  Using Monte Carlo simulation methods in the canonical and grand canonical
ensembles, we discuss the melting and the formation of ordered structures of
mixed Ar-Xe submonolayer films on graphite. The calculations have been
performed using two- as well as three-dimensional models of the systems
studied. It is demonstrated that out-of plane motion does not affect the
properties of the adsorbed films as long as the total density is not close to
the monolayer completion. On the other hand, close to the monolayer completion,
the promotion of particles to the second layer considerably affects the
properties of mixed films. It has been shown that the mixture exhibits complete
mixing in the liquid phase and freezes into solid phases of the structure
depending upon the film composition. For submonolayer densities, the melting
temperature exhibits non-monotonous changes with the film composition. In
particular, the melting temperature initially increases when the xenon
concentration increases up to about 20%, then it decreases and reaches minimum
for the xenon concentration of about 40%. For still higher xenon
concentrations, the melting point gradually increases to the temperature
corresponding to pure xenon film. It has been also demonstrated that the
topology of phase diagrams of mixed films is sensitive to the composition of
adsorbed layers.
