Compression mechanism in aluminosilicate melts: Raman and XANES spectroscopy of glasses quenched from pressures up to 10 Gpa

Raman and XANES spectroscopy were carried out on a series of glasses of composition 44CaO–12Al2O3–44SiO2, formed at pressures up to 10 GPa by isobaric quench from a temperature of 2200°C. The most significant changes in the Raman spectrum as a function of the synthesis pressure, or density, of the glass occur in the low-frequency region (300–700 cm-1), associated with T–O–T bending vibrations. With increasing density of the glass, the overall intensity at low frequencies decreases relative to the high-frequency portion of the spectrum. Relative intensities of bands within the low-frequency region of the Raman spectrum are also very sensitive to synthesis pressure, whereas there is little evidence that pressure influences Q-speciation as the high-frequency region of the spectrum remains virtually unchanged. With initial compression (V/V0=1–0.96), the severe loss in intensity near 500 cm-1 indicates coordination of bridging oxygen atoms to an additional cation, which inhibits the vibrational motion that gives rise to this band normally observed for silicate glasses formed at ambient pressure. At higher densities(V/V0<0.96), bands in the low-frequency region are shifted to higher frequencies, indicative of narrower T–O–T angles. No significant changes are observed in the Si and Ca K-edge XANES spectra with increasing densification of the glass. The Al K-edge spectra also show no significant changes among the lower density glasses (V/V0=1–0.96), but reveal a feature near 1570 eV that dramatically increases in relative intensity with increasing densification beyond V/V0=0.96. The observations from both Raman and XANES spectroscopy are consistent with two different compression mechanisms operating in different pressure ranges. At lower pressures, the spectroscopic data are characterized by features that we attribute to the presence of triclusters(OT3 units) in the quenched melt. At higher pressures, T–O–T angle reduction and also an increase in the average coordination number of Al are likely to occur to further reduce the volume of the melt. The complex response of the structure of aluminosilicate melts to compression suggests that their physical properties will also behave complexly as a function of pressure.