Franck Fayon

Chemical bonding differences evidenced from J-coupling in solid state NMR experiments involving quadrupolar nuclei

Small scalar J-coupling between quadrupolar nuclei and spin 1/2 can be measured in inorganic solids using J-Resolved experiments and further used to acquire 2D J-HQMC heteronuclear correlation, giving detailed insight into the chemical bonding scheme.

Pb2+ environment in lead silicate glasses probed by Pb-LIII edge XAFS and 207Pb NMR

The local structural environment of lead (Pb) atoms in lead silicate glasses has been studied by two complementary spectroscopic techniques: Pb-LIII edge X-ray absorption fine structure (XAFS) and 207Pb solid state nuclear magnetic resonance (NMR). XAFS results have been obtained with two kinds of experimental devices: a synchrotron radiation source and a laboratory spectrometer. The weak photon flux is then compensated by a higher stability of the source and a longer acquisition time. The experiments were carried out on selected compositions of lead silicate glasses in which we observed PbO3 and PbO4 pyramidal units with short Pb–O bond lengths typical of a covalent bonding state.

29Si and 207Pb NMR study of local order in lead silicate glasses

Binary lead silicate glasses have been investigated using 207 Pb and 29 Si high resolution solid state Nuclear Magnetic Resonance (NMR) spectroscopy. From these experiments we have evidenced that lead forms covalent PbO 4 and PbO 3 pyramids over a large compositional range. The distribution of the Q n silicate species is determined. The calculated equilibrium constant of the reactions 2Q n = Q n+1 + Q n-1 are 1 order of magnitude larger than those determined for alkali silicates. For both lead and silicon, there is a distribution of local environments. At larger lead contents, we observe the existence of a lead oxide based network. For <60 mol% PbO content, silicon is the main glass former. As the lead content increases, the silicate network is broken and the chains constituted of PbO n pyramids connect together.

Through-space contributions to two-dimensional double-quantum J correlation NMR spectra of magic-angle-spinning solids

A routinely used assumption when interpreting two-dimensional NMR spectra obtained with a commonly used double-quantum (DQ) magic-angle-spining (MAS) pulse sequence referred to as the refocused incredible natural abundance double-quantum transfer experiment (INADEQUATE) [A. Lesage, M. Bardet, and L. Emsley, J. Am. Chem. Soc. 121, 10987 (1999)] has been that correlation peaks are only observed for pairs of nuclei with a through-bond connectivity. The validity of this assumption is addressed here by theory, experiment, and computer simulations. If the isotropic chemical shifts of the two nuclei are different and the MAS frequency is far from rotational resonance, the theoretical description demonstrates that DQ correlation peaks are indeed indicative of a J coupling. However, if the isotropic chemical shifts are the same, it is shown that DQ peaks can appear for pairs of nuclei even in the absence of a through-bond J coupling. These peaks appear in the specific case of a pair of nuclei with a nonzero through-space dipole-dipole coupling and chemical shift anisotropy tensors having different principal magnitudes or orientations, provided that the MAS frequency is comparable to or smaller than the chemical shift anisotropies. Experimental 31P spectra recorded on a sample of TiP2O7 and computer simulations show that the magnitude of these anomalous peaks increases with increasing B0 magnetic field and that they decrease with increasing MAS frequency. This behavior is explained theoretically.

Through-bond phosphorus-phosphorus connectivities in crystalline and disordered phosphates by solid-state NMR.

2D 31P refocused INADEQUATE NMR experiments have been used to determine through-bond P-O-P connectivities in crystalline and disordered phosphates.

