NMR study of paramagnetic nano-checkerboard superlattices

With the ubiquity of electronic devices, finding ways to improve quality or fabrication methods of components is an important area of study. This dissertation looks at two sets of materials that may be used to address this need. The first is a series of disordered perovskites of the form Nd⅔xLi3 xTiO3. These materials are notable for the way the lithium becomes spontaneously patterned during synthesis into square planar regions, the dimensions of which are only dependent upon the initial concentration of lithium. Through the use of point-charge calculations, the paramagnetic and first-order quadrupole interaction tensors for each of the 28 unique lithium sites of the x = 0.083 concentration were calculated and used to accurately simulate the experimental spectra. From this, it was observed that the 28 crystallographically distinct sites present in that particular concentration could be grouped into three sets based on the principal values of the paramagnetic interaction tensors. Qualitative analysis of spectra from the other concentrations suggests that this grouping holds for other concentrations, with only the relative number of sites in each group changing. Additionally, jump dynamics were incorporated into the simulations of one of the sites in order to explain the broadening that occurs at lower temperatures.;The second study included in this dissertation is focused on lithium in a pair of high-dielectric microwave ceramics, Ca(Li1/3Nb 2/3)O3 and (Ca2/3La1/3)(Li1/3 Nb2/3)O3. Experimental results are reported for the temperature-dependence of both the spin-lattice relaxation rate and the isotropic chemical shift for each material. For both samples, the isotropic shift was linear with temperature, with the isotropic shift of Ca(Li 1/3Nb2/3)O3 having a stronger temperature dependence (3.53 Hz??K-1 compared to 2.65 Hz??K -1). The spin-lattice relaxation rates of both samples follow an Arrhenius relationship with temperature, with Ca(Li1/3Nb 2/3)O3 sample having an activation energy of 5.08 kJ ?? (mol ?? K)-1 and (Ca2/3La1/3)(Li 1/3Nb2/3)O3 having an activation energy of 2.21kJ ?? (mol ?? K)-1. In addition to the lithium study, there were also spectra acquired that observed the niobium nucleus in each material, which has a noticeably more complex spectrum. For the (Ca2/3 La1/3)(Li1/3Nb2/3)O3 sample, a double-quantum satellite-transition magic angle spinning pulse sequence was used to determine the isotropic chemical shift as well as the quadrupole product of each of the five resolved sites

Local structure analysis of solid state ionic conductors, perovskite-derived structures by NMR and computational studies

The full text of this thesis is embargoed until end November 2015 for publication reasons. Supporting data is held on a separate record at https://www.repository.cam.ac.uk/handle/1810/244979In this work, local environments of ions in solid oxide fuel cell (SOFC) electrolyte materials with perovskite and perovskite-derived crystallographic structures, i.e. $Ba_{2}In_{2}O_{5}$, $Ba_{2}(In_{1-x}Ga_{x})_{2}O_{5}$ and $Ba_{2}In_{2}O_{4}(OH)_{2}$, were investigated for their high ionic (O2– and H+) mobility at elevated temperatures. Two general methods were employed in this investigation; first, computational methods, such as density functional theory (DFT), gauge including projector augmented wave (GIPAW), cluster expansion (CE) and Monte Carlo simulations (MC); second, experimental methods, such as nuclear magnetic resonance (NMR), X-ray scattering (both powder diffraction and pair distribution function (PDF) analysis) and thermo-gravimetric analysis (TGA). The parent material, $Ba_{2}In_{2}O_{5}$, has inherent oxygen vacancies which allow for fast O2– ion mobility at elevated temperatures and for hydration of the material needless of doping. We improve a previous NMR study of $Ba_{2}In_{2}O_{5}$ by Adler et al. [1], assigning all three oxygen crystallographic sites to their relevant NMR peaks and investigate the high temperature structure. We then study the iso-valent doping of Ga into the In site resulting in Ba2(In1-xGax)2O5. While Yao et al. [2] find that Ga doping levels higher than 20% form a stable cubic structure, our findings indicate that Ga doping results in a phase segregation. However our findings for quenched samples are no different than those of Yao et al. [2]. Lastly we study the hydrated form of the parent material, $Ba_{2}In_{2}O_{4}(OH)_{2}$, which has high H+ ion mobility above 180C. We observe at least three possible hydrogen sites with local environments slightly different from the previous neutron diffraction study by Jayaraman et al. [3]. In contrast to the observation by Jayaraman et al. [3] of the hydrogen presence in all O2 layers we find an alternating occupancy of hydrogens in those layers

Alteration mechanisms of spent nuclear fuel and characterization of potential uranium secondary phases.

