Thermodynamics of Fluid Polyamorphism

TL;DR

This paper presents a universal phenomenological framework for understanding fluid polyamorphism, describing how different amorphous states can coexist or interconvert in single-component fluids under various conditions.

Contribution

It introduces a generic thermodynamic model that explains fluid polyamorphism regardless of molecular details, unifying different scenarios and linking them to phase transitions and molecular interconversion.

Findings

Polyamorphism can occur with or without phase separation depending on order parameter symmetry.

A two-state thermodynamic model describes equilibrium between interconvertible molecular structures.

The framework explains anomalous fluid properties and unifies diverse polyamorphic phenomena.

Abstract

"Fluid polyamorphism" is the existence of different condensed amorphous states in a single-component fluid. It is either found or predicted, usually at extreme conditions, for a broad group of very different substances, including helium, carbon, silicon, phosphorous, sulfur,tellurium, cerium, hydrogen and tin tetraiodide. This phenomenon is also hypothesized for metastable and deeply supercooled water, presumably located a few degrees below the experimental limit of homogeneous ice formation. We present a generic phenomenological approach to describe polyamorphism in a single-component fluid, which is completely independent of the molecular origin of the phenomenon. We show that fluid polyamorphism may occur either in the presence or the absence of fluid phase separation depending on the symmetry of the order parameter. In the latter case, it is associated with a second-order transition, such as in liquid helium or liquid sulfur. To specify the phenomenology, we consider a fluid with thermodynamic equilibrium between two distinct interconvertible states or molecular structures. A fundamental signature of this concept is the identification of the equilibrium fraction of molecules involved in each of these alternative states. However, the existence of the alternative structures may result in polyamorphic fluid phase separation only if mixing of these structures is not ideal. The two-state thermodynamics unifies all the debated scenarios of fluid polyamorphism in different areas of condensed-matter physics, with or without phase separation, and even goes beyond the phenomenon of polyamorphism by generically describing the anomalous properties of fluids exhibiting interconversion of alternative molecular states.