Thermodynamic properties of a system of interacting particles adsorbed on a lattice with two nonequivalent sites

We investigate the thermodynamic properties of a system of particles adsorbed on a lattice with two nonequivalent sites, with different unit cell numbers and adsorption energies. We include lateral interactions between particles in the nearest-neighbor nonequivalent sites. We use the decoration-iteration transformation in order to reduce the problem to the Ising spin square ferromagnet in a magnetic field. The phase diagram of the system for the case of repulsive interaction differs considerably from the phase diagram of an ordinary lattice gas with equivalent sites and shows nonmonotonic dependence of the critical temperature Tc on adparticle surface coverage θ. Using the real-space renormalization group method we also calculate adsorption isotherms and coverage dependencies of the isothermal susceptibility for attractive and repulsive interactions. These functions exhibit features characteristic for systems with simultaneous repulsive and attractive interactions. Results from Monte Carlo simulations are in good agreement with theory.