Alkali and halogen dependence of optical absorption bands of alkali-borateglasses

We examined the alkali and halogen ion concentration dependence of the opti cal absorption in alkali borate glasses. When potassium iodide was added to alkali borate glasses, I-3(-) molecules were formed and induced two bands with peaks at 287 and 354 nm. These bands increased with K2O concentrations until 10 mol%. and decreased with K2O from 20 to 25 mol%. In glasses with additions of KBr and KCI, the bands were observed at 266 and 259 ran, respe ctively. These positions were negligibly influenced by the kind of alkali i ons. It is confirmed that all of these bands are attributed to the X-3 ions (X: halogen), by the comparison with absorption spectra of thin films prep ared by simultaneous evaporation of alkali halide and a small amount of hal ogen. Molecular orbital (MO) calculations were also carried out to examine the charge distribution and structures. From the experimental results and M O calculations, we suggest that the X-3 ion binds to a boroxol ring that is coupled with a triborate group through an alkali ion. (C) 2001 Elsevier Sc ience B.V. All rights reserved.