Amorphous-amorphous transitions in silica glass. II. Irreversible transitions and densification limit

In part I, we showed that the thermomechanical anomalies of silica glass are due to the reversible structural transitions that affect the intermediate-range order and that are brought about by atomic displacement modes similar to those underlying the alpha-to- beta phase transformations in cristobalite silica. In this paper, we show that polyamorphic transitions can be irreversible, in particular under large compressive stresses, and provided adequate thermal activation for the necessary bond exchanges to take place. Under these conditions a stable high-density form of amorphous silica develops, which is characterized by larger ring sizes but unchanged near-range structural order. In this high-density amorphous state another anomalous feature of silica glass, i.e., negative thermal expansion, is accentuated. Pressure and temperature effects on the permanent densification of silica, as well as the nature of the newly discovered amorphous phases and its implications on the physical properties of amorphous silica are discussed.