The zero-temperature phase diagram of soft-repulsive particle fluids

TL;DR

This study maps the zero-temperature phase diagrams of soft-repulsive particle models, revealing universal trends and complex behaviors in crystal stability and fluid reentrance relevant to soft matter and elemental solids.

Contribution

It provides the first comprehensive analysis of zero-temperature phase diagrams for various soft-repulsive potentials, including non-Bravais lattices and reentrant fluid phases.

Findings

Unbounded potentials show a transition from high- to low-coordinated crystal phases.

Bounded potentials exhibit intermittent compact structures with no stable phase.

Fluid phases reenter at intermediate pressures, indicating complex phase behavior.

Abstract

Effective pair interactions with a soft-repulsive component are a well-known feature of polymer solutions and colloidal suspensions, but they also provide a key to interpret the high-pressure behaviour of simple elements. We have computed the zero-temperature phase diagram of four different model potentials with various degrees of core softness. Among the reviewed crystal structures, there are also a number of non-Bravais lattices, chosen among those observed in real systems. Some of these crystals are indeed found to be stable for the selected potentials. We recognize an apparently universal trend for unbounded potentials, going from high- to low-coordinated crystal phases and back upon increasing the pressure. Conversely, a bounded repulsion may lead to intermittent appearance of compact structures with compression and no eventual settling down in a specific phase. In both cases, the fluid phase repeatedly reenters at intermediate pressures, as suggested by a cell-theory treatment of the solids. These findings are of relevance for soft matter in general, but they also offer fresh insight into the mechanisms subtended to solid polymorphism in elemental substances.