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Ab initio slab simulations have been performed for silver adhesion to the perfect and defective MgO(001) surfaces. For 1/4 Ag monolayer (ML) coverage of perfect substrate, we observe small silver adhesion energies over both O2- and Mg2+ ions on a regular MgO(001) substrate (0.23 and 0.22 eV per Ag atom, respectively), with negligible interfacial charge transfer towards metal atoms. For larger Ag coverages (beginning with 1/2 ML), silver adsorption over regular O2- ions is much more favorable. We demonstrate that point surface defects on a magnesia surface increase markedly the metal adhesion energy and cause a redistribution of the electron density across the interface. The results for electron (Fs° = O vacancy with two trapped electrons) and hole (Vs° = Mg vacancy with two holes trapped by nearest O2- ions) centers in the Ag atom adhesion at different surface coverages are analyzed. For Ag atoms positioned over the point surface defects, the substrate binding energies increase by more than an order of magnitude (to 7.6 and 12.7 eV, repsectively) compared to a regular interface and are associated with marked charge transfer (approximately 1 e towards a Ag atom over a Fs center and approximately 1.5 e towards the nearest O2- ions from a Ag atom over a Vs center). A comparison of these results with silver adhesion on α-Al2O3(0001) surfaces shows two similarities in the nature of the adhesion for Ag adsorption: (1) on the perfect MgO(001) and Al-terminated (stoichiometric) corundum (0001) substrates we observe physisorption with a weak atomic polarization, whereas (2) over a Vs center on the defective MgO and for O-terminated corundum, strong interfacial ionic bonding takes place.
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