On the nature of adsorbate phase diagrams: beyond lattice gas models

Abstract

Lattice gas models have frequently been used to study adsorbate phase diagrams. Experimental phase diagrams exhibit particle-hole asymmetry while the lattice gas model with pairwise interactions predicts a symmetric phase diagram. One way to remove this discrepancy between theory and experiment is to assume that non-negligible three-body interactions occur. In this work I present another explanation based on allowing the adsorbates to move away from the substrate symmetry sites, i.e., removing the lattice gas restriction. I show that even in the simplest case of only nearest neighbour adsorbate-adsorbate interactions, this generalization introduces a remarkable richness in the coverage-temperature phase diagram, including incommensurate structures and commensurate domain-wall superstructures, in good agreement with experiment. I also show that the models discussed in this work are equivalent to lattice gas models with three-body interaction terms.