Effect of fluoride ion incorporation on the structural aspects of barium–sodium borosilicate glasses

Barium–sodium borosilicate glasses containing upto 6 wt% fluoride ions were prepared by conventional melt quench method and characterized by 19F, 29Si and 11B nuclear magnetic resonance (NMR) techniques.19F NMR studies have confirmed the presence of mainly linkages like F–Si(n) or F–B(n) along with F–Ba(n). Their relative concentrations are unaffected by F− content in the glass. Incorporation of fluoride ions in the glass is associated with significant reduction in the nonbridging oxygen concentration attached to silicon, as revealed by the increase in the concentration of Q3 structural units of silicon at the expense of Q2 structural units. 11B NMR studies have established that the relative concentrations of BO3 structural units are higher for F− ion containing glasses compared to the one without F− ion incorporation. The observed increase in the relative concentrations of Q3 structural units of silicon and BO3 structural units with fluoride ion incorporation in the glass has been attributed to the formation of F–Ba(n) type of linkages, thereby reducing the concentration of network modifying cations for breaking the Si–O–Si/B–O–B linkages. Formation of such structural units weakens the glass network thereby decreasing the glass transition temperatures.