Borate vs. Silicate Glasses: Why are they so Different ?

Most attempts to explain the difference between the properties of borate and silicate glasses, {i.e. the so-called borate (or boron oxide) anomaly} have focussed on the change in co-ordination number of some of the boron atoms from three to four. The present paper discusses the origin of this co-ordination number change at a fundamental level, together with the important role played, both by superstructural units and by network-modifying cations, in determining borate glass properties. In addition, the Krogh-Moe-Griscom superstructural unit model for the structure of borate glasses is updated in line with more modern experimental data, and used to account for the way in which the fraction of 4-fold co-ordinated boron atoms in lithium, sodium and silver borate glasses varies with composition. It is concluded that the "anomalous" properties of vitreous B2O3 and binary M2O-B2O3 glasses are entirely consistent with respect to the electron configuration and bonding of the boron atoms and to the presence of superstructural units. Extrema in the properties of the binary glasses, when plotted as a function of composition, arise due to the interplay between two (or more) opposing factors. The major remaining question concerns the origin and relative magnitudes of the stabilisation energies for the various superstructural unit species.