Inverse melting in lattice-gas models

TL;DR

This paper explores inverse melting in lattice-gas models on a triangular lattice, revealing geometrical mechanisms behind the phenomenon through computational and analytical methods, and drawing parallels to Helium's behavior.

Contribution

It introduces discrete lattice models exhibiting inverse melting driven by geometric factors, expanding understanding beyond previous models.

Findings

Loose-packed phase extends to zero temperature at low pressure

Melting of close-packed solid occurs upon cooling at constant pressure

Geometrical factors primarily drive the inverse melting phenomenon

Abstract

Inverse melting is the phenomenon, observed in both Helium isotopes, by which a crystal melts when cooled at constant pressure. I investigate discrete-space analogs of inverse melting by means of two instances of a triangular-lattice-gas system endowed with a soft-core repulsion and a short-ranged attraction. To reconstruct the phase diagram, I use both transfer-matrix and Monte Carlo methods, as well as low-temperature series expansions. In one case, a phase behavior reminiscent of Helium emerges, with a loose-packed phase (which is solid-like for low temperatures and liquid-like for high temperatures) extending down to zero temperature for low pressures and the possibility of melting the close-packed solid by isobaric cooling. At variance with previous model studies of inverse melting, the driving mechanism of the present phenomenon is mainly geometrical, related to the larger free-energy cost of a ``vacancy'' in the loose-packed solid than in the close-packed one.