CONNECTIVITIES AND CATION DISTRIBUTIONS IN OXIDE GLASSES - NEW RESULTS FROM SOLID-STATE NMR

Modern solid stale nuclear magnetic resonance (NMR) techniques afford powerful experimental strategies for elucidating detailed structural i nformation in noncrystalline materials. While previous research emphas is has been on the identification and quantification of local environm ents (short-range-order), primarily by using the technique of magic-an gle spinning (MAS) NMR, correlations beyond the nearest neighbor coord ination sphere (intermediate-range order) can be studied by more sophi sticated methods. This contribution focuses on recent advances made fo r a variety of oxide glasses, using methods of dipolar spectroscopy: C onnectivities among the structural building blocks present in sodium a luminoborate glasses have been exposed using B-11{Al-27} and Al-27{B-1 1} rotational echo double resonance (REDOR) NMR. Similarly, Si-29{P-31 } and Si-29{Li-7} REDOR data form the basis of a structural model inte grating six-coordinated silicon into the network structure of alkali s ilicophosphate glasses. The spatial cation distributions in sodium sil icate glasses are studied by Na-23 spin echo decay spectroscopy, offer ing experimental evidence for inhomogeneous cation distributions at lo w alkaline contents. Finally, for mixed alkali sodium lithium silicate glasses, Na-23{Li-7} spin echo double resonance (SEDOR) and Si-29{Na- 23} and Si-29{Li-7} REDOR spectroscopies provide important experimenta l criteria for testing hypothesized relative Na-Li cation ordering sce narios.