Intermediate state in pressurized silica glass: Reversibility window analogue

We propose that silica glass exists in a special intermediate state for pressures between {~} 3 and 5 GPa, which can be identified as an analogue of the ``reversibility window'' observed in chalcogenide glasses. In this state there is a gradual reduction of the degree of low-energy flexibility of the structure, which matches a gradual change in the average coordination number. At lower pressures (P<~3GPa) the structure can accommodate buckling of the structure network without any deformations of the SiO₄ polyhedra, and at higher pressures (P>~5GPa) the structure is rigid and deforms through rearrangements of the bonding on a local scale. For pressures within the 3-5-GPa window, there is balance between the two mechanisms, with the flexibility assisting in the global rebonding processes. The most dramatic manifestation of the window is seen at high temperature, where there is a much greater relative volume decrease on heating at pressures within the window than for other pressures, with the window widening at higher temperatures.