A Raman study of the pressure-induced densification of SiO₂-based glass-ceramics

Here we report a structural characterization study of glass-ceramics as a function of pressure by Raman spectroscopy. We selected a glass-ceramics of Li₂O-Al₂O₃-nSiO₂ (n  ≈  8) doped with ZrO₂ and TiO₂. This composition induces slight structural modification in Si-O and Si-O-Si bonds in the glass matrix that are crucial to stabilize metastable hexagonal SiO₂ (β-quartz) solid-solution nano crystals corresponding to γ-LiAlSi₂O₆. This structure yields a more compact β-quartz-type phase that eventually stabilizes it in wider pressure range than pure SiO₂. Raman spectroscopy allows to unravel at least two pressure-induced structural phase transitions at about 6 and 15 GPa that could not be previously revealed by x-ray diffraction. We show that the phase-transition sequence can tentatively be described in terms of SiO₂-type structure as: β-quartz  →  Coesite I  →  Coesite II. The measured transition pressures are consistent with the larger cell volume attained in the γ-LiAlSi₂O₆ yielding a wider metastability pressure range of the β-quartz-type phase.