COORDINATION CHANGES AND THE VIBRATIONAL-SPECTRUM OF SIO2 GLASS AT HIGH-PRESSURE

We investigated the persistence of tetrahedral coordination in SiO2 glass under high pressures by comparing experimental infrared spectra with the calculated spectra of Monte Carlo simulations of SiO2 glass. The calculations are self-consistent: we used the same potential model, which prevents bonded Si-O coordinations higher than four, to generate the simulated structures and to calculate the eigenfrequencies and eigenvectors. The simulated, tetrahedrally coordinated structures agree well with available structure and equation-of-state data and, at higher pressures, are expected to differ from the real glass only if the latter undergoes coordination changes. The calculated vibrational density of states and infrared spectrum at zero pressure agree well with the data. The calculations reproduce most of the important features of the observed high-pressure infrared spectrum, including the features thought to be indicative of the onset of octahedrally coordinated Si at high pressures. We therefore suggest that the infrared spectra do not yield a unique structural interpretation and leave open the possibility that SiO2 glass preserves its tetrahedral coordination to at least 40 GPa.