V. Physicochemical Methods of Glass Characterization. Connectivities and cation distributions in oxide glasses: New results from solid state NMR

Modern solid state nuclear magnetic resonance (NMR) techniques afford powerful experimental strategies for elucidating detailed structural information in noncrystalline materials. While previous research emphasis has been on the identification and quantification of local environments (short-range-order), primarily by using the technique of magic-angle spinning (MAS) NMR, correlations beyond the nearest neighbor coordination sphere (intermediate-range order) can be studied by more sophisticated methods. This contribution focuses on recent advances made for a variety of oxide glasses, using methods of dipolar spectroscopy: Connectivities among the structural building blocks present in sodium aluminoborate glasses have been exposed using 11B{27Al} and 27Al{11 B} rotational echo double resonance (REDOR) NMR. Similarly, 29Si{31P} and 29Si{7Li} REDOR data form the basis of a structural model integrating six-coordinated silicon into the network structure of alkali silicophosphate glasses. The spatial cation distributions in sodium silicate glasses are studied by 23Na spin echo decay spectroscopy, offering experimental evidence for inhomogeneous cation distributions at low alkaline contents. Finally, for mixed alkali sodium lithium silicate glasses, 23Na{7Li} spin echo double resonance (SEDOR) and 29Si{23Na} and 29Si{7Li} REDOR spectroscopies provide important experimental criteria for testing hypothesized relative Na-Li cation ordering scenarios.