Cation site occupation by Ag+/Na+ ion-exchange in R2O-Al2O3-SiO2 glasses

Ag+/Na+ ion-exchanged R2O-Al2O3-SiO2 glasses with uniform concentration pro file of Ag+ and Na+ were prepared by heat treatment in molten silver salt f ollowed by holding at the same temperature in an ambient atmosphere. Their glass transition temperature (T-g) and thermal expansion coefficient (TEC) were measured and structures were investigated using Si-29-MAS NMR, Al-27-M AS NMR, IR and Raman spectroscopies. Both T-g and TEC decreased with increa se of the exchange ratio, but T-g was still above the ion-exchange temperat ure of 400 degrees C even for the fully exchanged sample. The Si-29- and Al -27-MAS NMR spectra were mostly unchanged and no sign of the structural alt eration of the glass network was observed. On the other hand, the vibration al spectra showed remarkable peak shifts depending on the exchange ratio. F rom these structural results, it was found that when the exchange ratio was low, the introduced Ag+ ions were stabilized at the non-bridging oxygen (N BO) site, and then Na+ ions in AlO4 site were exchanged by Ag+ ions after f ull replacement of NBO sites, where O represents the bridging oxygen. (C) 2 000 Elsevier Science B.V. All rights reserved.