Structural transitions between epitaxially ordered phases in adsorbed submonolayers

The global phase diagram of a triangular lattice-gas model for submonolayers adsorbed epitaxially on basal graphite is studied using a position-space renormalization method. This model has nearest-neighbor exclusion, and accomodates dominant third-neighbor interaction. Each cell of 12 sites is mapped onto a single local degree of freedom with a single-triplet-quadruplet structure. The lattice gas, with up to 20th-neighbor interactions, is thereby transformed into a nearest-neighbor model, which is then analyzed by a Migdal-Kadanoff renormalization transformation. At low temperatures, as coverage is increased from zero, gas, 2 × 2 solid, and $\sqrt{3}\times \sqrt{3}$ solid phases can be encountered, separated by first-order transitions. These solids undergo first-or higher-order transitions into fluid phases as temperature is increased at given density. Triple points, multicritical points, and/or critical end-points occur for various relative strengths of interactions. For certain plausible potentials, the 2 × 2 solid occurs at finite temperature, but not at zero temperature. Distinct liquid and gas phases, with a solid-liquid-gas triple point, are found in some cases. Contact is made with the phase diagram of methane physisorbed on basal graphite, suggesting that the effective hard-core radius of methane is increased by adsorption. A phase diagram very similar to that exhibited by oxygen chemisorbed on nickel (111), with both 2 × 2 and $\sqrt{3}\times \sqrt{3}$ structures, is also obtained.