Structural investigation of sodium borate glasses and melts by Raman spectroscopy. II. Conversion between BO 4 and BO 2O − units at high temperature

Abstract

Raman scattering spectra were recorded for pure boron oxide and sodium borate glasses and their melts at the temperature ranging from room temperature to 1200 °C to investigate the structural changes occurring in the melts. The amounts of short-range order structures (SRO), BO 2O − and BO 3, were estimated from the high frequency bands at 1100–1600 cm −1. The ratio of 4-fold coordinated boron oxide BO 4, N 4, at high temperature could be derived for the glass melts as a function of Na 2O concentration. In Na 2O ⩽20 mol% region, N 4 showed a slight decrease while the remarkable decrease of N 4 was found in the region of Na 2O ⩾25 mol% with increasing temperature. The enthalpy of the equilibrium reaction, BO 4↔ BO 3+ NBO monotonously increased from 20 to 80 kJ/mol with an increase in Na 2O. A characteristic temperature T X SRO was found to exist, which defines the temperature sensitive–nonsensitive transition point of the above conversion of the structures of SRO, and did not necessarily coincide with the glass transition temperature. T X SRO plots were located between the liquidus line and the immiscibility dome in the phase diagram. Using this temperature and composition relationship, anomalous behavior of isothermal viscosity and strong-fragile characteristics of melts could be explained quantitatively.