂VO₃FeAs, a rare example of an “undoped” superconductor (T_c = 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 Sr₂VO₃FeAs, however μSR and powder neutron diffraction studies suggest that this system is some way from commensurate long range order. In contrast to Sr₂CrO₃FeAs, electron-doping strategies in Sr₂VO₃FeAs have the effect of decreasing T_c and ultimately suppressing superconductivity entirely as Sr₂V_1-xTi_xO₃FeAs and Sr₂VO₃Fe_1-xCo_xAs materials are over electron-doped. Sr₂V_1-xMg_xO₃FeAs samples were also prepared, but rather than this strategy hole-doping the FeAs layer it preferentially oxidises vanadium towards V⁴⁺. This substitution also has a considerable effect on the superconducting critical temperature (T_c) which is raised as high as 31 K in Sr₂V_0.775Mg_0.225O₃FeAs. The isovalent substitution of Sr²⁺ by Ca²⁺ in Sr_2-xCa_xVO₃FeAs has been shown to strongly influence the superconducting properties of this material and a clear correlation between the evolution of T_c and the shape of the FeAs₄ 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 Mn²⁺ 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.

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.

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.

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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.