Density-functional approach to phase transitions of submonolayer films. I. The role of the intrinsic and extrinsic ordering forces

A density-functional theory for the submonolayer adsorbed film is developed. The adsorbate interactions are restricted to hard repulsions and described with a nonlocal density functional for the free energy, which allows for the presence of intrinsic crystalline order. The competition with the substrate ordering is studied as a function of the temperature and the coverage for different values of the substrate lattice parameter. The phase diagrams obtained include fluid, commensurate, and incommensurate phases.