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Spin-cycloid instability as the origin of weak ferromagnetism in the disordered perovskite Bi0.8La0.2Fe0.5Mn0.5O3
No full textPowder neutron diffraction and magnetometry studies have been conducted to investigate the crystallographic and magnetic structure of Bi0.8La0.2Fe0.5Mn0.5O3. The compound stabilizes in the Imma orthorhombic crystal symmetry in the measured temperature range of 5 to 380 K, with a transition to antiferromagnetic order at TN≈240 K. The spin cycloid present for BiFeO3 is found to be absent with 50% Mn3+ cation substitution, leading to G-type antiferromagnetic order with an enhanced out-of-plane canted ferromagnetic component, evident from measurable weak-ferromagnetic hysteresis. Structural modifications do not solely explain this behavior, indicating that modified electron exchange interactions must be taken into account. A classical spin simulation was developed to investigate the effect of random substitution in a disordered pseudocubic perovskite. The calculations took into account the nearest-neighbor, next-nearest-neighbor, and Dzyaloshinskii-Moriya interactions, along with the local spin anisotropy. Using this framework to extend the established Hamiltonian model for BiFeO3, we show that only certain types of perturbations at a magnetic defect and the surrounding molecular fields trigger a simultaneous collapse of cycloidal order and the emergence of the long-range weak-ferromagnetic component. By adopting values for the Mn molecular fields appropriate for REMnO3 (RE= rare earth), simulations of BiMn0.5Fe0.5O3 exhibit the key magnetic properties of our experimental observations.© 2014, American Physical Society
Characterising terrestrial influences on Antarctic air masses using Radon-222 measurements at King George Island
Get PDFWe report on one year of high-precision direct hourly radon observations at King Sejong Station (King George Island) beginning in February 2013. Findings are compared with historic and ongoing radon measurements from other Antarctic sites. Monthly median concentrations reduced from 72 mBq m−3 in late-summer to 44 mBq m−3 in late winter and early spring. Monthly 10th percentiles, ranging from 29 to 49 mBq m−3, were typical of oceanic baseline values. Diurnal cycles were rarely evident and local influences were minor, consistent with regional radon flux estimates one tenth of the global average for ice-free land. The predominant fetch region for terrestrially influenced air masses was South America (47–53° S), with minor influences also attributed to aged Australian air masses and local sources. Plume dilution factors of 2.8–4.0 were estimated for the most terrestrially influenced (South American) air masses, and a seasonal cycle in terrestrial influence on tropospheric air descending at the pole was identified and characterised. © Author(s) 201
Microfluidic implementation of Ru-catalyzed methylation of amines using CO2 as carbon source
No full textCarbon dioxide chemistry is an area of continuing growth in recent times, due to socioeconomic and environmental reasons. Several methods have now been reported for obtaining N-methylation on primary and secondary amines directly from CO2. We have translated in two microfluidic setups (Slug Flow [SF] and Tube-in-Tube [TiT]) a ruthenium (Ru)-catalyzed process previously reported using a pressure vessel. Here, we demonstrate how the SF approach is more efficient but requires more input to reach a steady state, while the TiT system is less efficient but more tuneable.We have tested these processes on three model amines and two radiopharmaceutical precursors that are routinely used in 11C chemistry. The microfluidic processes tested are also potentially more efficient than the pressure vessel counterpart, in terms of amount of Ru catalyst needed (1% vs. 10%) and projected reaction completion time. © 2016 Akadémiai Kiadó Zrt
Radiocarbon age constraints for a Pleistocene–Holocene transition rock art style: The Northern Running Figures of the East Alligator River region, western Arnhem Land, Australia
No full textIn this article we present nine radiocarbon age determinations producing a minimum age and a minimum age range for a regionally distinct rock art style known as the Northern Running Figures from Red Lily Lagoon, western Arnhem Land Australia. These radiocarbon determinations provide age constraints for both Pleistocene and early Holocene rock art in western Arnhem Land. The radiocarbon age determinations are produced from extracting calcium oxalate contained within mineral crusts associated with the rock art. Significantly this study employs a novel separation technique, designed to effectively isolate the oxalate compounds from the mineral crust sample using chemical pre-treatment, and demonstrates significant time offsets between radiocarbon age determinations for the calcium oxalates and other carbon inclusions contained within mineral crusts. © 2016, Elsevier Ltd
Challenges in understanding the sources of bioaccumulated metals in biota inhabiting turbid river systems
No full textBioaccumulation of As, Cd, Cu, Pb and Zn by Macrobrachium prawns was observed to occur in the Strickland River downstream of a gold mine at Porgera, Papua New Guinea. This was despite the total metal concentrations of waters and sediments indicating no difference from reference sites within tributaries. To provide information on potential sources and bioavailability of metals to prawns, an extensive range of analyses were made on waters, suspended solids, deposited sediments and plant materials within the river system. Dissolved metal concentrations were mostly sub-micrograms per liter and no major differences existed in concentrations or speciation between sites within the Strickland River or its tributaries. Similarly, no differences were detected between sites for total or dilute acid-extractable metal concentrations in bed sediments and plant materials, which may be ingested by the prawns. However, the rivers in this region are highly turbid and the dilute acid-extractable cadmium and zinc concentrations in suspended solids were greater at sites in the Strickland River than at sites in tributaries. The results indicated that mine-derived inputs increased the proportion of these forms of metals or metalloids in the Strickland River. These less strongly bound metals and metalloids would be more bioavailable to the prawns via the dietary pathway. The results highlighted many of the difficulties in using routine monitoring data without information on metal speciation to describe metal uptake and predict potential effects when concentrations are low and similar to background. The study indicated that the monitoring of contaminant concentrations in organisms that integrate the exposure from multiple exposure routes and durations may often be more effective for detecting impacts than intermittent monitoring of contaminants in waters and sediments
