Joël Emery

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.

Application of solid-state NMR (13C and 29Si CP/MAS NMR) spectroscopy to the characterization of alkenyltrialkoxysilane and trialkoxysilyl-terminated polyisoprene grafting onto silica microparticles

The solid-state Nuclear Magnetic Resonance (NMR) was used to characterize surfaces of silica gels chemically modified by alkenyltrialkoxysilanes and trialkoxysilyl terminated 1,4-polyisoprenes. The formation of covalent bonds created between alkoxy functional groups from alkenyltrialkoxysilane or trialkoxysilyl-terminated 1,4-polyisoprene and silanol groups on silica was clearly demonstrated by means of 13C and 29Si CP/MAS NMR spectroscopy. Quantitative data, including calculation of the grafting yields in relation with the initial silanol concentrations, were also obtained by using solid-state 29Si-NMR leading to a final well-defined characterization of the silica surfaces. A relatively good agreement was noticed between the grafting yields calculated from 29Si-NMR spectra and those determined from other analytical techniques such as Wijs titration or elementary analysis. The reactivity of the various silica silanols towards each coupling agent was clearly characterized and estimated, as were the proportions of the various grafted structures formed at the silica surface.

Anomalies in Li+ ion dynamics observed by impedance spectroscopy and 7Li NMR in the perovskite fast ion conductor (Li3xLa2/3-x□1/3-2x)TiO3

Abstract A previous study, performed on the fast ionic conductor Li 3 x La 2/3- x TiO 3 by 7 Li and 6 Li Nuclear Magnetic Resonance (NMR), has shown that Li + ions undergo two different motions: a fast motion ( τ c ≈10 −9 s) inside the A-cage of the perovskite structure and a slower one ( τ c ≈10 −6 s) from one A-cage to a next vacant one. Furthermore, a change of these two motion mechanisms is observed around 200 K. Apart from NMR, impedance spectroscopy may also afford information on the ionic motion mechanism. Lithium motion in Li 3 x La 2/3- x TiO 3 is then studied by impedance spectroscopy in the 1 Hz–10 MHz frequency range and in the 140–500 K temperature range. The results obtained by these two techniques, i.e. 7 Li NMR and impedance spectroscopy, are then compared in the 140–270 K temperature range. As observed in NMR, the dc conductivity shows a change in the mechanism of ionic motion around 200 K. Apart from the dc plateau, the real part of conductivity ( σ ′) displays a dispersive behavior at high frequencies. Plotting the ac data in terms of impedance and modulus reveals the presence, in the mechanism of conduction, of both a nonlocalized conduction (long-range motion of the mobile ions) and a localized one (dipolar relaxation). According to these experimental observations, an equivalent electrical circuit is proposed, taking into account the physical processes assumed to be present when a small electrical signal is applied to the oxide. Both dipole polarization and long-range motion of the mobile ions are included in the electrical circuit of the conductive pathways. A complex nonlinear least squares fitting procedure (CNLS) is used to fit this electrical model to the experimental conductivity vs. frequency response ( σ ′ and σ ″). This procedure shows that all the parameters linked to the conductive pathways undergo a sudden change around 200 K, suggesting that a change in the ionic motion mechanism occurs at this temperature. This result is discussed in relation to both the crystallographic structure of the ionic conductor and the results previously obtained by 7 Li NMR.

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.

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 and Mössbauer effect studies in iron-doped 57Fe isotope enriched phosphate glasses

Abstract Electron paramagnetic resonance and Mossbauer spectroscopy measurements of phosphate glasses doped with 57Fe isotope enriched Fe2O3 have been carried out. The electron paramagnetic resonance spectra have been computer simulated using an approach based on the eigenfield method applied to the ‘rhombic’ spin Hamiltonian, which contains only the Zeeman and quadrupole fine structure terms. In order to account for the structural disorder in glass, different distribution densities of fine structure parameters D and E have been tried: a two-dimensional Gaussian function of D and λ = | E D | and ‘Czjzeks and related functions’. An agreement between the experimental and computer simulated spectra found with the Gaussian distribution density suggests the presence of a large fraction of Fe3+ sites with axial or feebly rhombic distortions (λ ≤ 0.08). The mean value of the axial fine structure parameter D is consistent with a highly distorted environment of Fe3+ ions in the phosphate glasses (but somewhat less in comparison with the borate glasses). It is shown that the gef = 2.0 absorption in the phosphate glasses consists of two different features: a narrow one results from Fe3+ ions isolated in the glass matrix, while the broad one is due to crystallite inclusions.

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.

A Brillouin scattering investigation of relaxation versus crosslink density in glass- and gel-forming polymers

A Brillouin scattering study of the relaxational processes above the calorimetric glass transition temperature (in the 1.2 - 1.6 temperature range) on polyurethane gels is presented. The specificity of this work is to investigate several samples corresponding to various crosslink densities. The relaxational process observed by Brillouin scattering is compared to low-frequency measurements of the relaxational processes by other techniques: in contrast to low-frequency determinations, the Brillouin scattering relaxation time exhibits an Arrhenius behaviour for all the samples; however, the characteristic parameter of the time distribution width is the same for the two processes. The low-frequency relaxation and the high-frequency Brillouin scattering relaxation seem to merge for temperatures higher than 1.6 and these data are discussed on the basis of different models for the glass transition. The influence of the crosslink density yields an increase of the mean relaxation time with increasing connectivity and small variations of the apparent activation energy. In contrast, the distribution time width is not composition dependent. Nevertheless, the most striking results are that a rescaling to fully accounts for the variations of the mean relaxation time with the crosslink density and a correspondence between the crosslink density and the inverse of the temperature is suggested.

Li+ ionic conduction in the layered perovskite Li2La2/3Ta2O7

The Li+ ionic conduction properties of the Li2La2/3Ta2O7 layered perovskite compound have been investigated by complementary techniques: impedance spectroscopy, 7Li NMR and thermal neutron powder diffraction. Up to 770 K, the results are consistent with an electrical conductivity dominated by Li+ ions jumping between Li1 and Li2 sites, the adjacent centers of the two kinds of LiO4 tetrahedra constituting the interlayer region of the structure. The sudden event, observed near 770 K on the curve log(σT) = f(1000/T), is associated with significant changes to the 7Li NMR signal and to a structural modification followed by neutron powder diffraction up to 973 K. All these facts are consistent with the displacement at 770 K of the lithium ions residing in the Li1 sites. Neutron powder diffraction showed that, in the interlayer region, the location of the lithium ions in the Li2 sites remains unchanged. However, 17% of the Li1 population leaves its tetrahedral position to occupy a new Li3 site inside the perovskite cages, very close to their four O2− bottlenecks. The other Li+ ions (83%) remain in the interlayer but their coordination changes from tetrahedral to a five fold one.

The structure of a boron oxyfluoride glass, an inorganic cross-linked chain polymer

Abstract A combination of 19 F and 11 B NMR studies as well as ion dynamics computer simulation was used to establish the structure of the recently described vitreous material BO1.3F0.4. The cross-linked spaghetti structure suggested earlier is confirmed.