No abstract

Local order in silicate glasses has been observed by many experimental techniques to be similar to that in crystalline materials. Details of the intermediate-range order are more elusive, but essential for understanding the lack of long-range symmetry in glasses and the effect of composition on glass structure. Two-dimensional 17O dynamic-angle-spinning nuclear magnetic resonance experiments reveal intermediate-range order in the distribution of inter-tetrahedral (Si-O-Si) bond angles and a high degree of order in the disposition of oxygen atoms around the network-modifying cations.

Using 207Pb magic angle spinning and static NMR, we have resolved and assigned different lead sites in crystalline lead oxides and lead silicates to their isotropic chemical shifts. Chemical shift anisotropies were also obtained for lead sites from the intensities of spinning sidebands. Empirical correlations between 207Pb isotropic chemical shifts and structural parameters are proposed. For ionic compounds, we show good correlations between chemical shift and coordination number or mean bond length. For more covalent compounds, the best empirical correlation has been obtained by using the degree of oxygen s−p hybridization and second neighbor electronegativity similar to that previously used to characterize 29Si shifts in aluminosilicates. The Pb2+ chemical shift anisotropies increase with more positive chemical shifts. These correlations, established for simple crystalline compounds, should allow better characterization of lead environments in disordered materials of complex composition.