DISORDER AMONG NETWORK-MODIFIER CATIONS IN SILICATE-GLASSES - NEW CONSTRAINTS FROM TRIPLE-QUANTUM O-17 NMR

The state of ordering among network-modifier cations in molten silicat es has a potentially large effect on their overall configurational ent ropy, on foe energies of mixing, and on viscosity and diffusivity. Mos t models of thermodynamic and transport properties assume random mixin g, but there is relatively little direct microstructural information t o constrain the real extent of this disorder. Two-dimensional O-17 NMR can produce spectra that are free of quadrupolar broadening and thus in which peak widths for non-bridging oxygen sites directly reflect th e extent of disorder in the local structural environment. In this repo rt, we describe new data from triple-quantum magic-angle-spinning (3QM AS) NMR for a series of barium and calcium silicate glasses. Results a re best explained by completely random mixing of Ba and Ca, confirming conventional modeling assumptions. Other recent data show. however th at significant ordering may be present (at least at the temperature of the glass transition) for modifier cations with greater differences i n size or charge, and among network-forming cations.