Layered Intergrowth Phases Bi4MO8X (X=Cl, M=Ta and X=Br, M=Ta or Nb): Structural and Electrophysical Characterization

The high-temperature structural behavior of the layered intergrowth phase Bi₄TaO₈Cl, belonging to the Sillén-Aurivillius family, has been studied by powder neutron diffraction. This material is ferroelectric, space group P2₁cn, at T_C<640 K. An order-disorder transition to centrosymmetric space group Pmcn is found around 640 K, which involves disordering of TaO₆ octahedral tilts. A second phase transition, of a first-order nature, to space group P4/mmm occurs at a temperature of ∼1038 K. The crystal structures of the bromide analogs Bi₄MO₈Br (M=Nb, Ta) have also been determined at room temperature; both are isomorphous with Bi₄TaO₈Cl and exhibit maxima in dielectric constant at temperatures of approximately 588 and 450 K, respectively.     

Structural Behavior of the Four-Layer Aurivillius-Phase Ferroelectrics SrBi4Ti4O15 and Bi5Ti3FeO15

Rietveld refinement of powder neutron diffraction data has been used to study the crystal structures of the four-layer Aurivillius-phase ferroelectrics Bi₅Ti₃FeO₁₅ (at 25°C) and SrBi₄Ti₄O₁₅ (at a series of temperatures up to 800°C). At 25°C both materials adopt the polar orthorhombic space group A2₁am, in common with two-layer analogues such as SrBi₂Ta₂O₉. At temperatures well above the ferroelectric Curie temperature (i.e., at temperatures of 650°C and above, with T_c∼550°C) SrBi₄Ti₄O₁₅ transforms to the centrosymmetric tetragonal space group I4/mmm. However, there is good evidence from the raw diffraction data of a very subtle intermediate paraelectric orthorhombic phase, of Amam symmetry, in the region 550>650°C. The distortion in the ferroelectric phase can be traced to displacements of the cations in the A site of the perovskite block, with cooperative tilting of the BO₆ octahedra. The nature of the octahedral tilt system, cation disorder at the perovskite A and B sites, and the phase transition sequence in SrBi₄Ti₄O₁₅, which parallels that found in SrBi₂Ta₂O₉, are discussed.     

Adsorption and desorption behavior of plasmid DNA on ion-exchange membranes. Effect of salt valence and compaction agents

In this study, we report the effect of salt type and compaction agents on adsorption and desorption behavior of plasmid DNA on strong anion-exchange membranes. Both divalent cations and compaction agents are known to reduce the effective charge density of plasmid DNA in solution, and compaction agents decrease the radius of gyration of plasmids. Differences in the batch uptake adsorption of a 6.1 kilo base pair plasmid in solution with sodium and magnesium salts were observed at low ionic strengths. Recoveries at high salt conditions, however, were independent of the cation, and measured only 63-76%. Similarly, no improvement in recoveries were observed when using sulfate rather than chloride anions as displacers. The compaction agents, spermine and spermidine, showed no strong effect on the uptake adsorption, capacity, or recovery of three different-sized plasmids on membrane sheets. It is recommended that further efforts to improve plasmid recoveries from anion-exchange membranes focus on properties of the adsorbent surface.     

The location and ordering of fluoride ions in pure silica zeolites with framework types IFR and STF; implications for the mechanism of zeolite synthesis in fluoride media

Single-crystal X-ray diffraction studies carried out at a synchrotron radiation source have allowed the structure solution and location of fluoride ions inside as-made pure silica zeolites with the IFR and STF framework structures. The local environment of the fluoride has been identified, and unusual ordering of the fluoride ions has been discovered in both cases. The details of the crystal structures are used to suggest structural features that are important in determining the ordering of fluoride ions in zeolites. A mechanism for how the fluoride ordering occurs is suggested for IFR and STF based on the local structure of small cages that make up these zeolites, and the implications for the mechanism of crystal growth are discussed.     

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Analytical and Bioanalytical Chemistry -     

A stereoselective synthesis of 1,6-diphenyl-1,3,5-hexatrienes utilising 4,4,6-trimethyl-2-vinyl-1,3,2-dioxaborinane as a two-carbon alkenyl building block

A number of 1,6-diphenyl-1,3,5-hexatrienes of varying alkene geometries were stereoselectively prepared from just two starting materials: iodobenzene and 4,4,6-trimethyl-2-vinyl-1,3,2-dioxaborinane via a series of Heck, Suzuki-Miyaura and stereocontrolled iododeboronation reactions. These results demonstrate how 4,4,6-trimethyl-2-vinyl-1,3,2-dioxaborinane can be used as a genuine two-carbon vinyl-dianion building block in stereocontrolled polyene synthesis.     

Performance and scale-up of adsorptive membrane chromatography

Separation efficiency and scalability of Pall Corporation's new Mustang stacked membrane chromatographic devices were investigated, using both the 10-ml and l(-1) models and comparing the responses of tracer pulses obtained for conventional and reverse-flow operation. Tracers included AMP, lysozyme, and thyroglobulin, which vary in relative molecular mass from less than 1000 up to 650000. Both devices showed marked insensitivity to tracer size and flow-rate and gave sharper peaks than would have been expected from conventional 15-microm bead packings. However, reverse-flow peaks were always significantly sharper than those for conventional operation, and the differences were ascribed primarily to non-uniform header residence times. Numerical simulations of the macroscopic flow confirmed that this was indeed the case. This problem was much less pronounced for the l(-1) device so scale-up is conservative.     

Structure solution of a novel aluminium methylphosphonate using a new simulated annealing program and powder X-ray diffraction data

The structure of a novel layered aluminium methylphosphonate, formula Al2(CH3PO3)3, has been solved from laboratory X-ray powder diffraction data by simulated annealing of five independent structural sub-units, revealing a combination of four- and five-fold coordinated aluminiums within the inorganic lamellae that is unique for this kind of solid.