Phase transitions in a two-dimensional lattice gas model of orientable diatomic molecules. II. Order-disorder transitions of superantiferromagnetic and c(2×2)_AF phases

The nature of phase transitions occurring in the two-dimensional spin-1 lattice model with the first- and second-nearest-neighbor interactions is studied using Monte Carlo methods. This system models the adsorption of rigid diatomic molecules (A-B) on (100) surfaces of crystals, assuming that each molecule is oriented perpendicularly to the crystal surface, and the binding energy depends on whether the A atom or B atom is adsorbed. It is shown that under the condition of a fully filled lattice, the order-disorder transition of the superantiferromagnetic (SAF) phase may occur via a continuous as well as a first-order phase transition, depending on the model parameters. The continuous order-disorder transition of the SAF phase is found to be nonuniversal. The nature of the various phase transitions is analyzed by the finite size scaling method. The application of this method is also demonstrated for the case of a second-order transition from one ordered phase to another ordered phase. We have also demonstrated that the phase diagram topology changes with the model parameters. While at low temperatures a first-order transition between the SAF phase and the c(2×2)_AF-ordered phase corresponding to a half-filled lattice occurs, this transition line at higher temperatures may terminate either by a triple point or a tricritical point. For the latter case a phase diagram with a different type of multicritical point is suggested.