Crystal structures of Ln4Ni3O8 (Ln = La, Nd) triple layer T'-type nickelates

Single-phase Ln4Ni3O8 (Ln = La, Nd) nickelates were synthesized and their crystal structures were determined by Rietveld refinement of powder neutron diffraction data. The crystal structures of these mixed-valent Ni1+/Ni2+ phases belong to the T'-type and are built by intergrowth of LnO2 fluorite layers with triple NiO2 infinite-layer structural blocks. The major driving force of transformation of the LnO rock-salt block of the parent Ln4Ni3O10-delta Ruddlesden-Popper phases to the fluorite arrangement in the reduced Ln4Ni3O8 phases is attributed to internal structural stress. This transformation allows longer Ni-O bonds in Ln4Ni3O8 without overstretching of the Ln-O bonds, especially in the equatorial plane. The observed displacement of Ni atoms from the outer NiO2 planes toward the Ni atom of the central NiO2 plane in Ln4Ni3O8 is ascribed to large electrostatic repulsion from the fluorite part of the structure. X-ray absorption spectra near the K-edge of Ni suggest that the charge density on the nickel ion is similar for all members of the T'-type Lnn+1NinO2n+2 homologous series, which correlates with nearly constant Ni-O bond lengths observed in all the reduced nickelates. This suggests that the formal changes in the valence state of Ni affect the covalency of the Ni-O bond.     

Synthetic entries to substituted bicyclic pyridones

The synthesis of 6,6- and 5,6-bicyclic pyridone scaffolds has been completed using (i) an intramolecular Mitsunobu reaction and/or (ii) hydrolysis of a bicyclic pyridinium salt intermediate. Regioselective functionalization of the pyridone ring has been achieved via either direct lithiation or use of the "halogen dance" reaction. Suzuki coupling then allows introduction of aryl units at C(7)/C(9) or C(8) onto the bicyclic pyridone scaffold at either an early or late stage in the synthetic sequence. Suzuki couplings involving iodopyridinium intermediates are particularly effective.     

'One-pot' Hammett plots: a general method for the rapid acquisition of relative rate data

A general 'one-pot' method for determining relative rates of reaction in complex mixtures has been established using free energy relationships to demonstrate its utility. Competition experiments involving as many as seven species gave relative rate constants that are in good agreement to those obtained from individual kinetic analyses.     

The influence of flow acceleration on tidal stream turbine wake dynamics: A numerical study using a coupled BEM–CFD model

Studies of tidal stream turbine performance and of wake development are often conducted in tow-tanks or in regulated flumes with uniform flows across the turbine. Whilst such studies can be very useful, it is questionable as to what extent the results would differ if the flows were more complex in nature, for instance if the flows were unsteady or non-uniform or even both. This study aims to explore whether the results would be affected once we move away from the uniform flow scenario. A numerical modelling study is presented in which tidal stream turbine performance and wake development in non-uniform flow conditions are assessed. The model implements the Blade Element Momentum method for characterising turbine rotor source terms which are used within a computational fluid dynamics model for predicting the interaction between the turbines and the surrounding flow. The model is applied to a rectangular domain and a range of slopes are implemented for the water surface to instigate an increase in flow velocity along the domain. Within an accelerated flow domain wake recovery occurred more rapidly although rotor performance was not affected.     

UNISOR on-line nuclear orientation facility (UNISOR/NOF)

The UNISOR on-line nuclear orientation facility (UNISOR/NOF) consists of a₃He−₄He dilution refrigerator on line to the isotope separator. Nuclei are implanted directly into a target foil which is soldered to the bottom accessed cold finger of the refrigerator. A 1.5 T superconducting magnet polarizes the ferromagnetic target foils and determines the axis of symmetry. Up to eight gamma detectors can be positioned around the refrigerator, each 9 cm from the target. A unique feature of this system is that the k=4 term in the directional distribution function can be directly and unambigously deduced so that a single solution for the mixing ratio can be found. The first on-line experiment at this facility reported here was a study of the decay of the¹⁹¹Hg and¹⁹³Hg isotopes.     

