I18-Microfocus Spectroscopy
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Feb 2012
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I18-Microfocus Spectroscopy
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Feb 2012
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I18-Microfocus Spectroscopy
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Abstract: Recently glasses based on gallate and aluminate networks have aroused interest in laser technology, for example, for the use as the host for .Iaser active ions. As these glasses are not good glass-formers other ingredients such as silica are normally added to improve their glass abilities. In this work I have succeeded in producing these glasses without the need for these additions. The structures of these pure rare-earth gallate and aluminate glasses made by aerodynamic levita- tion and laser heating techniques have been studied including neutron and X-ray diffraction and spectroscopy. The following results have been obtained. The structures of rare-earth gallate glasses, R2Ga6012 and R3Ga5012 where R = Pr and Nd, were studied using neutron diffrac- tion with the isomorphic substitution technique. A good agreement between the structural models from MD simulation and MD-RMC for the difference functions and the full experimental data sets was achieved. The mean Ga-O coordination number was found to be 4.1(1). The results also show a mixture of 6, 7 and 8-fold coordinated sites for the rare-earth ions with an average coordination number of 7.7(1). A more detailed study using a combination of neutron diffraction, Extended X-ray Absorption Fine Structure (EXAFS), MD simulation and MD-RMC refinement was applied to obtain the detail of the local structure of Pr3Ga5012 glasses at Pr and Ga K-edges. The nucleation and phase separation in the (Y203)x(Al203)1-x glassy systems produced by an aerodynamic levitation and laser heating was studied using micro-focus EXAFS. Turbidity was found to occur in the x < 0.25 and x > 0.35 glasses. At x < 0.25, even though no large inclusions (> 1?m) were seen in the turbid glasses, a phase separation into a polycrystalline sample of YAP (x = 0.50) and pure alumina was found. It is concluded that the turbidity in glassy samples at x ? 0.25 is due to the formation of nano meter size crystallites. For x = 0.36 and 0.375 (known as YAG), a nucleation of YAG crystals as spherical inclusions was found in a glass matrix giving rise to the turbidity in these glasses. Finally, a study of the structures of BaTiAl206 glasses was made in order to understand the processes giving rise to their unusual properties. Black and opaque, and clear and transparent BaTiAb06 glasses produced by aerodynamic levitation and laser heating by fast and slow quench rates were studied. Neutron and X-ray diffraction, MD simulation, MD-RMC refinement and X-ray absorption spectroscopy have been used and combined to determine the structure of the glass especially with regard to the coordination structure around the Ti ions. Evidence is found to show that the Ti ions occur in four fold and higher fold oxygen coordinated sites while the Al ions remain to- tally four fold coordinated. Very small differences in the structure of the two glasses are observed confirming that the opacity arises largely due to a small number of optical defects in present in the same overall glass structure.
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Feb 2012
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I18-Microfocus Spectroscopy
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Abstract: Quantitative determination of element species is crucial for understanding geological processes, particularly changing redox conditions. As Ti3+ is stable under very reducing conditions, Ti3+/Ti4+ oxybarometers are suitable for investigating early Solar System processes. The systematics of Ti3+/Ti4+ in Ti-bearing CaO-MgO-Al2O3-SiO2 (Ti-CMAS) glasses and both synthetic and meteoritic hibonite (Ca(Al,Ti,Mg)12O19) were investigated using X-ray absorption spectromicroscopy techniques. For this purpose suites of synthetic samples with 0-100% Ti3+/?Ti (where ?Ti = Ti3+ + Ti4+) were prepared in a 1 atm gas mixing furnace. The samples were characterised by electron microscopy, and when relevant, by X-ray diffraction and neutron powder diffraction (NPD). The crystal structure of hibonite has five distinct Al sites onto which Ti may substitute. Structural refinements of NPD data using the Rietveld refinement method were used to determine Ti3+ and Ti4+ site occupancy in hibonite. Ti3+ was found to occupy the M4 octahedral site, whereas Ti4+ partitions between the M2 trigonal bipyramidal site and the M4 octahedral site. For hibonite with <1 Ti per formula unit Ti4+ partitions between the M2 and M4 site in a roughly constant ratio of 1:4.3, irrespective of the Ti3+ content. Ti K-edge X-ray absorption near edge structure (XANES) spectra were collected from Ti-CMAS glasses and both synthetic and meteoritic hibonite. Ti3+/Ti4+ oxybarometers were developed using the synthetic samples, and the Ti3+/Ti4+ K-edge oxybarometer was used to quantify Ti3+/?Ti within hibonite from six different meteorites. Ti3+ hibonite was found in CAIs from the Acfer094 (unique), El Djouf001 (CR2), Murchison (CM2) and NWA1476 (R3) meteorites, whereas hibonite within CAIs from the NWA2446 (R3) and Hughes030 (R3-6) meteorites were Ti3+ poor. Furthermore, the Ti3+/?Ti varied within hibonite grains in CAIs from Acfer094 and El Djouf001. The presence of Ti3+ within hibonite in CAIs with no alteration minerals, or those that only experienced low temperature alteration, suggests they formed and/or were reprocessed under very reducing conditions (
