Alkali sites in glass

In this paper we study the nature of sites occupied by alkali metal ions in oxide and organic glasses by probing the localised metal-site vibrations which are active in the far-infrared. Analysis of the far-infrared profiles of the glasses considered in this investigation indicates the presence of one or two distributions of cation hosting sites. It is argued that the energetics of these sites are determined by both the bonding requirements of the metal cation and the versatility of the anionic glass network to fulfil these requirements. This is demonstrated by comparing earlier data on alkali borates and more recent results on alkali germanates and ionic organic glasses, as well as by studying the effect of the method of glass formation (fast melt quenching, slow cooling, sol-gel) on the far-infrared absorption envelope. The alkali metal sites inferred from far-infrared have been compared to those probed by ultraviolet spectroscopy using Tl+ and Pb2+ ions electrolysed into borate glasses. The results suggest that mobile alkali ions are mainly those occupying sites of high basicity.