Borate and related glasses

Boron (B) is in group III of the Mendeleev table and is a network former with strong Dietzel field. It tends to bond to three different oxygen ions adopting a triangular coordination.

Tetrahedral coordination is also found. As regards the three coordinated B atoms, it has been suggested that these form boroxyl groups B3O6.These groups are planar and linked to form a three-dimensional network by boron–oxygen–boron bonds. Such a structure is in good agreement with X-ray diffraction results. Such a structure is then much different from that of silica. When adding alkali metals to borates the induced changes are opposite to those generated in silica; this is called in the literature the ‘boron oxide anomaly’. In fact, the viscosity may increase and the thermal expansion decrease when adding alkali metal, contrary to what is generally observed in silica-based glasses. It is believed that such a trend could be related to a change in the coordination of boron atoms. Borosilicate glasses play an important role in glass manufacturing since they are corrosion resistant to water and allow for low thermal expansion. They are composed of Si and B atoms which form the network, Si being four coordinated and B three coordinated. The resulting network is softer than silica with lower viscosity and lower melting temperature. Contrary to what happens when modifiers are introduced in silica, the bond potential remains symmetrical in borosilicates inducing low variation in the mean distances with increasing temperature, that is, a low thermal expansion. In borosilicate glasses, alkali metal ions like Na+ ions will either modify the network introducing NBOs or change the boron coordination (from3 to 4). The latter phenomenon is believed to be responsible for the ‘boron oxide anomaly’ where viscosity may increase with increasing alkali metal concentration as stated for borate glasses.