Molecular dynamics study of the structure of borate glasses

Boron plays an important role in numerous glasses. However, its role in glass structure is far from being completely understood. This may be due to the complexity of the boron-oxygen bond such as its partial convalency, p bond characteristics and easy structural conversion from BO3 triangle to BO4 tetrahedron.;The borate glass systems, vitreous B2O3, alkali borate and borosilicate, are investigated by molecular dynamics (MD) computer simulations, adopting a coordination dependent potential scheme.;The simulated vitreous B2O3 is comprised mainly of randomly connected BO3 triangles and some boroxol rings. The pair distribution functions and bond angle distributions are well matched to the experimental data. The fraction of boron atoms in boroxol rings in the simulated glass is 13--18%, which supports the interpretation of the reverse Monte Carlo and Monte Carlo calculations, but it is somewhat lower than that inferred from neutron diffraction results where values up to 80% are claimed.;The simulated sodium borate and lithium borate glasses consist of not only BO3 and BO4 basic structural units, but also superstructural groupings such as boroxol rings, triborate and di-triborate units, etc. The addition of alkali converts BO3 triangles to BO4 tetrahedra and generates nonbridging oxygens at the same time. N4, the fraction of tetrahedrally coordinated boron, reaches a maximum at 40 mol% alkali oxide. Sodium and lithium metal ions are found near nonbridging oxygens and nonbridging oxygens are associated mainly with BO3 triangles rather than BO 4 tetrahedra. The boron anomaly seems to be related to a formation of nonbridging oxygen around 20--30 mol% of alkali metal oxide. The difference in behavior between lithium and sodium ions is explained using acidity/basicity concepts.;To simulate alkali borosilicate glasses, a two-body potential for silicon-oxygen and three-body potentials for silicon-oxygen-silicon and oxygen-silicon-oxygen bond angles were developed for the oxygen-oxygen potential parameter used in the model of vitreous B2O3. Simulated pure (alkali-free) borosilicate glass consists of BO3 triangles and SiO4 tetrahedron. The structure of simulated sodium borosilicate glass depends on R and K, where R is the ratio of alkali oxide to boric oxide and K, that of silica to boric oxide.