Short and intermediate range order in sodium borosilicate glasses: a quantitative thermodynamic approach

It is shown that the short range and intermediate range order in the structure of sodium borosilicate glasses can be quantitatively described using the thermodynamic concept of the chemical structure. This approach enables the distribution of the basic structural units (BO3, BO4-, BO2O-, BOO22-, Q(4), Q(3) and Q(2)), superstructural units (boroxol, pentaborate, triborate, diborate rings and cyclic metaborate anions) and 4-membered borosilicate rings (danburite and reedmergnerite units) to be calculated, without use of adjustable parameters, over the entire glass forming region. The calculations have been made considering structural changes in terms of the minimum Gibbs free energy of the system. A critical analysis has been performed of the fractions of 4-fold co-ordinated boron atoms, BO4-, and the glass transition temperatures, T-g, calculated by Mauro and co-workers on the basis of the topological principles of borosilicate glass chemistry.