SODIUM AND SILVER ENVIRONMENTS AND ION-EXCHANGE PROCESSES IN SILICATE AND ALUMINOSILICATE GLASSES

X-ray absorption fine structure (XAFS) was used to determine the local environments of sodium in sodium silicate and aluminosilicate glasses. Silver environments were also measured after Ag/Na ion exchange in the same glasses. Na-O distances were found at a characteristic first-shell distance of 2.30-2.32 angstrom. The coordination number of approximately 5 oxygen in binary silicates dropped, however, when Al2O3 was added to the glass. We ascribe this to changes in the geometry of available oxygen cages in the glass structure as nonbridging oxygens are replaced by bridging oxygens. In comparison, ion exchange with silver led to cation environments with much shorter cation-oxygen distances of 2.08-2.23 angstrom and even smaller coordination numbers approaching 2 oxygen, resembling the immediate environment of Ag in Ag2O. This geometry was shown to persist in one of the ion-exchanged glasses despite a variety of thermal history and processing conditions. In detail, though, as the glass composition was varied, disorder in the oxygen shell surrouding Ag was affected. This increased as Al2O3 was added, indicating a more rigid environment as the network polymerized. The different ion-exchange rates of the sodium and silver have been compared and are reconciled in terms of the effects different cation oxygen ratii and the number of available nonbridging oxygen have on diffusion mechanisms in the glass.