Regular Rings in Glasses

Abstract

An energy minimization argument has been proposed and refined to predict that small order rings of bonds in a glass are likely to be regular. When applied to vitreous (v-) SiO₂ the argument predicts substantial (_~1%) concentrations of planar 3-fold and regular 4-fold rings of Si-O bonds. These hexagonlike and puckered octagon-like structures have been identified respectively with the 606 cm^-1 Raman lines, and are especially interesting because they do not exist in any known form of crystalline SiO₂. Numerous experimental results supporting the identification will be discussed, including recent isotopic substitution and annealing experiments. Arguments disfavoring several earlier identifications for the defect lines will also be given. The energy minimization argument will be applied to several other glasses and the predictions compared with appropriate experiments, where the latter are available. For example, the energy argument predicts that nearly planar 3-fold rings should play a large role in the structure of v-GeO₂, and this is consistent with our Raman studies involving isotopic substitution.