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Jan 2012
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I18-Microfocus Spectroscopy
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Abstract: The poorly controlled disposal of chromium ore processing residue (COPR) is a globally widespread problem due to its potential to form chromium contaminated hyperalkaline (pH > 12) leachates. These highly oxidising leachates typically contain chromium in the Cr(VI) oxidation state as its chromate anion (CrO42-). This anion is highly mobile, toxic, carcinogenic, and exhibits a high degree of bioavailability. Under reducing conditions chromium exists in the non-toxic and poorly soluble Cr(III) oxidation state. Thus, the reduction of Cr(VI) to Cr(III) is often the goal of remediative strategies. In anaerobic subsurface environments where reducing conditions are established by the indigenous microbial population, chromium reduction can occur naturally. The microbial transformation of Cr(VI) to Cr(III) can be both a result of its direct use in microbial metabolism, or through its indirect reaction with microbially produced reduced species, e.g. Fe(II). This study has used a multidisciplinary approach to investigate the biogeochemical influences on the fate and stability of Cr(VI) leaching from a site of COPR in the north of England. Reducing sediments encountered directly beneath the COPR waste were found contain elevated concentrations of chromium. These sediments were shown to be able to remove aqueous Cr(VI) from solution when incubated with contaminated site groundwater in microcosm incubation experiments. This removal is likely a result of the abiotic reduction by soil associated microbially produced Fe(II), followed by precipitation as insoluble Cr(III) hydroxides. X-ray absorption spectroscopy (XAS) and electron microscopy confirms the association of chromium as Cr(III) with iron in these soils, hosted as a mixed Cr(III)-Fe(III) oxyhydroxide phase. Upon air oxidation, only minor amounts of chromium was remobilised from these sediments as Cr(VI). A diverse population of alkaliphilic microorganisms are indigenous to this horizon, capable of successful metabolism despite elevated pH values. This population was found to contain a consortium of microorganisms capable of iron reduction when incubated at pH 9 to 9.5. Microbial community analysis found taxonomic similarity to several known metal reducing alkaliphiles from the phylum Firmicutes. These results suggest that the novel action of iron reducing alkaliphiles indigenous to reducing sediments beneath COPR sites may provide zones of natural chromium attenuation via microbially mediated mechanisms of Cr(VI) transformation.
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Jan 2012
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I18-Microfocus Spectroscopy
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Diamond Proposal Number(s):
[1205]
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Nov 2011
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I18-Microfocus Spectroscopy
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Diamond Proposal Number(s):
[1125]
Abstract: There is a well-established literature indicating a relationship between iron in brain tissue and Alzheimer's disease (AD). More recently, it has become clear that AD is associated with neuroinflammatory and oxidative changes which probably result from microglial activation. In this study, we investigated the correlative changes in microglial activation, oxidative stress, and iron dysregulation in a mouse model of AD which exhibits early-stage amyloid deposition. Microfocus X-ray absorption spectroscopy analysis of intact brain tissue sections prepared from A?PP/PS1 transgenic mice revealed the presence of magnetite, a mixed-valence iron oxide, and local elevations in iron levels in tissue associated with amyloid-?-containing plaques. The evidence indicates that the expression of markers of microglial activation, CD11b and CD68, and astrocytic activation, GFAP, were increased, and were histochemically determined to be adjacent to amyloid-?-containing plaques. These findings support the contention that, in addition to glial activation and oxidative stress, iron dysregulation is an early event in AD pathology.
