Generic Maximum-Valence Model for Fluid Polyamorphism

TL;DR

This paper introduces a versatile maximum-valence model that captures liquid polyamorphism across various fluids by incorporating attractive, covalent, and repulsive interactions, explaining phase transitions and anomalies.

Contribution

It presents a generic maximum-valence framework applicable to any coordination number, unifying the modeling of fluid polyamorphism and phase behavior.

Findings

Liquid-liquid phase transitions occur for various coordination numbers.

The model reproduces density anomalies and negative thermal expansion regions.

Phase diagrams show transitions for dimerization, polymerization, and network formation.

Abstract

Recently, maximal valence model has been proposed to model liquid-liquid phase transition induced by polymerization in sulfur. In this paper we present a simple generic model to describe liquid polyamorphism in single-component fluids using a maximum-valence approach for any arbitrary coordination number. The model contains three types of interactions: i) atoms attract each other by van der Waals forces that generate a liquid-gas transition at low pressures, ii) atoms may form covalent bonds that induce association, and iii) additional repulsive forces between atoms with maximal valence and atoms with any valence. This additional repulsion generates liquid-liquid phase separation and the region of negative heat expansion coefficient (density anomaly) on a P-T phase diagram. We show the existence of liquid-liquid phase transitions for dimerization, polymerization, gelation and network formation for corresponding coordination numbers z = 1, 2, ..6 and discuss the limits of this generic model for producing fluid polyamorphism.