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 Q¹², Q²¹ 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 a₃ = 1, a ₂ = 1.26, a₁ = 1.53 and a₀ = 2.56 are allowed in the determination of Qⁿ-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 2Qⁿ Q ^{n - 1} + Q^{n + 1} was determined. It was found that equilibrium constants for di- and metasilicate glasses, k₃ = 0.015 and k₂ = 0.07 are in close agreement with NMR studies. The k ₃ and k₂ 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 2Q¹ Q ² + Q⁰ was found to be 0.13 ± 0.03.