G Silly

Superposition model for isotropic chemical shift in ionic fluorides: from basic metal fluorides to transition metal fluoride glasses

Abstract Experimental measurements of 19 F isotropic chemical shifts on a large set of mainly ionic fluoride compounds (from simple metal fluorides to transition metal fluoride glasses) obtained by MAS NMR at 15 kHz spinning rate are simultaneously investigated. First, Ramseys theory of the chemical shift with molecular orbitals obtained by Lodwins orthogonalisation method is used to evaluate the isotropic part of the 19 F chemical shift in ionic fluorides for which the crystallographic structure and the atomic radial wavefunctions are known. Then, assuming that the paramagnetic part of the 19 F shielding in a given material is simply the summation of the paramagnetic contributions due to all the cations in the neighbourhood of the considered fluorine, a superposition model of the 19 F isotropic chemical shift is developed. Finally, this empirical approach is applied to complex fluoride compounds of unknown structure and it is demonstrated that it allows to obtain reliable structural informations. More generally, it can be applied to all the ionic fluorides.

The local field distribution of in transition metal fluoride glasses investigated by electron paramagnetic resonance

The EPR spectra of in two transition metal fluoride glasses (PZG and PBI) have been extensively studied as functions of temperature, concentration and microwave frequency (S, X, K and Q bands). The marked changes observed in the EPR spectra with temperature and concentration imply the existence of pairs of ions. The spectra of isolated ions were obtained at low temperature for samples with low doping concentration. Isolated ions are characterized by a fine-structure -parameter distribution within the range . The simulations of these spectra are computed with a Czjzek fine-structure parameter distribution (this solution prohibits the existence of axial symmetry sites). It indicates a decrease of the number of geometrical constraints compared to those for and (octahedral coordination in a fluoride medium), in agreement with the higher coordination numbers. -ion sites are found to be more distorted in PZG than in PBI, suggesting that the glass network has an influence on the polyhedron distortion amplitude.

Solid state 77Se NMR investigations on arsenic-selenium glasses and crystals

Abstract Some resolved solid state 77 Se NMR spectra are presented in the As x Se 1− x glass family at ambient temperature. They exhibit three different kinds of Se environments. A comparison with the parent crystalline phases permits to assign the lines to Se- Se -Se, Se- Se -As and As- Se -As Se atom neighborhoods. The measurements of the relative intensities of the lines prove the validity of the intermediate range order structural model known as the “chains crossing model” which is based on AsSe 3 pyramids homogeneously distributed among the divalent Se atoms network. In particular, any scenario involving a selenium clustering process is refuted.

Local structural orders in nanostructured Al2O3 prepared by high-energy ball milling

Nanostructured Al2O3 powders were prepared by high-energy ball milling of corundum. Both the solid state nuclear magnetic resonance spectra of the Al3+ ions and the solid state electron paramagnetic resonance spectra of incorporated Fe3+ ions are governed by noticeable spectral changes dependent on the duration of the mechanical treatment. The quadrupolar parameters of the 27Al nuclei and the zero-field splitting parameters of the Fe3+ ions as well as their statistical distributions were determined as functions of the milling time. Structural changes of the Al2O3 matrix were also followed by powder x-ray diffraction and transmission electron microscopy measurements. Direct relations between the structural disorder as obtained by x-ray data and the spin Hamiltonian parameters of both ions could be established. These results suggest that the milled powders consist of nanocrystalline grains embedded in amorphous grain boundaries even for the longest milling time. The grains can be described in terms of ordered AlO6-octahedra as in the starting crystalline material exhibiting a slight rhombic distortion. The grain boundaries look like random arrangements of these octahedral units. The specific behaviour of the environment of the Fe3+ paramagnetic probe points out that such a point defect acts as an activation centre of the amorphization process.

AF fluoropolymer for optical use: spectroscopic and surface energystudies; comparison with other fluoropolymers

Four fluoropolymers are studied by Infra-Red, Raman and solid state NMR; one of them (Teflon AF) is optically transparent thanks to the presence of large ether cycles which prevent crystallization. Detailed investigations of the different spectra lead to the interpretation of the different lines characteristic of the constitutive fluoro-based sites.

