We are grateful to Diamond Light Source for awarding beamtime for experiments on beamline I16. We thank EPSRC (studentships to B.A.P. and A.M.R.) and Cardiff University for financial support.
Structural properties of the bromocyclohexane/thiourea
inclusion
compound have been determined using both single-crystal and powder
X-ray diffraction over a range of temperatures above and below a first-order
phase transition at 233 K in this material. The results demonstrate
marked contrasts to the phase transition behavior in the prototypical
cyclohexane/thiourea inclusion compound, demonstrating that relatively
small changes in molecular geometry (in this case bromine substitution)
can have a profound influence on the structural properties of the
low-temperature phase in such materials.
Barium cycling in the ocean is associated with a number of processes, including the production and recycling of organic matter, freshwater fluxes, and phenomena that affect alkalinity. As a result, the biogeochemical cycle of barium offers insights into past and present oceanic conditions, with barium currently used in various forms as a palaeoproxy for components of organic and inorganic carbon storage, and as a quasi-conservative water mass tracer. However, the nature of the oceanic barium cycle is not fully understood, particularly in cases where multiple processes may be interacting simultaneously with the dissolved and particulate barium pools. This is particularly the case in coastal polar regions such as the West Antarctic Peninsula, where biological drawdown and remineralisation occur in tandem with sea ice formation and melting, glacial meltwater input, and potential fluxes from shelf sediments. Here, we use a high-precision dataset of dissolved barium (Ba d) from a grid of stations adjacent to the West Antarctic Peninsula in conjunction with silicic acid (Si(OH) 4), the oxygen isotope composition of water, and salinity measurements, to determine the relative control of various coastal processes on the barium cycle throughout the water column. There is a strong correlation between Ba d and Si(OH) 4 present in deeper samples, but nevertheless persists significantly in surface waters. This indicates that the link between biogenic opal and barium is not solely due to barite precipitation and dissolution at depth, but is supplemented by an association between Ba d and diatom tests in surface waters, possibly due to barite formation within diatom-dominated phytodetritus present in the photic zone. Sea-ice meltwater appears to exert a significant secondary control on barium concentrations, likely due to non-conservative biotic or abiotic processes acting as a sink for Ba d within the sea ice itself, or sea-ice meltwater stimulating nonsiliceous productivity that acts as a Ba d sink. Meteoric water input, conversely, exerts little or no control on
While the phenomenon of birefringence is well-established in the case of visible radiation and is exploited in many fields (e.g., through the use of the polarizing optical microscope), the analogous phenomenon for X-rays has been a virtually neglected topic. Here, we demonstrate the scope and potential for exploiting X-ray birefringence to determine the orientational properties of specific types of bonds in solids. Specifically, orientational characteristics of C-Br bonds in the bromocyclohexane/thiourea inclusion compound are elucidated from X-ray birefringence measurements at energies close to the bromine K-edge, revealing inter alia the changes in the orientational distribution of the C-Br bonds associated with a low-temperature order-disorder phase transition. From fitting a theoretical model to the experimental data, reliable quantitative information on the orientational properties of the C-Br bonds is determined. The experimental strategy reported here represents the basis of a new approach for gaining insights into the orientational properties of molecules in anisotropic materials.
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