Optical absorption due to tetrahedral and octahedral ferric iron in silicate glasses

Abstract

The optical absorption spectra of commercial soda-lime-silicate glasses is strongly influenced by the presence of ferrous and ferric iron. The absorption spectrum for iron arises from charge transfer and ligand field absorptions due to ferrous and ferric iron in octahedral and/or tetrahedral coordination. The present study was undertaken to use optical absorption spectra (300–2500 nm) to measure the levels of iron and other colorants in commercial glasses. In accord with Mossbauer analyses, the optical data from this work indicates that octahedral, as well as tetrahedral, ferric iron exists in soda-lime-silicate glasses. The observed behavior of the ligand field and charge transfer absorptions suggests that the ratio of octahedral to tetrahedral ferric iron changes with iron redox. Absorption in the ultraviolet had previously been explained by a combination of both ferrous and ferric charge transfer. The present study indicates that charge transfer absorption due to ferrous iron and both octahedral and tetrahedral ferric iron must be taken into account in order to explain the ultraviolet absorption of soda-lime-silicate glasses.