Origins of anomalous mixed-alkali effects in ion-exchanged glasses

Abstract

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 cationinduced relaxations of the network (CIRON). This process enables the glass to accommodate larger ions at the expense of empty Κ 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 Tg and the effects of viscous flow can be neglected.