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Oct 2011
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I18-Microfocus Spectroscopy
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Diamond Proposal Number(s):
[1215]
Abstract: The luminescence emission of quartz is used in optically stimulated luminescence dating (OSL), however the precise origins of the emission are unclear. A suite of quartz samples were analysed using X-ray excited optical luminescence (XEOL). Radiation dose effects were observed whereby the UV emissions (3.8 and 3.4 eV) were depleted to the benefit of the red emission (1.9–2.0 eV). Samples were excited at ?7 keV. Understanding why some quartz emit light more brightly than others will increase the efficiency and precision of OSL analyses.
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Oct 2011
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I18-Microfocus Spectroscopy
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Oct 2011
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I18-Microfocus Spectroscopy
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Diamond Proposal Number(s):
[1205]
Abstract: This thesis reports a study of the sulfur problem which is the production of sulfuric acid via the oxidation of iron sulfide in the timbers of the Mary Rose a flagship of Henry VIII 's navy which sunk in 1535 and was raised in 1982. The work has involved a range of chemical and physical techniques with particular use of synchrotron sources to measure the iron and sulfur speciation in the timbers with X-ray absorption spectroscopy (XAS) measurements. XAS measurements are almost unique in providing the speciation of atoms in a sample via the XANES. They are particularly useful for sulfur, which has a wide range of oxidation states. However, the current work has shown the need to use the bulk and microfocus XAS measurements in parallel, particularly for archaeological samples. It is clear that the iron and sulfur contents and speciation can vary widely from sample to sample of the Mary Rose timbers. In the study on the effectiveness of chelating agents in the removal of iron species from the timbers care was taken, wherever possible, to ensure that the same samples and sample positions were used for the before and after treatment measurements. The nature of the iron and su~ur species is of extreme importance because it is assumed that it is Fe" that gives rise to the production of su~uric acid. The current study has shown that in the timbers that had not been PEG treated contained iron in the surface regions that was predominantly Fe", similar to the findings of other workers. Most of the work in this thesis used samples close to the surface of the timbers. It was only in samples taken deep into the timbers experiments that there were significant concentrations of Fe". A range of sulfur species were found in the samples. The predominant species were reduced sulfur species, elemental sulfur and sulfate. Very little pyrite was found -in the timbers studied, but it should be noted that these timbers had not been PEG treated. Some pyrite was found in the cell walls. The present studies were predominantly on the surface regions of the timbers and the conclusion is that the bulk of the pyrite which may have been present had oxidised in the moist, oxygen containing environment in which they had been stored after recovery from the sea bed. A key finding of the present study is the co-location of iron and sulfate in the timbers. This had been suggested but had not been experimentally verified. The production of sulfuric acid in the timbers is thought to involve the oxidation of iron sulfides in the presence of water to produce is iron sulfate and sulfuric acid. The fact that the present experiments show iron and sulfate in the same positions i'] the XANES maps strongly supports the proposed oxidation mechanism of iron sulfides. The bulk and microfocus XAS experiments show that a large fraction of the iron in the current samples was in the form of an oxide. This is most likely to be goethite (FeO(OH)). The XANES analysis and the fitting of the EXAFS are consistent with this identification. All the four chelating agents used in this work (EDTA, DTPA, ammonium citrate and calcium phytate) were effective in removing iron from the timbers. However, the more efficient are DTPA and calcium phytate in terms of amount removed at fixed molarity. For samples that had been treated with PEG the current work showed that the chelating agents were less effective. This is presumably due to the PEG blocking the penetration of the solutions of the chelating agents into the wood.
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Oct 2011
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