A magic Pd-Ag Binary Cluster on the Fs-defected MgO(100) Surface

The structure of Pd1AgN clusters (N ) 1-8), both in the gas-phase and adsorbed on an Fs-center of an MgO(100) terrace, is investigated via a density-functional basin-hopping (DF-BH) approach. A structural transition from planar to noncrystalline 5-fold symmetric configurations is found for both free and adsorbed clusters in this size range. The Pd-Ag clusters are highly fluxional. In addition, Pd1Ag6 is found to be a magic cluster, exhibiting a large HOMO-LUMO gap (both in the gas-phase and adsorbed on the defect) and a peculiar structural stability (when adsorbed on the defect). To our knowledge, this is the first example of a magic metal cluster on an oxide surface. This is rationalized in terms of an electronic shell-closure, involving also the electrons trapped in the oxygen vacancy, coupled with a good adhesion to the defected surface. The low-energy Pd1Ag6 isomers (with excitation energies around 0.1-0.3 eV) adsorbed on the Fs-center also exhibit a large HOMO-LUMO gap.