Thesis submitted in 2019 and examined in Feb 2020.This research thesis progresses along two pathways, alteration of spent nuclear fuel and identification of said alteration. The relative abundance of uranium as an energy resource, coupled with the high costs of spent nuclear fuel reprocessing and the associated risks of nuclear proliferation make a strong case for direct disposal of SNF in deep underground geological disposal facilities. The escape of radionuclides from underground spent nuclear fuel disposal facilities will likely result from anoxic dissolution of spent nuclear fuel by intruding groundwater with potential high alpha radioactivity even after hundreds of years. Considering the lack of oxygen at repository depths of 500 m to few km below the Earth, anoxic dissolution experiments with uranium dioxide in various solid forms was conducted to investigate secondary phases formation, the escape of radioactivity in the form of dissolved uranium and electrochemistry evolution to understand the redox changes happening in the surface and solution of our experiments due to the interaction of water and spent nuclear fuel. The other research thrust in this thesis is the analysis of potential secondary phases via non-destructive scientific techniques such as scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX), electron- backscattered diffraction (EBSD), X Ray Diffraction (XRD), Raman spectroscopy with a chapter dedicated on 17O NMR. For the latter, given the long alteration timeline for uranium dioxide, it is challenging to achieve sufficient alteration products for analysis within a short 4-year PhD program. Some known uranium compounds were synthesized and enriched with 17O, an NMR active isotope with spin 5/2 for investigation of their properties. The many advantages of NMR over conventional x- ray diffraction technique render this an important chapter, especially when alteration products may not be single- phase and crystalline.Singapore Nuclear Research and Safety Initiative Cambridge Philosophical Society Wolfson College Scholarshi

52nd Rocky Mountain Conference on Analytical Chemistry

Final program, abstracts, and information about the 52nd annual meeting of the Rocky Mountain Conference on Analytical Chemistry, co-endorsed by the Colorado Section of the American Chemical Society and the Society for Applied Spectroscopy. Held in Snowmass, Colorado, August 1-5, 2010

Multinuclear Solid-State NMR Investigation of Structure, Dynamics, and Formation of Porous Materials

The work described herein demonstrates the utility of solid-state nuclear magnetic resonance (SSNMR) spectroscopy for the characterization of molecular-level structure and dynamics in porous materials, including the determination of the reaction pathways involved in the formation of porous solids made via solid-state synthetic techniques, a study of the motion of dynamic components of metal-organic frameworks (MOFs) that are prototypes for future molecular machines, and the structural characterization of a surface-supported catalyst. In Chapters 2 and 3, accelerated aging and mechanochemical reactions are used to synthesize cadmium-containing zeolitic imidazolate frameworks (ZIFs). These techniques provide a means for clean and efficient syntheses of these materials; however, little is known about the reaction kinetics and mechanisms underlying their production. First, the structure of a new cadmium-imidazolate framework (CdIF) is determined using a combination of powder X-ray diffraction (PXRD) and SSNMR, a methodology known as NMR-assisted crystallography. SSNMR experiments are also used to monitor the formation of ZIFs made using mechanochemical synthesis, providing information on the intermediates and products of the reactions. It is revealed that the initial mechanochemical ball milling provides the activation energy for the formation of ZIFs, but aging reactions within the milling jars drive the reaction to completion. As demonstrated here, milling times as short as five seconds provide enough energy for the initiation of the reactions, allowing for extremely low-energy synthesis of these materials. In Chapter 4, series of metal-organic frameworks (MOFs) with dynamic, interlocked crown ether rings are investigated to determine the factors that influence the motion of the rings. It is demonstrated that the size of the rings and the framework structure affect the motion. 13C variable temperature SSNMR is used to confirm the shuttling motion of rings between recognition sites on an axle that is incorporated into a MOF. Next, a study on a series of simple inorganic molecular rotors is described. It is shown that some of these compounds act as rotors with very low energy barriers that exhibit random rotational dynamics at temperatures below 75 K, while other structurally similar compounds do not display any motions over a wide range of temperatures. It is posited that steric and electronic effects from the coordinating ligands are responsible for the observed dynamics. 2H SSNMR is shown to be essential for classifying and understanding the dynamics of these low-energy molecular rotors Finally, 35Cl SSNMR is used to elucidate the structure of a transition-metal compound bound to the surface of a porous silica material. It is demonstrated that ultra-wideline (UW) 35Cl SSNMR spectra for transition-metal complexes can be rapidly acquired using a combination of high magnetic fields and specialized pulse sequences. These spectra allow for the differentiation of different Cl bonding environments (i.e., bridging, terminal axial, and terminal equatorial). Density functional theory (DFT) calculations and an accompany molecular-orbital analysis allow for an understanding of the origin of the observed 35Cl electric field gradient (EFG) parameters, which influence the 35Cl quadrupolar interactions. The structure of a surface-supported complex is then proposed, demonstrating the applicability of these techniques to the study of very dilute catalytic species