Structural characterization of water-bearing silicate and aluminosilicate glasses by high-resolution solid-state NMR

Various one- and two-dimensional high-resolution solid-state NMR techniques have been applied to hydrous silicate and aluminosilicate glasses: simple acquisition, cross-polarization (CP-MAS), heteronuclear correlation (HETCOR), dipolar dephasing, spin counting, double-quantum correlation, and rotational echo double resonance (REDOR). The comparison of the results obtained for sodium tetrasilicate and phonolite glasses suggests that the water incorporation mechanisms are qualitatively similar for these two compositions. From proton NMR experiments, we observe no evidence of proton clustering and a wide range of chemical shifts ranging from 0 to 16 ppm, even for the aluminosilicate phonolite glass, identifying at least three types of hydroxyl (OH) protons in addition to molecular H2O. This variety of OH groups can be discussed in terms of hydrogen bonding strength. For both compositions, the results indicate some depolymerization of the tetrahedral network, but the picture cannot be so simple as to completely exclude any of the different previously proposed models for water incorporation in silicate glasses.

31P NMR study of magnesium phosphate glasses

Abstract 1-D magic angle spinning (MAS) and 2-D double quantum solid-state 31 P nuclear magnetic resonance (NMR) investigations have been carried out on (MgO)x(P2O5)1−x glasses over compositional range from x=0.44 to 0.59. The distribution of the Qn phosphate units, obtained from the simulation of 31 P MAS spectra, is close to a binary distribution of bridging and non-bridging oxygen atoms, indicating that the Mg2+ cations act as network modifiers. High-resolution 31 P double quantum NMR is used to probe the intermediate range order in phosphate glasses and suggests that the anomalous behavior near the metaphosphate composition is due to a change from ring structures to phosphate chains.

Order-resolved sideband separation in magic angle spinning NMR of half integer quadrupolar nuclei

Abstract We describe a new pulse sequence that separates spinning sidebands by order for MAS NMR spectra of half integer quadrupolar nuclei broadened to second order. This two dimensional experiment is based on the modulation of each spinning sideband by its order with nine pulses (QPASS: quadrupolar phase adjusted spinning sidebands). The experimental spectra are compared with simulation for the case of 71 Ga in β-Ga 2 O 3 .

Migration and segregation of sodium under β-irradiation in nuclear glasses

Raman, 11B Nuclear Magnetic Resonance (NMR) and X-ray Photoelectron (XPS) spectroscopy investigations have been made on a series of β-irradiated aluminoborosilicate glasses. This work shows the following changes of the glass structure under β-irradiation: (i) an increase of glass polymerization, (ii) an increase of boron in a trigonal environment and (iii) no modification in the aluminum environment during irradiation. These glass modifications under β-irradiation can be correlated with the migration of sodium outside of the Si and B glass network formers. Moreover, XPS experiments show a strong decrease of the sodium concentration at the surface of these irradiated glasses. This study presents therefore strong evidences of migration and segregation processes of sodium during a β-irradiation in the bulk for these simplified aluminoborosilicate glass compositions.

Highly Transparent BaAl4O7 Polycrystalline Ceramic Obtained by Full Crystallization from Glass

Transparent polycrystalline ceramics are an emerging class of photonic quality materials competing with single crystal technology for a diverse range of applications including high-energy lasers, scintillating devices, optical lenses, and transparent armour. Polycrystalline ceramics offer several advantages, particularly in the fabrication of complex shapes and large-scale industrial production, and enable greater and more homogenous doping of optically active ions than is possible in single crystals. A limited number of either cubic or nanocrystalline transparent polycrystalline ceramics are known, but require complex and time-consuming synthetic approaches. Here, we show for the fi rst time that fully dense transparent polycrystalline ceramics can be simply obtained by direct and complete crystallization from glass. This is demonstrated for the previously unreported composition, BaAl 4 O 7 , which exhibits two orthorhombic polymorphs with micrometer grain size, both optically transparent in the visible range. This innovative synthetic route to transparent polycrystalline ceramics should facilitate the discovery of new, cost-effective chemical methods for transparent ceramic applications. Conventional optically transparent single crystal materials are widely used in numerous photonic applications. However, these materials face several technological and economical challenges, including a restricted list of appropriate single crystal compounds, limitations on the type and level of chemical doping, and mechanical and manufacturing requirements for large and complex physical shapes. Many of these obstacles can be avoided through the use of ceramic materials, which afford a wider range