Structure of alkali-silicate and germania glasses at high temperature and pressure

Abstract

Advanced multi-channel micro-Raman spectrometry has been used for investigating the structure of alkali-silicate and GeO 2 glasses and melts at ambient pressure and high temperatures as well as the structure of Rb 2Si 4O 9 glass at pressures to 247 kbar at room temperature. The results of the high-temperature Raman study show that below the respective glass transition, T g, there is little change in the structure of glasses except for a small decrease in Raman band frequencies with increasing temperature. Above T g the so-called ‘defect’ bands, attributed to either broken TO bonds (where T is Si, Ge, Al) or to three-membered rings of TO 4 tetrahedra, show a sudden increase in intensity with increasing temperature. The rate of this intensity increase with temperature is inversely related to viscosity. Another important change observed in GeO 2 (and most likely SiO 2) glass above T g is the decrease in the relative intensity of the highest frequency band of the LO-TO couple, suggesting partial decrease in intermediate range order in the melt relative to the glass. Changes in the high-frequency band envelope of Rb 2Si 4O 9 glass/melt suggest homogenization of the environment of Q 3 species with increasing temperature. High-pressure Raman spectra of Rb 2Si 4O 9 glass exhibit a significant increase in the relative intensity of the 595 cm −1 ‘defect’ band with pressure, especially in the pressure range 100–150 kbar where the intensities of Raman bands associated with non-bridging and bridging oxygen vibrations of SiO 4 tetrahedra rapidly diminish. Simultaneous with these intensity changes is the decrease in frequency of the SiO stretching envelope, indicating an increase in the average coordination of silicon at pressres ≥ 100 kbar.