First-principles modeling of metal layer adsorption on CaF₂(1 1 1)

Recently, an unusual wetting enhancement was observed for the CaF₂/(In-Ti) and CaF₂/(Cu-Ti) systems. This enhancement cannot be attributed to a chemical reaction between the melt and the substrate, but may occur due to the dissimilarity of the adsorption of the metallic atoms to the substrate. In the present study, ab initio simulations of adsorption of one monolayer of various metal (Me) atoms on the CaF₂(1 1 1) surface have been performed for three different atomic sites, in order to understand the nature of the wetting enhancement, to find the reason for the adsorption dissimilarity and to predict other systems that can wet this compound. Each adsorption site was modeled using a periodic two-dimensional slab, a vacuum separation, and a Me layer at a distance of 1-4 Å from the surface. An attractive adsorption is obtained only when the Me adsorbs above the terminal F atoms. When transition metals with partly occupied d-states are adsorbed on CaF₂, a relatively high electron density is observed in the Me-F bonds, and the local density of states exhibits high peaks in the vicinity of the Fermi energy. This is reflected by the high adsorption energy of these atoms, which may serve as an important detail in understanding the wetting enhancement.