Monte Carlo and density functional theory analysis of the distribution of gold and palladium atoms on Au/Pd(111) alloys

The total energies of various Au/Pd(111) alloys are calculated using density functional theory and the results parametrized by a model that includes the energy to exchange gold and palladium atoms within the first and second layers of the alloy, and between the first and second layers. The model is in excellent agreement with the results of density functional theory calculations giving a net repulsive energy between gold and palladium in the first layer, denoted epsilon(11), of similar to 0.06 eV. The effect of the repulsion between gold and palladium is explored using Monte Carlo simulations, where it is found that ordered structures appear only for values of epsilon(11)/kT>1.4. The results are compared with previously published scanning tunneling microscopy images of gold/Pd(111) alloys, and the measured gold distributions yield good agreement with the results of the Monte Carlo simulations for epsilon(11)/kT similar to 0.64. This suggests that the experimental value of epsilon(11) is similar to 0.02 eV, in the same range, but slightly lower than predicted by density functional theory.