Solid-State NMR of Metallic and Paramagnetic Systems

Solid-state nuclear magnetic resonance (ssNMR) is a powerful element-specific technique to study local atomic environments in many different classes of materials; however, ssNMRbased methodologies have primarily focussed on diamagnetic systems without any unpaired electrons. In paramagnetic or metallic materials localised or delocalised unpaired electrons, respectively, couple with the nuclear magnetic moments and introduce significantly greater spectral broadening, often combined with very fast nuclear relaxation, so that these systems are challenging to study. However, these same hyperfine interactions can also provide important details of the electronic and magnetic structure for a sample. In this work ssNMR methodologies are developed to study different paramagnetic and metallic systems, and thereby demonstrate the information that can be obtained. These strategies include investigating the temperature dependence of the NMR spectra, to distinguish paramagnetic and metallic shifts, and exploiting differences in relaxation rates to afford spectral selectivity and extract further information. Specifically, the following studies have been performed: 1) The 17O NMR of Sm2O3, Eu2O3 and Sm/Eu-substituted CeO2, for which the lanthanide ions induce paramagnetic shifts with unusual temperature dependences due to the presence of low-lying excited electronic states. The spectra of the monoclinic polymorphs of the sesquioxides are assigned and the paramagnetic shifts of the cubic polymorphs are investigated over a wide temperature range. Different local environments in the substituted CeO2 are identified due to nearest-neighbour lanthanide ions and oxygen vacancies, and the activation energy for oxygen motion is determined from variable temperature T1 measurements. 2) The surface-selective direct 17O dynamic nuclear polarisation (DNP) NMR of CeO2 nanoparticles. In this case exogenous paramagnetic biradicals are deliberately introduced and exploited to selectively hyperpolarise the surface of CeO2, so that the first three (sub-)surface 17O environments can be identified with high specificity. Polarisation build-up curves show that this selectivity is due to faster polarisation of the surface relative to the bulk. 3) The structure and mechanism of electrochemically metallised VO2. By comparison with catalytically hydrogenated VO2, electrochemical metallisation is shown to be associated with hydrogen intercalation, and the presence of metallic and paramagnetic phases is explored with 1H, 2H, 17O and 51V NMR. By selectively deuterating the ionic liquid electrolyte, hydrogenation is then shown to arise from electrolyte breakdown, and the degree of hydrogenation and resultant phases are investigated as a function of the particle size and the temperature of electrochemical metallisation.Oppenheimer Studentshi

Atomic and Electronic Structure of Complex Metal Oxides during Electrochemical Reaction with Lithium

