Temperature dependence of the refractive index of Al2O3-Na2O-SiO2 melts: Role of electronic polarizability of oxygon controlled by network structure

Refractive indexes for the Al2O3-Na2O-SiO2 system have been measured using an ellipsometer for a wavelength of 632.8 nm over a wide temperature range (1100 to 1800 K). Two kinds of sample were used: xAl2O3-(40-x)Na2O-60SiO2 and yAl2O3-yNa2O-(100-2y)SiO2, where x ranged between 6 and 20 mol pct and y between 12.5 and 25 mol pct. In the former samples, the temperature coefficient of refractive indexes changed from negative to positive on increasing the concentration of Al2O3. In the latter samples, the refractive indexes increased monotonically with decreasing concentration of SiO2, and the temperature coefficient was always positive. It has been found that the temperature dependence of refractive indexes in these melts is determined by the coefficient of thermal expansion, which would be relevant to the degree of polymerization of the melts. In addition, the electronic polarizability of oxygen derived from the refractive indexes increased with increasing temperature in each melt. This suggests that the basicity of the alumino-silicate melts increases as temperature increases. The positive temperature coefficient of the electronic polarizability of oxygen can be attributed to an increase in the distance between cation and oxygen ion due to thermal expansion. The dependence of the electronic polarizability of oxygen on the concentration of Al2O3 has also been discussed in terms of the electronic polarizabilities of three types of oxygen contained in the melts.