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

• We present systematic investigation of sodium silicate melts from O to 67 mol% Na2O by in-situ Raman spectroscopy.
• We used a curve-fitting procedure taking into account the second coordination sphere of Si atoms.
• The conversion coefficients from characteristic Raman bands to concentrations of structural units were calculated.
• Qn-abundance in sodium silicate melts in the range from 0 to 67 mol% Na2O was determined.
• The equilibrium constants kn of the reactions 2Qn ↔ Qn − 1 + Qn + 1 were determined.

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 Q32 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, a2 = 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 kn of the reactions 2Qn ↔ Qn − 1 + Qn + 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 k3 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 ↔ Q2 + Q0 was found to be 0.13 ± 0.03.