Optical basicities and structural dynamics in glassy materials

There is a general question within the theory of optical basicity, which is why basicities determined by probe cations (typically Tl+ and Pb2+) fail to lie on the 'theoretical' curve linking the average basicity to glass composition. An explanation for this discrepancy is presented here, based in part on the 'two-site' model of Kamitsos ct al. and on a dynamic site model which assumes that mobile ions (Li+, Na+, etc.) create their own sites lying along the pathways for ion migration. The experimental spread in optical basicities is attributed to this dynamic structure, and the 'hole-digging' capabilities of both host and probe cations. Mobile ions in singly-occupied 'double sites' are in relatively shallow energy wells (the L-sites), while those in doubly-occupied double sites are in relatively deep energy wells (the H-sites). Probe cations enter both H- and L-sites when their motions become coupled to those of the host cations. (C) 1997 Elsevier Science B.V.