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

Ag+/Na+ ion-exchanged R2O-Al2O3-SiO2 glasses with uniform concentration profile of Ag+ and Na+ were prepared by heat treatment in molten silver salt followed 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-MAS NMR, IR and Raman spectroscopies. Both T-g and TEC decreased with increase of the exchange ratio, but T-g was still above the ion-exchange temperature 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 alteration of the glass network was observed. On the other hand, the vibrational spectra showed remarkable peak shifts depending on the exchange ratio. From these structural results, it was found that when the exchange ratio was low, the introduced Ag+ ions were stabilized at the non-bridging oxygen (NBO) site, and then Na+ ions in AlO4 site were exchanged by Ag+ ions after full replacement of NBO sites, where O represents the bridging oxygen. (C) 2000 Elsevier Science B.V. All rights reserved.