Abstract
Glasses in the system 2 B2O3-Na2O-Na2F2 were heat-treated at 450° and 500°C in order to locate the position of F ions in the glass. The crystallization rate was measured as a function of Na2O fraction, and precipitated phases were identified by the X-ray diffraction method. From glasses of Na2O rich composition the diborate (2 B2O3⋅Na2O) phase precipitated predominantly, and from NaF rich compositions the B2O3⋅NaF phase precipitated along with NaF crystal. B2O3⋅NaF phase was considered to have the diborate structure and to include F ions attached to 4-fold coordinated boron atoms in the diborate structure. It was implied that some fraction of NaF component is isolated in the glass network. Crystal growth rate became minimum at intermediate compositions. Theoretical analysis has shown that the fusion entropy ΔSf becomes maximum at these compositions and that oxygen atoms and fluorine atoms are randomly distributed in the glass.
