Borate melt structure: a short review

The historical development of the study of borate melt structure is briefly reviewed. Focus is on the short range ordering, and in particular the temperature dependence of the boronoxygen coordination number, n(BO)(T), and its relationship to melt composition. Early in the 20th century measurements of borosilicate melt viscosities revealed rich compositiontemperature behavior, and by mid-century it was suggested that this was related to a decline in n(BO)(T) with T increasing. Measurements of n(BO)(T) began as early as 1972 when Stepanov & Novikov unwittingly demonstrated the fictive temperature, Tf, dependence of n(BO) in aluminoborosilicate glass fibres using B-11 NMR. Since that time the number of studies of n(BO)(T) grew rapidly, including further ex-situ B-11 NMR, but also spurred by the development of in-situ high temperature Raman and NMR spectroscopies, x-ray and neutron diffraction, and statistical, thermodynamic and molecular dynamics modelling methodologies. In recent years ab-initio and polarizable ion molecular dynamics, based on density functional theory, and non-resonant inelastic x-ray scattering techniques have been applied to borate melts. This wealth of effort has led to the understanding that B-O coordination typically declines with temperature increasing. The magnitude of such changes are strongly composition dependent, ranging from Delta n(BO)/Delta T approximate to 0 in e.g. aluminoborates and low modifier sodium borates, up to |Delta n(BO)/Delta T| <= 1 x 10(-3) K-1 in e.g. alkali diborates and borosilicates. The possibility of Delta n(BO)/Delta T > 0 remains, and although not observed to date, has been predicted for compositions in the vicinity of sodium metaborate. Directions for future research include: in-situ study of the competing effects of temperature and pressure on n(BO)(P,T); the interaction of temperature induced B-O coordination change with other variable coordination network cations (Al3+, Ga3+, Ge4+, Ti4+); further accurate and extensive measurements of n(BO)(T) to provide stringent tests of models.