Structural rules of phase separation in alkali silicate melts analyzed by high-temperature Raman spectroscopy

The structure of alkali silicate melts with the compositions around the immiscibility dome, i.e. xR2O-(100 − x)SiO2 (R = Li, Na; x = 15, 25, 33) were investigated by high-temperature Raman spectroscopy. The distributions of structural units of Qn were estimated as a function of temperature and composition through the curve fitting of spectra based on the Qn equilibration 2Q3 ↔ Q2 + Q4. The Qn distributions of lithium silicate melts were insensitive to temperature, while those of sodium silicate melts showed a temperature dependence especially in the high sodium concentration region. By using the temperature and composition dependences of the Qn distribution, a new expression of the non-ideal entropy of mixing (ΔSmix) for the silicate system is proposed. In order to clarify the relationship between the Qn equilibration and the entropy change of alkali silicate melts, the Qn distributions were summarized as isothermal Qn curves on the Q2–Q3–Q4 ternary diagram (Qn diagram) on a contour map of ΔSmix. The Qn diagrams reveal unique characteristics of the melt under the immiscibility condition. It is suggested that phase separation takes place in a specific composition range, where the system cannot decrease entropy by structural change via the equilibration of Qn species.