STRUCTURE OF VITREOUS SILICA

A new study of the structure of vitreous silica has been made under greatly improved conditions. Using Rh K ! radiation with the method of fluorescence excitation, reliable intensity values were measured. The interpretation was in terms of pair functions, thereby eliminating the approximations in earlier work. Each silicon is tetrahedrally surrounded by four oxygen atoms, with a Si-O distance which is closely 1. 62 A. Each oxygen atom is bonded to two silicon atoms. The Si-O- Si bond angle ! shows a distribution V(!) extending all the way from 120 to 180**0, with a maximum at ! equals 144**0. This wide variation in ! is an important distinction between the vitreous and the crystalline forms of silica, and itfurnishes an important criterion for any proposed model of vitreous silica. Good agreement with the measured pair function distribution curve was obtained by assuming a random orientation about the Si-O bond directions. The interpretation leads to the familiar random network model, but with the new results the model is more precise.