High-pressure phase transformations in liquids and amorphous solids

Abstract

We outline the current state of experimental study and basic ideas fordescribing phase transitions in topologically disordered condensed matter, suchas liquids and amorphous solids. Reviewing briefly the study of moltenelementary substances under pressure, we pay primary attention to the resultsfor liquid Se, S, and P and also to those substances that have not beenrepresented in previous publications, mainly the liquid oxides B2O3 andGeO2.The experimental data reveal the possibility of rather sharp transformations inrelatively simple liquids that are smoothed at high temperatures. Comparing thetransitions in amorphous solids and in liquids, one should emphasize the metastableand non-ergodic nature of amorphous substances and the existence of static localatomic stresses fluctuating in thermally frozen amorphous networks. In particular,the kinetic study of amorphous–amorphous transformations (AATs) in SiO2 and GeO2 glasses andamorphous H2Oice under pressure highlights a number of anomalous features that distinguish theAATs from ordinary first-order transitions and from transformations in liquids.The recent in situ study of the volume changes in glassy silica a-SiO2upon compression at high temperatures provides a new conclusion asregards the existence of two pressure-induced AATs in a-SiO2with different microscopic mechanisms of structural rearrangements. We alsoperform the analysis of two possible kinetic scenarios for AATs, including sharpand diffuse transitions. The key relation determining the transformation scenariois the relationship between the radius of structural correlations in amorphous solidand the size of the critical nucleus of the growing disordered modification.The comparative analysis emphasizes the main difference between thetransformations in liquids and amorphous solids that consists in the factthat the transitions in liquids are mainly determined by thermodynamicrelationships, whereas the transitions in amorphous solids take place faraway from equilibrium and are governed by the corresponding kinetics.