Raman spectroscopy of fully polymerized oxide, halide, and aluminosilicate glasses at high pressure

Under isothermal compression at 77 K, amorphous ice is known to undergo an apparent first-order transition between two distinct amorphous phases that differ only in density. In this dissertation this unusual “polyamorphic” transformation between two amorphous phases is investigated for several open framework glasses using Raman spectroscopy and the high-pressure diamond anvil cell. Careful measurements of the absolute Raman scattering intensity as a function of pressure for silica (SiO2) and germania (GeO2 ) glasses show a dramatic decrease of the main symmetric stretching band at the onset of the gradual formation of high coordinate oxygen species and the corresponding pressure interval in which permanent structural changes occur in both glasses. By comparison of the in situ Raman spectra to that of silica, the inferred coordination change of two “charge-balanced” aluminosilicate glasses, NaAlSi3O6 and NaAlSi2 O6, is reduced. In contrast to SiO2 and GeO 2 glasses, the aluminosilicate samples recovered from pressures as high as 15 GPa show no densification in their spectra. A mechanism for the moderate-pressure “memory” effect as well as for the densification which takes place at significantly higher pressure is proposed. For vitreous zinc chloride, a “weakened” silica analog, the abrupt crystallization of the high-pressure CdCl2-structured phase precludes a polyamorphic transition. However, a transition from a low-density glass phase to a high-density glass phase is directly observed in vitreous ZnCl2 at low temperature (∼77 K) where nucleation of the high-pressure crystalline phase is suppressed, suggesting that a density-driven phase separation in the underlying free energy surface of the liquid controls the high-pressure behavior of zinc chloride and other glasses which have competing structural configurations. Although a “strong-fragile” transition in glassforming melts is proposed to be responsible for the transition observed in vitreous ZnCl2 and has been implied by computer simulations for silica, direct experimental evidence is lacking. Using the correlation of the low-frequency vibrations in glasses to the strong-fragile behavior, the dynamic correlation length calculated from the low-frequency Raman scattering of SiO2 and GeO2 as a function of pressure indicates possible dramatic changes in the melt properties of silica at pressure and further supports a liquid-liquid phase separation in the supercooled regime of tetrahedral framework glasses.