Structure of Na2O-SiO2 melt as a function of composition: In situ Raman spectroscopic study

The structure of x%Na2O . (100 - x)%SiO2 glasses and melts (x = 33, 40, 50, 55, 60, 67) was studied by in-situ Raman spectroscopy at temperatures from 293 to 1462 K. The curve-fitting procedure was applied for data analysis taking into account the second coordination sphere of Si atoms. To obtain a correct interpretation of all bands in Raman spectra, additional types of structural units Q(12), Q(21) and Q(32) were introduced into a description of the glass and melt structures. The conversion coefficients from characteristic Raman bands to concentrations of the corresponding structural units alpha(3) = 1, alpha(2) = 1.26, alpha(1) = 1.53 and alpha(0) = 2.56 are allowed in the determination of Q(n)-abundance in sodium silicate melts in the range from 0 to 67 mol% Na2O. From these data the dependence of the equilibrium constants k(n), of the reactions 2Q(n) <-> Q(n - 1) Q(n + 1) was determined. It was found that equilibrium constants for di- and metasilicate glasses, k(3) = 0.015 and k(2) = 0.07 are in close agreement with NMR studies. The k(3) and k(2) increase with increasing temperature up to 0.046 at 1243 K and 0.14 at 1462 K, respectively, which indicates an increase of the system disorder. The equilibrium constant of the reaction 2Q(1) <-> Q(2) + Q(0) was found to be 0.13 +/- 0.03. (c) 2013 Elsevier B.V. All rights reserved.