Coexistence of nonregistered monolayer and bilayer solid films

A statistical thermodynamic theory is formulated for monolayer and bilayer solid films of inert gases in physical adsorption. At temperatures where the lattice vibrations are anharmonic, and allowing for thermally activated vacancies and dilute overlying gas layers, the Helmholtz free energies of the adsorbed phases are evaluated with a cell model approximation which includes part of the correlation energy. Three physical systems are treated, using realistic models for adatom interactions and substrate holding potentials: Xe/Ag(111), Ar/Gr, and Kr/Ag(111). Phase diagrams and thermodynamic functions of the solid films are calculated. The lattice constant discontinuity at the transition under compression from the monolayer to the bilayer is found to be small, in accordance with experiments. The lattice constants at the transition are distinctly larger than that of the corresponding bulk solid under its own vapor pressure. The calculated difference between the latent heats of adsorption of the monolayer and the bilayer is in good agreement with experimental data for Xe/Ag(111) and Ar/Gr, but it is significantly smaller than the experimental difference for Kr/Ag(111).