Structural modifications induced by Na+/K+ ion exchange in silicate glasses: A multinuclear NMR spectroscopic study

The structural mechanisms of stress relaxation during Na+/K+ ion exchange is studied in a variety of Na silicate glasses with and without alkaline-earth modifiers, using one and two-dimensional 23Na and 29Si nuclear magnetic resonance (NMR) spectroscopy. The results suggest that significant structural modifications accompany the Na+/K+ ion exchange process in the form of a shortening of the average NaO and SiNBO distances and an opening of the SiOSi angles without any detectable change in the Qn speciation. These trends are similar to those observed in analogous mixed Na,K glasses derived via the melt-quench route, with increasing K:Na ratio. Consequently, the ion-exchange-induced reorganization of the glass network is accompanied by a partial relaxation of the stress generated by the exchange of smaller Na+ by the larger K+ and better accommodation of the latter ion as a modifier.