Phase diagrams in the lattice RPM model: from order-disorder to gas-liquid phase transition

TL;DR

This study explores the phase behavior of a lattice ionic model with short-range repulsions, revealing a rich phase diagram that includes gas-liquid separation and order-disorder transitions, depending on the strength of the repulsive interactions.

Contribution

It provides the first detailed Monte Carlo simulation analysis of the lattice RPM with varying nearest neighbor repulsions, connecting lattice and continuum behaviors.

Findings

Identified gas-liquid phase separation with Ising criticality at high nn strength.

Observed a transition from order-disorder to continuum-like phase behavior as nn strength increases.

Mapped the phase diagram showing the evolution of phase transitions with nn interaction strength.

Abstract

The phase behavior of the lattice restricted primitive model (RPM) for ionic systems with additional short-range nearest neighbor (nn) repulsive interactions has been studied by grand canonical Monte Carlo simulations. We obtain a rich phase behavior as the nn strength is varied. In particular, the phase diagram is very similar to the continuum RPM model for high nn strength. Specifically, we have found both gas-liquid phase separation, with associated Ising critical point, and first-order liquid-solid transition. We discuss how the line of continuous order-disorder transitions present for the low nn strength changes into the continuum-space behavior as one increases the nn strength and compare our findings with recent theoretical results by Ciach and Stell [Phys. Rev. Lett. {\bf 91}, 060601 (2003)].