Abstract
The structure of AgN clusters (N = 1 - 4, 6, 8, 10), both in the gas-phase
and grown on the Fs-defected MgO(100) surface, is investigated via a density
functional basin-hopping (DF-BH) approach. In analogy with what observed
in the case of gold clusters, it is found that the presence of the defect implies
a double-frustration and a cylindrical invariance of the metal-surface
interaction, causing small Ag clusters growing around the Fs defect to be
highly
fluxional. Nevertheless, two different structural cross-overs are found
to be induced by the metal-defect interaction for the adsorbed clusters such
that: (a) planar structures prevail for N<=4 (as in the gas-phase); (b) noncrystalline
(five-fold symmetric) structures, which are the lowest-energy ones
in the gas-phase for medium sized AgN clusters (N>=7), prevail for N = 6 and
N = 8; (c) distorted fcc structures grown pseudomorphically on the defected
surface prevail for N = 10. The transition (c) from five-fold to fcc motifs is rationalized
in terms of the double-frustration effect, which increases the bond
strain of the non-crystalline structures. Detrapping energies from the defect
are also calculated, finding that the lowest-energy pathway corresponds to
the detachment of a dimer.