Lithium-ion batteries have transformed energy storage and technological applications. They stand poised to convert transportation from combustion to electric engines. The discharge/charge rate is a key parameter that determines battery power output and recharge time; typically, operation is on the timescale of hours but reducing this would improve existing applications and open up new possibilities. Conventionally, the rate at which a battery can operate has been improved by synthetic strategies to decrease the solid-state diffusion length of lithium ions by decreasing particle sizes down to the nanoscale. In this work, a different approach is taken toward next-generation high-power and fast charging lithium-ion battery electrode materials. The phenomenon of high-rate charge storage without nanostructuring is discovered in niobium oxide and the mechanism is explained in the context of the structure–property relationships of Nb2O5. Three polymorphs, T-Nb2O5, B-Nb2O5, and H-Nb2O5, take bronze-like, rutile-like, and crystallographic shear structures, respectively. The bronze and crystallographic shear compounds, with unique electrochemical properties, can be described as ordered, anion-deficient nonstoichiometric defect structures derived from ReO3. The lessons learned in niobia serve as a platform to identify other compounds with related structural motifs that apparently facilitate high-rate lithium insertion and extraction. This leads to the synthesis, characterisation, and electrochemical evaluation of the even more complicated composition–structure–property relationships in ternary TiO2–Nb2O5 and Nb2O5–WO3 phases. Advanced structural characterisation including multinuclear solid-state nuclear magnetic resonance spectroscopy, density functional theory, X-ray absorption spectroscopy, operando high-rate X-ray diffraction, and neutron diffraction is conducted throughout to understand the evolution of local and long-range atomic structure and changes in electronic states.Churchill Scholarship - Winston Churchill Foundation of the United States of America Herchel Smith Scholarship Cambridge International Trus

Probing Structure and Dynamics in Advanced Molecular Materials by Solid State Nuclear Magnetic Resonance

Probing and determining the structure and dynamics of advanced molecular materials is crucial to aid our understanding of their properties. Solid state NMR is capabale of probing short-range order and dynamics. Therefore this analytical technique (often used in conjunction with computational studies) is able to provide structural characterisation at the atomic level as well as probing local order and therefore has great potential to study these motions. In this thesis, advanced solid state NMR approaches have been used to access the temperature dependence site-selective dynamics of guest-free and -adsorbed tubular covalent cages and pillar[n]arenes, accessing understanding of their flexibility behaviours, and determine the structures of a new class of amorphous paramagnetic hybrid perovskites glasses. Firstly, 2H static NMR spectra has identified tubular covalent cages as ultra-fast molecular rotors and smart materials capable of adsorbing iodine and its release upon the application of an external stimuli. Secondly, correlation times and proton detected local field NMR experiments found that the extruding ethoxy group of perethylated pillar[n]arenes has significant dynamics when compared to the dynamics associated within the core. Using these techniques we also show the strong dipolar coupling present between para-xylene and the EtP6 host, providing insights into the guest’s location inside the host. Finally, spectral analysis of paramagnetic hybrid perovskites was completed and NMR methods were able to confirm that the materials studied melt at low temperatures and can be quenched into a glass form. It is the dynamics and flexibility of these structures that controls the selectivity of molecules in the voids located in the frameworks and hence enable them to be used for molecular separation

New Trends in Lithium Niobate

The present volume “New Trends in Lithium Niobate: From Bulk to Nanocrystals” contains the materials of a Special Issue of the MDPI journal Crystals dedicated to the memory of Prof. Dr. Ortwin F. Schirmer and provides a new synopsis of his research focusing on LiNbO3. It also includes recent developments, exemplifying the continued interest in this outstanding ferroelectric, non-linear optical and holographic crystal as a workhorse for testing and realizing new ideas and applications.This book starts with reviews on intrinsic and extrinsic crystal defects in LiNbO3 of single-crystal, thin-film or nano-powder forms, studied by various optical, magnetic resonance and nuclear methods, clarifying in particular the reasons for the suppression of anion vacancy formation upon thermal reduction, mechano-chemical processing or irradiations of various types. The reviews are followed by research papers on the experimental and theoretical investigation of small polarons, together with recent results on the properties of Li(Nb,Ta)O3 mixed crystals. Among the various contributions dealing with nonlinear optical applications, papers on device development, entangled photon pair generation and thin films on the Lithium Niobate On Insulator (LNOI) platform can also be found