Resolving the different dynamics of the fluorine sublattices in the anionic conductor BaSnF(4) by using high-resolution MAS NMR techniques

19F and (119)Sn MAS NMR spectroscopy have been used to investigate the fluoride ion conductor, BaSnF(4), a member of the MSnF(4) family of fluorite-related anionic conductors containing double layers of Sn(2+) and M(2+) cations. Two fluorine sublattices were observed by (19)F MAS NMR, which could be assigned to specific sites in the lattice. The first sublattice is due to fluorine atoms located in Ba(2+) double layers and is rigid on the MAS NMR time scale at room temperature. The second sublattice comprises the fluoride ions between the Ba(2+) and Sn(2+) layers, and the few fluorine atoms that inhabit the Sn(2+)-Sn(2+) double layers. These ions are in rapid exchange with each other, and an extremely short correlation time tau(C) for the motion of these ions of <3 x 10(-)(5) s is obtained at -100 degrees C. T(1) measurements indicate that tau(C) approaches 10(-)(8) s at room temperature. (19)F-to-(119)Sn cross-polarization (CP) experiments confirmed the assignments of the resonances, and that the fluorine atoms located next to the tin atoms are extremely mobile at room temperature (and thus do not contribute to the CP process). Two-dimensional (19)F exchange experiments showed that exchange between the rigid and mobile lattice does occur, but at a much slower rate (tau(C) approximately 10 ms at 250 degrees C). Low-temperature (19)F MAS and (19)F-to-(119)Sn CP NMR spectra demonstrate that the motion of the fluoride ions has almost completely frozen out by -150 degrees C. The results are consistent with rapid two-dimensional (anisotropic) conductivity involving the fluoride ions between the Ba and Sn layers. Conductivity in three dimensions requires hops between the ions in the BaF(2)-like layers and the mobile ions. This process does occur, but with exchange rates that are at least 6-7 orders of magnitude slower.     

Preparation and chain extension of C100dicarboxylic acid

The selective degradation of polyethylene crystals with fuming nitric acid has been used to prepare a C₁₀₀ chain with terminal functional groups. After treating this product with concentrated sulphuric acid, it is shown by average molecular weight, titration and infra-red spectroscopy measurements that each molecule contains two carboxylic acid groups. This C₁₀₀ dicarboxylic acid is chain extended with several low molecular weight difunctional coupling agents. Block copolymers of this acid and poly(propylene glycol) have been prepared.     

Effect of pressure on coupled electronic ground and excited states determined from luminescence spectra of trans-dioxorhenium(V) complexes

Pressure-dependent luminescence spectra of trans-dioxo complexes of rhenium(V) with ancillary ethylenediamine ligands exhibit resolved vibronic structure in the O=Re=O symmetric stretching mode at room temperature. The intensity distribution within the vibronic progression changes with pressure, leading to band shapes that are also pressure-dependent. These spectroscopic features arise from coupled electronic states and depend on the energy differences between ground and excited states, which vary by 2500 cm(-1) for the three complexes with ethylenediamine, tetramethylethylenediamine, and tetraethylethylenediamine ancillary ligands. We describe the pressure-dependent vibronic structure and band shapes with anharmonic adiabatic potential energy surfaces for the ground states of all complexes. The calculated spectra reveal the pressure dependence of the energies of electronic origins, luminescence band maximums, offsets between ground- and emitting-state potential minimums, and vibrational frequencies. The largest pressure effects are observed where the coupled electronic states are close in energy.     

An X-ray and Mössbauer study of the FeTi2O5Ti3O5 system

Mössbauer spectra and X-ray diffraction data have been recorded for compounds in the solid solution series Fe_xTi_3?xO₅, 0 ? x ? 1. Compounds with x > 0.35 have the orthorhombic pseudobrookite structure, and those with x < 0.35 a monoclinic distortion of it. Ferrous ions in the fourfold and eightfold sites give rise to separate quadrupole doublets in the Mössbauer spectra, and thus it has been possible to determine the site occupancy as a function of composition by a least-square analysis of the spectra. The Mössbauer method has also proved to be a sensitive tool for the investigation of the orthorhombic-monoclinic distortion.     

