High-pressure phase transformations in liquids and amorphous solids

We outline the current state of experimental study and basic ideas for describing phase transitions in topologically disordered condensed matter, such as liquids and amorphous solids. Reviewing briefly the study of molten elementary substances under pressure, we pay primary attention to the results for liquid Se, S, and P and also to those substances that have not been represented in previous publications, mainly the liquid oxides B2O3 and GeO2. The experimental data reveal the possibility of rather sharp transformations in relatively simple liquids that are smoothed at high temperatures. Comparing the transitions in amorphous solids and in liquids, one should emphasize the metastable and non-ergodic nature of amorphous substances and the existence of static local atomic stresses fluctuating in thermally frozen amorphous networks. In particular, the kinetic study of amorphous-amorphous transformations (AATs) in SiO2 and GeO2 glasses and amorphous H2O ice under pressure highlights a number of anomalous features that distinguish the AATs from ordinary first-order transitions and from transformations in liquids. The recent in situ study of the volume changes in glassy silica a-SiO2 upon compression at high temperatures provides a new conclusion as regards the existence of two pressure-induced AATs in a-SiO2 with different microscopic mechanisms of structural rearrangements. We also perform the analysis of two possible kinetic scenarios for AATs, including sharp and diffuse transitions. The key relation determining the transformation scenario is the relationship between the radius of structural correlations in amorphous solid and the size of the critical nucleus of the growing disordered modification. The comparative analysis emphasizes the main difference between the transformations in liquids and amorphous solids that consists in the fact that the transitions in liquids are mainly determined by thermodynamic relationships, whereas the transitions in amorphous solids take place far away from equilibrium and are governed by the corresponding kinetics.