Bog burst in the eastern Netherlands triggered by the 2.8 kyr BP climate event
No full textThe nature and cause of the so-called 2.8 kyr BP event have been a subject of much debate. Peat sequences have provided much of the evidence for this event, but the process link between climate and peatland response is not well understood. Multiproxy, high-resolution analysis of a core from Bargerveen in the eastern Netherlands based on pollen, non-pollen palynomorphs, testate amoebae and geochemistry identified an abrupt shift from relatively dry to extremely wet conditions. Radiocarbon-based wiggle-match dating (WMD) and biostratigraphy based on the pollen record show that this shift in local hydrology occurred around 2800 cal. yr BP. We interpret an erosional hiatus lasting up to 950 years immediately prior to this, as the effect of a bog burst after excessive rainfall. This phenomenon was not limited to our sampling location but occurred over a large part of the former Bargerveen. Peat at the hiatus contains microfossils that reflect temporary eutrophication as a consequence of local fires and secondary decomposition because of increased drainage after the erosion event. Our data show how detailed multiproxy analyses can elucidate the past response of peatlands to changing climate and suggest that the climatic change in northwest Europe at this time caused major non-linear disruption to these ecosystems. © 2014, © SAGE Publications
Cation antisite disorder in uranium-doped gadolinium zirconate pyrochlores
No full textThe incorporation of uranium into gadolinium zirconate (Gd2Zr2O7) is investigated by synchrotron X-ray powder diffraction and X-ray absorption near-edge structure (XANES) spectroscopy. The results suggest that the uranium cation is largely located on the pyrochlore B-site instead of the targeted A-site. Cation disorder in Gd2Zr2O7 and U-doped Gd2Zr2O7 is investigated by positron annihilation lifetime spectroscopy (PALS) which demonstrates the absence of cation vacancies in these systems. This work provides direct evidence for cation antisite (A- and B-site mixing) disorder in U-doped and off-stoichiometric Gd2Zr2O7 pyrochlore. © 2014, Elsevier B.V
Closing the data gap: Creating an open data environment
No full textPoor data management brought on by increasing volumes of complex data undermines both the integrity of the scientific process and the usefulness of datasets. Researchers should endeavour both to make their data citeable and to cite data whenever possible. The reusability of datasets is improved by community adoption of comprehensive metadata standards and public availability of reversibly reduced data. Where standards are not yet defined, as much information as possible about the experiment and samples should be preserved in datafiles written in a standard format. © 2013, Elsevier B.V
Small angle neutron scattering (SANS) studies on the structural evolution of pyromellitamide self-assembled gels
No full textThe kinetics of aggregation of two pyromellitamide gelators, tetrabutyl- (C4) and tetrahexyl-pyromellitamide (C6), in deuterated cyclohexane has been investigated by small angle neutron scattering (SANS) for up to 6 days. The purpose of this study was to improve our understanding of how self-assembled gels are formed. Short-term (< 3 h) time scales revealed multiple phases with the data for the tetrabutylpyromellitamide C4, indicating one-dimensional stacking and aggregation corresponding to a multifiber braided cluster arrangement that is about 35 Å in diameter. The corresponding tetrahexylpyromellitamide C6 data suggest that the C6 also forms one-dimensional stacks but that these aggregate to a thicker multifiber braided cluster that has a diameter of about 62 Å. Over a longer period of time, the radius, persistence length, and contour length all continue to increase in 6 days after cooling. These data suggest that structural changes in self-assembled gels occur over a period exceeding several days and that fairly subtle changes in the structure (e.g., tail-length) can influence the packing of molecules in self-assembled gels on the single-to-few fiber bundle stage © 2014, American Chemical Society
Relating urban airborne particle concentrations to shipping using carbon based elemental emission ratios
No full textThis study demonstrates a novel method for testing the hypothesis that variations in primary and secondary particle number concentration (PNC) in urban air are related to residual fuel oil combustion at a coastal port lying 30 km upwind, by examining the correlation between PNC and airborne particle composition signatures chosen for their sensitivity to the elemental contaminants present in residual fuel oil. Residual fuel oil combustion indicators were chosen by comparing the sensitivity of a range of concentration ratios to airborne emissions originating from the port. The most responsive were combinations of vanadium and sulphur concentration ([S], [V]) expressed as ratios with respect to black carbon concentration ([BC]). These correlated significantly with ship activity at the port and with the fraction of time during which the wind blew from the port.
The average [V] when the wind was predominantly from the port was 0.52 ng m−3 (87%) higher than the average for all wind directions and 0.83 ng m−3 (280%) higher than that for the lowest vanadium yielding wind direction considered to approximate the natural background. Shipping was found to be the main source of V impacting urban air quality in Brisbane.
However, contrary to the stated hypothesis, increases in PNC related measures did not correlate with ship emission indicators or ship traffic. Hence at this site ship emissions were not found to be a major contributor to PNC compared to other fossil fuel combustion sources such as road traffic, airport and refinery emissions. © 2014, Elsevier Ltd