A Crystallographic Guide to the Structure of Borate Glasses

Abstract

ABSTRACTBorate glasses are an enigma in that there is now increasing evidence that their structures are dominated by superstructural units, which comprise well defined arrangements of the basic BO3 and BO4 structural units, with no internal degrees of freedom in the form of variable bond or torsion angles. In the present paper, it is shown that considerable insight into the structure of borate glasses can be gained from a study of the corresponding crystalline polymorphs. A simple, model is proposed to predict the fraction, x4, of 4-fold co-ordinated boron atoms in vitreous borate networks and the topological criteria for the formation of such networks are discussed, taking into account the degrees of freedom necessary for conventional glass formation.