Magnetism and structural chemistry of the n=2 Ruddlesden-Popper phase La3LiMnO7

Polycrystalline samples of the n=2 Ruddlesden-Popper phase La₃LiMnO₇ have been prepared and characterized. X-ray and neutron diffraction suggest that the structure is tetragonal with a disordered distribution of Li and Mn cations over the octahedral sites, but ⁶Li MAS NMR shows that the Li and Mn are 1:1 ordered locally. Electron microscopy shows that the stacking of the cation-ordered, perovskite-like bilayers along the crystallographic z-axis is disordered on the distance scale sampled by X-ray and neutron diffraction. Magnetometry data and neutron diffraction data collected at 2 K together suggest that the Mn cations within each structural domain order antiferromagnetically at 14 K, but that the disorder along z prevents the establishment of long-range magnetic order.     

Catalytic oxidative degradation of polyethylene crystals

Linear polyethylene crystals have been degraded with oxygen in the presence of a catalyst composed of cobaltous and bromide ions at temperatures between 60 and 130°. The number of carbonyl groups introduced into polyethylene increases as the oxidation temperature is raised. Infra-red spectroscopy and molecular weight evidence suggests that ketone groups are formed by a non-chain scission reaction, whereas aldehyde and carboxylic acid groups result from chain scission reactions. Molecular weight distributions of the degradation products show that reactions occur selectively in the amorphous regions at the crystal surfaces, when oxidations are performed below 110°. At higher temperatures, the chains are degraded randomly. The carbonyl groups introduced into polyethylene were oxidized to carboxylic acid groups with concentrated (～ 70 per cent) nitric acid. The results demonstrate that this two-stage process yields a polydisperse long-chain linear paraffin with carboxylic acid end-groups, when the catalytic oxidations are performed between 115 and 130°.     

A new octahedral tilt system in the perovskite phase Ca3Nb2O8

The perovskite-related phase Ca₃Nb₂O₈, when grown as single crystals from a calcium vanadate flux, incorporates a small amount of vanadium from the flux to form the composition Ca₃Nb_2−xV_xO₈ with x=0.025. The crystals have pseudo-cubic symmetry with a=6×a_c(perovskite). The actual symmetry is rhombohedral, space group R3, with a_h=16.910(1) Å, c_h=41.500(2) Å. The structure was solved using a combination of single-crystal methods together with constrained refinements of powder X-ray and neutron powder data. The unit-cell composition is [Ca₁₃₈□₂₄]_A [Ca₄₂Nb₁₁₇V₃]_B[O₄₈₀□₆], with vacancies in both the anion sites and A-cation sites. The Ca and Nb atoms are fully ordered in the B-sites such that (001) layers containing only Nb-centered octahedra alternate with layers containing both Nb-centered and Ca-centered octahedra. At the origin B-site, ordered oxygen vacancies result in the octahedron being transformed to a tetrahedron, which, in the single crystals, is occupied by vanadium. The structure displays a new type of octahedral tilt system in which 3×3×3 blocks of (a⁺a⁺a⁺) tilts are periodically twinned on the pseudo-cubic {1 0 0}_c planes.     

Structural relations between weberite and zirconolite polytypes—refinements of doped 3T and 4M Ca2Ta2O7 and 3T CaZrTi2O7

New weberite-type Ca₂Ta₂O₇ and zirconolite-type CaZrTi₂O₇ polytypes have been prepared by doping with Nd/Zr and Th/Al, respectively, and their structures have been refined using single-crystal X-ray diffraction intensity data. The 3T zirconolite polytype, Ca_0.8Ti_1.35Zr_1.3Th_0.15Al_0.4O₇, has a=7.228(1), c=16.805(1) Å. The 3T weberite-type polytype, Ca_1.92Ta_1.92Nd_0.08Zr_0.08O₇, has a=7.356(1), c=18.116(1) Å. Both 3T polytypes have space group P3₁21, Z=6. The 4M Ca₂Ta₂O₇ polytype has the same composition, from electron microprobe analyses, as the 3T polytype, and has cell parameters: a=12.761(1), b=7.358(1), c=24.565(1) Å, β=100.17(1)°, space group C2, Z=16. The structural relationships between the different zirconolite and weberite polytypes are discussed. A consideration of the structures from the viewpoint of anion-centered tetrahedral arrays shows that zirconolite can be considered as an anion-deficient fluorite derivative phase. However, the fluorite-type topology of edge-shared OM₄ tetrahedra is not maintained in the Ca₂Ta₂O₇ weberite-type polytypes, even though they have a fluorite-like fcc packing of metal atoms. One of the oxygen atoms moves from a tetrahedral Ta₃Ca interstice to an adjacent Ta₂Ca₄ octahedral interstice in the weberite polytypes.