Origins Of Anomalous Mixed-Alkali Effects In Ion-Exchanged Glasses

Ion transport properties in ion-exchanged glasses are re-examined by a.c. impedance and infrared spectroscopies. In typical soda-lime-silica (float) glass it is shown that when Na+ ions are replaced by K+ ions, the region of highest resistivity resides in the surface, confirming the earlier result of Tomandl and Schaeffer. The existence of this anomalous impedance is correlated with cation-induced relaxations of the network (CIRON). This process enables the glass to accommodate larger K+ ions at the expense of empty K sites (which in melt-grown glasses provide the stepping stones for ion migration). As a consequence of the structural relaxations, the compressive stresses stored within the surface of the glass may be smaller than expected, even when ion exchange occurs far below T-g and the effects of viscous flow can be neglected.