Determination of refractive index and electronic polarisability of oxygen for lithium-silicate melts using ellipsometry

The refractive index of Li2O–SiO2 melts has been measured using the ellipsometer constructed for high-temperature use. Measurements were carried out on four samples having different compositions over a wide temperature range (1300–1800 K). The standard deviation for the values measured was within ±0.0005. Additions of Li2O were found to cause increases of refractive index in the temperature range investigated. At Li2O concentrations <25 mol% the refractive indices of the melts increased with increasing temperature, similar to SiO2. However, this temperature dependence became smaller with additions of Li2O and at Li2O concentrations more than 30 mol% the refractive indices of the melts decreased with increasing temperature. The temperature dependence of the refractive indices has been discussed from those of the density and the molar electronic polarisability on the basis of the Lorentz–Lorenz equation. In addition, the electronic polarisability of oxygen derived from the molar electronic polarisability increased with increasing temperature in each melt, suggesting that the ionicity of oxygen increases as temperature increases. Furthermore, additions of Li2O were found to cause increases in the electronic polarisability of oxygen, due to the formation of non-bridging oxygen ions.