Lateral interactions for CO/Ru(001): Order-disorder transitions of the 3 structure

The clean Ru(001) surface and the order-disorder transition of the √3 × √3 R30° structure of CO on this surface were investigated by LEED in the coverage range up to θ = 0.4. The investigated region of the clean surface consisted of terraces of an average size of more than 150 Å which are separated by monoatomic steps. At CO coverages between θ = 0.09 and θ = 0.18 and at temperatures below 150 K the √3 structure of CO orders into small domains in equilibrium with a disordered phase. The number of domains is decreasing with coverage, reaching a value of one per terrace only at coverages above θ = 0.27. At coverages above θ = 0.18 all CO molecules contribute to the ordered phase, but only at coverages above θ = 0.27 a homogeneous ordered phase is formed. The phase transition is continuous above θ = 0.18, whereas at lower coverages is could be first order, but smeared out by small particle effects. At low coverages a two-dimensional solid/gas equilibrium goes over to a fluid phase which in turn is probably in equilibrium with a gas phase. This process can be described by melting of small ordered islands. Up to θ = 0.18 the phase transition occurs around 150 K, rising sharply in temperature at higher θ. Above θ = 0.27 the phase transition is no longer observable with a stationary layer, as the layer stays ordered up to desorption. The overall topology of the experimental phase diagram is well described by theoretical models with only pairwise NN repulsion and NNN attraction; in order to reproduce details, however, it seems that longer range interactions have to be included.