Cluster variation theory of the triple point of the triangular lattice gas with the first nearest neighbor exclusion

Cluster variation calculations have been performed for a triangular lattice gas in which molecules exclude each other from the first nearest neighboring sites and attract each other at farther neighboring sites. The lattice gas undergoes an order–disorder phase transition, where the ordered phase has a structure of (3×3)R30°. When the attractive force is sufficiently strong at the third nearest neighbor site, the disordered phase is found to separate into two phases and a triple point appears. This finding combined with a realistic intermolecular potential function provides a satisfactory explanation for the experimental facts that CH4 adsorbed on the graphite basal plane has a triple point whereas N2 and Kr do not.