Polyamorphic Transitions in Network-Forming Liquids and Glasses

The occurrence of polyamorphism in stable liquids, supercooled liquids, and solid amorphous materials is reviewed. Tetrahedrally bonded network systems have been the primary focus of many experimental and computer simulation studies of amorphous phase transitions. Of these network-forming systems, H₂O and Si have been shown to undergo a first-order phase transition between two distinct amorphous phases. Nevertheless, many other tetrahedral networks, such as SiO₂ and GeO₂, show a much more gradual structural change. This brings up questions to the generality of polyamorphic behavior in network glasses. A proposed explanation for the variations in behavior among tetrahedral glasses is given in terms of the effective yield strength, percent volume collapse, and cooperativity. Comparison with crystal-to-amorphous phase transformations in compositionally similar systems yields additional insight.