Modeling Adsorption on fcc(\textit{nnm}) Surfaces

In general, fcc(\textit{nnm}) surfaces consist of very long armchair (111) terraces separated by steps. The number $M$ of atomic sites in the width of the terraces depends on the Miller indices (\textit{nnm}). The model considers values of $M \le $ 6, with adsorbate-substrate interaction energy on step-sites different from those on bulk sites, takes into account first- and second-neighbor adsorbate-adsorbate interactions, and specializes to the case of attractive first-neighbors. We obtain the complete low temperature, 3-dimensional, energy phase diagrams. The occupational configurations of the phases exhibit features similar to those of the phases obtained in the infinite-width limit, or flat fcc(111) surfaces. This yields a classification of the phases into types, and, within each type, the phases are grouped into families. This suggests a number of generalizations for any value of $M$ beyond 6, leading to a better understanding of the competing interaction energies and of the evolution of the phase diagrams with increasing width $M$ of the terraces. The relevance of these results to experiments is discussed within the context of preferential adsorption on step sites, and applied to the adsorption of water on Pt(335).