Determination of quinolones and fluoroquinolones in fish tissue and seafood by high-performance liquid chromatography with electrospray ionisation tandem mass spectrometric detection

A reversed-phase high-performance liquid chromatographic method with tandem mass-spectrometric detection was developed and validated for the simultaneous analysis of eight quinolones and fluoroquinolones (oxolinic acid, flumequine, piromidic acid, enrofloxacin, ciprofloxacin, danofloxacin, sarafloxacin and orbifloxacin) in trout tissue, prawns and abalone. The analytes were extracted from homogenised tissue using acetonitrile and the extracts subjected to an automated two-stage solid-phase extraction process involving polymeric reversed-phase and anion-exchange cartridges. Good recoveries were obtained for all analytes and the limit of quantification was 5 microg/kg (10 microg/kg for ciprofloxacin). The limit of detection was 1-3 microg/kg, depending on the analyte and matrix. Confirmation of the identity of a residue was achieved by further tandem mass-spectrometric analysis. A procedure for estimating the uncertainty associated with the measurement is presented.     

Calculation of peak-to-total ratios for high purity germanium detectors using Monte-Carlo modeling

Summary In order to estimate by calculation the magnitude of the true coincidence summing losses that may be affecting the observed gamma-ray spectrum of a given nuclide, measured using a spectrometer, knowledge of the total detection efficiencies at the gamma-ray energies within the cascades is essential. The total efficiency can be determined from the full energy peak efficiency, provided the peak-to-total ratio is known. For a given high purity germanium (HPGe) detector, one can establish an intrinsic peak-to-total (P/T) efficiency curve using a set of measurements performed with “single” (ideally monoenergetic) gamma-emitting nuclides (e.g., ²⁴¹Am, ¹⁰⁹Cd, ⁵⁷Co, ¹¹³Sn, ¹³⁷Cs, ⁶⁵Zn). Some of these nuclides are short lived and so have to be replaced periodically. Moreover, the presence of low energy gamma-rays and X-rays in most of the decay schemes complicate the empirical determination of the P/T ratios. This problem is especially severe if measurements are made using HPGe detectors that have a very thin dead layer. The problems posed by low energy gamma-rays and X-rays can be avoided by using absorbers, but then one has to be careful not to perturb the intrinsic value of the P/T ratio being sought. This paper addresses these problems. Measurement related limitations are avoided if one can use a computational technique instead. In the work presented here, the feasibility of using a Monte-Carlo based technique to determine the P/T ratios at a wide range of energies (60 keV to 2000 keV) is explored. The Monte-Carlo code MCNP (version 4B) is used to simulate gamma-ray spectra from various nuclides. Measured P/T ratios are compared to calculated ratios for several HPGe detectors to demonstrate the generality of the approach. Reasons for observed disagreement between the two are discussed.     

Low‐Temperature Cationic Rearrangement in a Bulk Metal Oxide

Cationic rearrangement is a compelling strategy for producing desirable physical properties by atomic‐scale manipulation. However, activating ionic diffusion typically requires high temperature, and in some cases also high pressure in bulk oxide materials. Herein, we present the cationic rearrangement in bulk Mn₂FeMoO₆ at unparalleled low temperatures of 150–300 ^oC. The irreversible ionic motion at ambient pressure, as evidenced by real‐time powder synchrotron X‐ray and neutron diffraction, and second harmonic generation, leads to a transition from a Ni₃TeO₆–type to an ordered‐ilmenite structure, and dramatic changes of the electrical and magnetic properties. This work demonstrates a remarkable cationic rearrangement, with corresponding large changes in the physical properties in a bulk oxide at unprecedented low temperatures.     

Comparison of list mode data acquisition and shift register measurements using the large epithermal neutron multiplicity counter (LEMC)

Neutron multiplicity analysis has been a valuable technique for safeguards measurements of plutonium oxide and mixed oxides. Historically, most of these measurements have been performed using shift register based electronics. The shift register data acquisition lacks certain flexibility because the basic coincidence parameters (e.g., pre-delay, gate width, and long delay) must be fixed prior to the start of the measurement and the values may potentially, therefore, be sub optimal. List mode or time stamped data acquisition records the arrival time of each pulse thereby preserving the history of the pulse stream and allowing analysis and reanalysis using software analogs to the shift register circuit with adjustable parameters. Until recently, the data rates encountered in the assay of modest amounts of plutonium using efficient multiplicity counters were beyond the capacity of readily available personal computers. The calibration of the large epithermal neutron multiplicity counter (LEMC) for assay of plutonium scrap materials is used as a vehicle to compare the performance of the multiplicity shift register and a commercially available list mode acquisition module.