New Kinds of Phase Transitions: Transformations in disordered Substances

The transitions in disordered substances are discussed briefly: liquid–liquid phase transitions, liquid–glass transition and the transformations of one amorphous form to another amorphous form of the same substances. A description of these transitions in terms of many–particle conditional distribution functions is proposed. The concept of a hidden long range order is proposed, which is connected with the broken symmetry of higher order distribution functions. The appearance of frustration in simple supercooled Lennard–Jones liquid is demonstrated. It is well known for a long time that there exist sharp phase transitions between different aggregate states and sharp polymorphic phase transitions between different crystalline phases of the same substances. During last two decades a lot of experimental data was obtained on complicated phase diagrams of liquids and amorphous solids, too. Some of these results were presented on the first international conference that took place here in Russia in 2001 [1] and that can be considered as a formal claim of a new direction of physical investigations – transformations in disordered substances: liquid–liquid transitions and transformations of one amorphous form to another amorphous form of the same substances. The liquid–glass transition, although has longer history, has to be considered in the same context. It is now firmly established by different experimental techniques that sharp liquid–liquid transitions under pressure, formally similar to first–order phase transitions, exist as well as reversible transformations between amorphous states involving changes in local order structures and density. Usually a crystal melts with a conservation of the short–range order (SRO) structure type or into denser liquid with SRO structure similar to that of high pressure crystalline phase. It should be emphasized that the transitions in liquids are true phase transitions mainly determined by thermodynamic relationships, whereas the transitions in amorphous solids take place far away from equilibrium and are governed by the corresponding kinetics. A useful microscopic theory of these transitions is not developed yet and only empirical models and computer simulations have been used in practice to date. For example, interesting results were obtained by Stanley through molecular dynamics study basing