From crystalline to glassy gallium fluoride materials: an NMR study of 69Ga and 71Ga quadrupolar nuclei

Owing to the implementation of acquisition techniques specific for nuclei with very large quadrupolar interaction (full shifted echo and variable offset cumulative spectra (VOCS)), NMR spectra of 69Ga and 71Ga are obtained in crystallised (PbGaF5, Pb3Ga2F12, Pb9Ga2F24 and CsZnGaF6) and glassy (PbF2-ZnF2-GaF3) gallium fluorides. Simulations of both static (full echo or VOCS) and 15 kHz MAS spectra allow to obtain consistent determinations of isotropic chemical shifts and very large quadrupolar parameters (nu(Q) up to 14 MHz). In the crystalline compounds whose structures are unknown, the number and the local symmetry of the different gallium sites are tentatively worked out. For the glassy systems, a continuous Czjzeks distribution of the NMR quadrupolar parameters accounts for the particular shape of the NMR spectrum.

Investigation of fluorine octahedron connectivities in transition metal fluoride glasses by solid-state magic-angle-spinning nuclear magnetic resonance spectroscopy

High-resolution magic-angle-spinning (MAS) NMR spectroscopy is used to investigate the structural properties of some transition metal fluoride glasses ( - - ) related to the fluorine network. Several glass compositions are investigated in order to vary the ratios. MAS NMR experiments are carried out on certain crystalline compounds selected as being the initial constituents or recrystallization compounds of glassy phases, or because they have some specific particular connectivities of the fluorine octahedron network (e.g. ). It is shown that three types of fluorine are involved in the glass network: free fluorines which are not connected to transition metal ions, and shared and unshared fluorines belonging to and octahedra. Quantitative information on these three different fluorine sites, their relative ratios in the glassy networks, and the degree of cross-linking of the fluorine octahedra is obtained. Our results prove the validity of the previously adopted assumption according to which the glass network is built up from corner-sharing fluorine octahedra centred on transition metal ions.

Electron paramagnetic resonance investigations of α-Al2O3 powders doped with Fe3+ ions: experiments and simulations

Electron paramagnetic resonance (EPR) of Fe3+ ions in Al2O3 is studied in powder samples prepared by different routes and/or modified by thermal or mechanical treatments, with different doping levels and grain sizes. The measurements are performed in various frequency bands (S, X, K, Q and W) and with bimodal detection in X-band. Simulations of the spectra are achieved with a code designed for computing EPR powder spectra described by any spin Hamiltonian including second-, fourth-and sixth-order ZFS terms (S ≤ 7/2). The linewidths, intensities and lineshapes are accounted for. The lineshape is Gaussian at low Fe3+ concentration whereas it is Lorentzian for higher concentration. The linewidths are interpreted as the superimposition of three main contributions: intrinsic linewidth, dipolar broadening and broadening due to lattice imperfections. The latter is tentatively interpreted in terms of quadrupolar spin Hamiltonian parameter distributions treated using first-order perturbation theory. Whatever the sample, only the b22 spin Hamiltonian parameter is found to be distributed around a mean zero value which corresponds to rhombic distortions. Angle and bond length distributions are tentatively extracted from the b22 distributions which gives some insight into the local order around the spin probe in relation to the preparation and treatment of the samples.

CP-MAS 207Pb with 19F decoupling NMR spectroscopy: medium range investigation in fluoride materials

The isotropic chemical shift of 207Pb is used to perform structural investigations of crystalline fluoride compounds (PbF2, Pb2ZnF6, PbGaF5, Pb3Ga2F12 and Pb9Ga2F24) and transition metal fluoride glasses (TMFG) of the PZG family (PbF2-ZnF2-GaF3). Using 207Pb Cross Polarisation Magic Angle Spinning (CP-MAS) NMR with 19F decoupling, it is shown that the isotropic chemical shift of 207Pb varies on a large scale (1000 ppm) and that the main changes of its value are not due to the nearest neighbour fluorines but may be related to the number of next nearest neighbour (nnn) Pb2+ ions. In this way, it is demonstrated that 207Pb chemical shift is an interesting probe to investigate medium range order in either crystalline or glassy fluoride systems. The 207Pb delta(iso) parameter has been linearly correlated to the number of nnn Pb2+ ions.

Aging effects on vulcanized natural rubber studied by high resolution solid state 13C-NMR

Abstract The vulcanization chemistry, network formation and thermal oxidative aging effects of unfilled and carbon-black filled natural rubber vulcanized with organic peroxide, efficient vulcanization (EV) system and conventional sulfur system are studied by solid state 13 C-NMR spectroscopy. Cis – trans isomerization and different sulfide structures are detected and identified through the chemical shift values of the extra NMR lines, which are observed in the various vulcanizates. It is shown that the kinetics of crosslink formation may be followed by this technique. A destruction of the polysulfidic crosslinks is clearly related to the aging process in the conventional system of cured unfilled or filled vulcanizates, whereas no significant modification is observed in the peroxide and the EV cured ones.