Gordon Oates

Water in poly(ethylene oxide): differential scanning calorimetry and nuclear magnetic resonance experiments

Non-freezable D2O in partially crystalline poly(ethylene oxide) shows residual deuterium quadrupolar splitting. Extrapolation of enthalpies of eutectic melting to zero yields an average hydration number of 0.9.

The effect of the NMR spin-echo sequence Py(90°)-τ-Py(180°) in solids containing different dipolar-coupled spin species: applications to KHF2 AND CsHF2

Abstract An experimental investigation of the effect of the NMR sequence P y (90°)-τ- P y (180°) in solids containing different types of dipolar-coupled spin species is reported. The experimental results suggest that the decay of the maximum spin-echo amplitude as a function of pulse spacing depends only on the strength of the homonuclear dipolar interactions. Polycrystalline samples of KHF 2 and CsHF 2 are used as model systems.

Preparation and characterization of some new compounds with nasicon-type structures

The new compounds CuTiZrP3O12, NbSnP3O12, FexTaTiP3O12 (x <0.33) and Snx,TaTiP3O12 (x < 0.5) have been prepared and characterized by X-ray powder diffraction. The lattice parameters are compared with those of related compounds. The copper K-edge extended X-ray absorption fine structure (EXAFS) indicates that Cu+ ions in materials of composition Cu+M23P3O12 are displaced along [001] from the centre of an octahedral interstitial site. The incorporation of iron and tin as Fe2+ and Sn2+ within the three dimensional interspace of compounds of the type M5+M4+P3O12 is discussed in terms of the amenability of pentavalent cations in the host lattice to reduction to a lower oxidation state, which is stable in octahedral oxygen coordination.

Crystal Chemistry and Cation Ordering in Zirconolite 2M

Structural studies of single phase or nearly single phase zirconolite ceramic samples have been conducted using electron microscopy and microanalysis, X-ray diffraction, neutron diffraction, and spectroscopic methods. We show that it is possible to produce a complete series of zirconolite 2M samples with substitution of 2Ti by Nb+Fe in the HTB layer. The samples are single phase up to about 80% Nb +Fe substitution, with the appearance of a minor perovskite phase at higher Nb+Fe levels. Electron probe microanalysis reveals that the samples are homogeneous and close to their nominal compositions, except for those containing perovskite, which have a slight excess of Zr and a deficiency in the Fe content. The lattice parameters and the positions of certain Raman bands are non-linear as a function of composition, suggesting the possibility of cation ordering over the three available Ti sites within the HTB layer. Rietveld refinement of Synchrotron X-ray powder data for the Nb+Fe end-member have been conducted for the disordered case and for six trial models each with a different ordering scheme. Results of this exercise indicate that Fe preferentially occupies the Ti2 (split) site with partial ordering of Nb and the remaining Fe over the Ti1 and Ti2 octahedra. The preference of Fe for the five coordinated Ti2 site has been confirmed by 57Fe Mossbauer spectroscopy.

Iron-57 Mössbauer spectroscopic studies of the weathering of L-chondrite meteorites

Some L-chondrite meteorites found in the arid desert region of Roosevelt County in New Mexico, USA, and14C dated to determine the terrestrial age have been examined by57Fe Mössbauer spectroscopy. The preliminary results reported here suggest that the initial weathering processes involve oxidation of iron in the iron-nickel alloy. After prolonged exposure to terrestrial weathering for approximately 36 000 years, the iron(II)-sulphide and -silicate phases are also oxidised. The corrosion products are complex and include paramagnetic Fe3+ species and macroscopic iron(III) oxide and/or oxyhydroxide phases. A meteorite which fell approximately 16 500 years ago at the end of the last glaciation showed extensive corrosion despite its relatively short terrestrial age. The result is associated with climatic changes which occurred at that time.

Iron-57 Mössbauer spectroscopy studies of meteorites recovered from Roosevelt County, USA

Some H5 ordinary chondrite meteorites recovered from the desert region of Roosevelt County in New Mexico, USA and14C dated to determine the terrestrial age have been examined by57Fe Mössbauer spectroscopy. The preliminary results suggest the increasing oxidation of Fe0 in the iron-nickel alloy phase and of Fe2+ in the iron-sulphide and -silicate phases as a result of terrestrial weathering over ca. 38 000 years and which induces the formation of macroscopic iron oxide/oxyhydroxide phases.

1 H and 19F nuclear magnetic relaxation study of the dynamics of the hydrogen difluoride ion, FHF–, in the α- and β-phases of caesium hydrogen difluoride

A 1H and 19F nuclear magnetic resonance spin–lattice relaxation investigation of polycrystalline caesium hydrogen difluoride, CsHF2, in the temperature range 175–390 K is reported. There is a discontinuous change in the relaxation behaviour in the region 330 ± 2 K which corresponds to the structural phase transition. In the low temperature, or α–phase, the relaxation measurements are interpreted in terms of a model in which the FHF– ions undergo a 180 °-flip motion for which the activation energy is found to be 24.8 ± 2.5 kJ mol–1. General relaxation expressions for a polycrystalline sample are developed for this motional model. A particular feature of the measurements is the cross-relaxation, or spin exchange, between the hydrogen and fluorine nuclei which results in non-exponential spin–lattice relaxation behaviour at temperatures below 200 K. In the cubic, or β-phase, the relaxation measurements are shown to be consistent with a restricted rotational model in which reorientations of the FHF– ion occur at random among the three cubic unit cell directions. The activation energy for this type of motional process is found to be 19.6 ± 1.5 kJ mol–1.

A 19F NMR study of the asymmetric short hydrogen bond in the crystalline adduct between potassium fluoride and succinic acid

Abstract An NMR method for investigating strong hydrogen bonding in solids is demonstrated for the adduct, KF·(CH 2 CO 2 H) 2 . A procedure for analysing the 19 F NMR FID for KF·(CH 2 CO 2 H) 2 is described and the results for the geometry of the OH…F… HO hydrogen-bonded system are compared with those of the X-ray structure. The value of the H…F bond length is found to be 0.148±0.003 nm which is significantly shorter than that determined by X-ray analysis.

Iron-57 Mössbauer spectroscopy study of iron oxide pillared clays synthesised by microwave heating

A novel method for the rapid preparation of iron oxide-pillared montmorillonite clay using microwave heating is described. The iron oxide pillared clay is progressively reduced in hydrogen at temperatures between 300 and 600°C. The metallic iron formed under reducing conditions is stable to oxygen at room temperature and partially oxidised when heated in oxygen at 300°C.