Polyamorphic transitions in silica glass

Polyamorphic transitions in silica are studied under pressure up to 26.2 GPa by Raman and Brillouin spectrocopies. The first reversible polyamorphic transition is characterized by an abrupt decrease of the Si–O–Si inter-tetrahedral angle θ at 2.5 GPa corresponding to the elastic anomaly of silica. This result is interpreted as a reversible transition between two Low Density Amorphous phases LDAβ ↔ LDAα similar to the β–α transition of cristobalite crystal under pressure. The second polyamorphic transition is observed for silica glass samples compressed up at different maximum pressures Pmax between 12 GPa and 26.2 GPa and recovered at ambient pressure. It corresponds to an irreversible and progressive transformation from the LDAα phase to a High Density Amorphous (HDA) phase. This HDA phase is characterized by an increase of the statistic distribution of 4- and 3-membered rings of SiO4 tetrahedra with a narrowing of inter-tetrahedral angle distribution. A vanishing of the elastic anomaly in the HDA phase is observed.