Monte Carlo study of a lattice gas model with non- additive lateral interactions

Abstract

We present a Monte Carlo study in which the traditional assumption of a strictly pairwise additive nearest neighbours interaction is replaced by a more general one, namely, that the bond linking a certain atom with any of its neighbours considerably depends on how many of them are actually present (or absent) on the sites in the first coordination shell of the atom. Two general cases, corrsponding to different types of bonding, are considered in more detail: (i) The lateral attraction decreases linearly with growing degree of occupation of the first coordination shell and (ii) the lateral interaction is strong as long as only one nearest neighbour is around, and it becomes weaker (and constant) if more than one atom surrounds the central one in the various configurations. A comparison of phase diagrams and critical parameters, following from the MC simulation, with those obtained by means of approximate analytical treatments - mean field approximation (MFA) or quasi-chemical approximation (QCA) - is qualitatively confirming the latter. It is shown that non-additivity leads to a considerable decrease in the critical temperature, T c , and in the critical degree of coverage, θ c , while the overall form of an Ising-type phase diagram is retained (case(i)). In case (ii), on the contrary the existence of a dimerized molecular phase, coexisting with the condensed monolayer, is observed and the possibility of additional phase transitions at low coverage is envisaged. The usefulness of some recently developed MC approaches, such as MC finite size scaling, is also examined in view of the strong asymmetry typical for the present model.