ALKALI FLUOROBORATE GLASSES

Alkali fluoroborate glasses are readily formed in the compositional ra nge from zero alkali fluoride to 45 mol% LiF or 55 mol% NaF or KF. The density refractive index, thermal expansion coefficient, glass transf ormation temperature, transformation range viscosity and electrical co nductivity of these glasses have been measured. Trends in the properti es of these glasses as a function of composition exhibit behaviour sim ilar to, but not identical with, that observed for alkali borate glass es including the berate anomaly in thermal expansion coefficient and g lass transformation temperature. A new berate anomaly has been found f or the refractive index of these glasses. A structural model based on the replacement of BO4 tetrahedra by BO3F tetrahedra is proposed which explains the differences in behaviour between these glasses and the c orresponding alkali berate glasses.