Vibrational spectroscopy of silicate liquids and glasses

The application of vibrational spectroscopy to the study of silicate liquids and glasses is described, and new Raman data for K₂Si₂O₅ and K₂Si₄O₉ compositions are presented. The timescale of the vibrational spectroscopic experiments relative to relaxation timescales in the melts is discussed. Silicate systems are usually described as "liquid" or "glassy" based on experimental measurements of viscosity or heat capacity. These have a much longer characteristic measurement timescale than the vibrational spectroscopic experiments. Because of the long structural relaxation times for silicate frameworks over the normal laboratory temperature range, silicate "glasses" and "liquids" always show the same, unrelaxed response to the vibrational spectroscopic experiment. This is one reason for the observed close similarity between "glass" and "melt" spectra. Vibrational spectroscopy can readily be used to investigate structural changes which occur within supercooled silicate liquids due to structural relaxation on the laboratory timescale, above the glass transition temperature, T_g. The vibrational spectroscopies are complementary to other spectroscopic methods, including nuclear magnetic resonance, for this type of study. Our new Raman spectroscopic results on K-disilicate and -tetrasilicate glasses and liquids show effects due to structural relaxation above T_g. The spectra for K₂Si₂O₅ show evidence for an increase in the concentration of Q² silicate species with increasing temperature. We have determined the enthalpy change for the 2Q³ Q² + Q⁴ speciation reaction in K₂Si₂O₅ to be ∼ 20 kJ mol^-1, of the same order of magnitude as those obtained for liquids near Na₂Si₂O₅ composition by previous workers. For K₂Si₄O₉ glass, the Raman data show evidence for a different type of structural relaxation. The intensity of a peak near 590 cm^-1 increases with increasing temperature above T_g, which is interpreted as an increase in the proportion of three-membered siloxane rings in the liquid. The enthalpy change for formation of these three-membered rings is also 20 kJ mol^-1, consistent with the results of a previous study on SiO₂ glass.