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. xR(2)O-(100-X)SiO2 (R = Li, Na; x = 15, 25, 33) were investigated by high-temperature Raman spectroscopy. The distributions of structural units of Q(n) were estimated as a function of temperature and composition through the curve fitting of spectra based on the Q(n) equilibration 2Q(3) <-> Q(2) + Q(4). The Q(n) 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 Q(n) distribution, a new expression of the non-ideal entropy of mixing (Delta S-mix) for the silicate system is proposed. In order to clarify the relationship between the Q(n) equilibration and the entropy change of-alkali silicate melts, the Q(n) distributions were summarized as isothermal Q(n) curves on the Q(2)-Q(3)-Q(4) ternary diagram (Q(n) diagram) on a contour map of Delta S-mix. The Q(n) 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 Q(n) species. (c) 2005 Elsevier B.V. All rights reserved.