Natalia M. Vedishcheva
Institute of Silicate Chemistry of the Russian Academy of Sciences,Nab. Makarova 2, St. Petersburg, 199034, Russia
It is known that, in alkali borate glasses and melts, the short-range order is determined by presence of basic structural units, viz. boron-oxygen tetrahedra and triangles with and without non-bridging oxygen atoms (NBOs), and the intermediate-range order is characterised by set of various superstructural units (rings). A thermodynamic approach based on the model of associated solutions, together with information about the structure of thermodynamically stable crystals existing in the system, is used for calculating the distribution of the basic structural units and superstructural rings in lithium borate glasses and melts over the glass-formation region. The calculated concentration dependences for the content of boron-oxygen tetrahedra and NBOs in glasses and melts are in good agreement with the experimental data available in the literature. An analysis of these dependences, in terms of the concept of chemical structure, enables some structural features experimentally found in glasses and melts as compared to crystals, to be quantitatively explained. Among them are an excess number of NBOs in glasses containing more than 20 mol. % Li2O, and a deficit of NBOs in glasses in the region above 50 mol. % Li2O. This approach also allows the polymerization degree of the borate network as a function of the glass composition to be analysed.