A Study Of Sio2 Glass And Supercooled Liquid To 1950 K Via High-Temperature Raman-Spectroscopy
We have measured in situ Raman spectra of SiO2 glass between room temperature through the glass transformation range to the supercooled liquid at 1950 K, using a new wire loop heating technique. These results provide valuable insights into the structural and dynamic behaviour of this prototype silicate system at magmatic temperatures. The principal low frequency Raman band in the glass shows an anomalous increase in frequency with increasing temperature, indicating that the average SiOSi angle narrows as temperature is increased, due to anharmonic vibrational effects. Configurational changes at temperatures above T-g result in further increases in the band frequency. These effects are responsible for the anomalous volume behaviour of SiO2 glass and supercooled liquid with increasing temperature, including the low thermal expansivity over most of the temperature range, and the occurrence of density maxima at low and high temperatures. From experiments on glasses with different fictive temperatures, there is an increase in the intensity of the 606 cm(-1) band, due to an increase in the proportion of three-membered ring ''defect'' species in the glass above T-g. This indicates that structural relaxation in the liquid state is accompanied by Si - O - Si bond breaking and reforming reactions, consistent with the measured magnitude of the activation energy for viscous flow. This observation is combined with recent results on the mechanism of structural relaxation in silicate liquids to propose an energetic model for viscous relaxation in SiO2, involving O2- transfer via formation of a Si-V intermediate. This model is then generalized to ternary aluminosilicate systems, to provide a rationalization for the observed viscosity maximum at constant silica content, in high silica rock-forming melt compositions.