A test for the existence of isomorphs in glass-forming materials

TL;DR

This paper introduces a method to identify isomorphs in glass-forming materials by analyzing thermodynamic invariances, challenging traditional correlation-based identification and providing insights into pressure densification behavior.

Contribution

It presents a novel approach to detect isomorphs in real materials beyond strong U-W correlation, supported by molecular dynamics simulations.

Findings

Systems with strong U-W correlation do not undergo pressure densification.

The method distinguishes materials with and without isomorphs based on thermodynamic invariance.

Simulations show pressure densification is absent in systems with isomorphs.

Abstract

We describe a method to determine whether a material has isomorphs in its thermodynamic phase diagram. Isomorphs are state points for which various properties are invariant in reduced units. Such materials are commonly identified from strong correlation between thermal fluctuations of the potential energy, U, and the virial W, but this identification is not generally applicable to real materials. We show from molecular dynamic simulations of atomic, molecular, and polymeric materials that systems with strong U-W correlation cannot be pressure densified; that is, the density obtained on cooling to the glassy state and releasing the pressure is independent of the pressure applied during cooling.