INTERATOMIC POTENTIALS AND THE PHASE-DIAGRAM OF XE/PT(111)

We present a microscopic model for the Xe/Pt (111) system that is consistent with the experimental desorption energy, the experimental vibrational frequency of the Xe atom in the direction normal to the Pt (111) surface, and salient features of the experimental phase diagram of Xe/Pt (111). The interatomic potentials in this model were obtained using a simple technique that we have developed for generalizing the typical pairwise-additive atom-atom central potentials used in modeling physisorption systems to make them noncentral and more flexible in their functional form. We applied this technique to the Lennard-Jones pair potential and fit the parameters to reproduce the experimental binding energy, the frequency for vibration of the adsorbate normal to the surface, and a reasonable choice of the binding distance. We adjusted the corrugation of the potential DELTAV, defined as the energy barrier for motion of an adsorbate atom from one binding site to another, in order to fit as much of the phase diagram as possible. Our model for the Xe-Pt interaction was constructed on the basis of the assumption that the binding site is located in the threefold site of Pt (111). When the Xe-Xe interaction was represented by the form appropriate for atoms in the gas phase, we were unable to find a stable commensurate phase for any choice of the corrugation that predicted a low temperature incommensurate phase. When a substrate mediated contribution to the Xe-Xe interaction was included in the model, we found that the commensurate phase was stable in an intermediate temperature range with an incommensurate phase stable at low temperature for a range of values of the corrugation. For a choice of DELTA V = 171 K, the striped incommensurate phase is stable at low temperature, there is a phase transition to a square-root 3 X square-root 3R30-degrees phase at T = 65 +/- 5 K, and the commensurate phase melts to become a liquid at T almost-equal-to 120 K. These results are in reasonably good agreement with the experiments.