RAMAN AND INFRARED STUDY OF PRESSURE-INDUCED STRUCTURAL-CHANGES IN MGSIO3 CAMGSI2O6 AND CASIO3 GLASSES

Raman and midinfrared spectra of MgSiO3, CaMgSi2O6, and CaSiO3 glasses have been measured to 45 GPa in a diamond cell. The principal pressure-induced changes in spectra observed are reversible, so that characterization of the high-density states of these materials requires in situ measurements at high pressure. The dominant change in all three glasses is a marked shift of the midfrequency Raman bands (at 625-650 cm-1) to higher frequencies with increasing pressure. This indicates that the main compression mechanism is associated with a decrease in the average intertetrahedral Si-O-Si angles, as found in other silicate materials. The intensities of high-frequency Raman and infrared bands at 900-1200 cm-1 decrease with pressure relative to those of lower frequency bands. This is attributed to distortions of SiO4 tetrahedra. However, the Raman and infrared spectra provide no direct evidence for a distinct [4]Si to [6]Si coordination change at room temperature in these glasses over the pressure range of this study. We present evidence that pressure-induced structural changes in glasses, including changes in cation coordination, can be affected by sample preparation and trace impurities.