Classification of Order-Disorder Transitions in Common Adsorbed Systems: Realization of the Four-State Potts Model

Order-disorder transitions on square, triangular, and honeycomb arrays are classified according to whether they can be continuous or must be first order by applying a criterion of Landau. In particular, the transitions to 2 × 2 arrays on triangular and honeybomb lattices are predicted to be in the universality class of the four-state Potts model. A physical realization of this model, ${\mathrm{N}}_2$ on Kr-plated graphite, is proposed.