INTERPRETATION OF HIGH-ORDER COMMENSURATE PHASES FOR AN ARGON MONOLAYER ADSORBED ON PT(111)

Recent helium atom scattering experiments have shown that Ar monolayer s form high-order commensurate (HOC) phases on Pt(111) substrate at lo w temperature and that HOC-HOC transitions proceed as functions of cov erage and temperature. This leads to the realization of a two-dimensio nal devil's staircase. Potential calculations using effective pairwise Pt-Ar and Ar-Ar interactions and substrate-mediated Ar-Ar/Pt contribu tions are performed to determine the equilibrium geometries of the mon olayer vs Ar coverage. The free energy calculated at finite temperatur e (T less than or equal to 50 K) shows that five metastable hexagonal HOC structures occur within the range 31 less than or equal to T less than or equal to 46 K. When T increases in this range, the nonrotated structures appear with increasing lattice parameters 3.71, 3.78, 3.82, 3.84, and 3.89 Angstrom. The most stable geometry is a (4 x 4) unit c ell with 9 Ar atoms. Rotated phases exhibit the same behavior with, ho wever, a slightly shifted temperature range and shifted values for the lattice parameter. These results are in close agreement with the expe rimental findings and they corroborate the devil's staircase event for such a system.