Structure and phase transformation of alkali silicate melts analysed by Raman spectroscopy

Raman spectra of binary alkali silicates were measured at various temperatures from 1300°C to room temperature to investigate the relation between structural change and phase transformation phenomena. Distribution of structural units of Q n was estimated at each temperature by the deconvolution of spectra based on the equilibrium 2Q3 ↔ Q2 + Q4. The Q n distributions of sodium and potassium silicate systems strongly depend on temperature and the equilibrium shifts to the left-hand side with decreasing temperature, but those of lithium silicate system were less sensitive to the temperature variation. In alkali disilicates (33 mol% R2O–67 mol% SiO2, where R=Li, Na or K), the Q n distributions near the melting point were independent of alkali ion species, and they held the relation [Q2] = [Q4] ≈ [Q3]/4. This means that two of 6Q3 units (six-memberd ring) in crystals are transformed into a pair of Q2 and Q4 in the melting process. Below the melting point, the Q n distribution in lithium disilicate melt remained while, in sodium and potassium disilicate melts, [Q3] increased with decreasing temperature. Crystallization of the alkali disilicate melts is discussed considering the configuration entropy of Q n units. In 25 mol% Li2O–75 mol% SiO2, which is in the range of the immiscibility dome, the Q n distribution was maintained even when phase separation occurs in the cooling process.