Comparative theoretical study of the Ag-MgO (100) and (110) interfaces

We have calculated the atomic and electronic structures of Ag-MgO(100) and (110) interfaces using a periodic (slab) model and an ab initio Hartree-Foc k approach with a posteriori electron correlation corrections. The electron ic structure information includes interatomic bond populations, effective c harges, and multipole moments of ions. This information is analyzed in conj unction with the interface binding energy and the equilibrium distances for both interfaces for various coverages. There are significant differences b etween partly covered surfaces and surfaces with several layers of metal, a nd these can be understood in terms of electrostatics and the electron dens ity changes. For complete monolayer (1:1) coverage of the perfect MgO(100) surface, the most favorable adsorption site energetically for the Ag atom is above the s urface oxygen. However, for partial (1:4) coverage of the same surface, the binding energies are very close for all the three likely adsorption positi ons (Ag over O, Ag over Mg, Ag over a gap position), For a complete (1:1) Ag monolayer coverage of the perfect MgO(110) interfac e, the preferable Ag adsorption site is over the interatomic gap position, whereas for an Ag bilayer coverage the preferred Ag site is above the subsu rface Mg2+ ion (the bridge site between two nearest surface O2- ions). In t he case of 1:2 layer coverage, both sites are energetically equivalent. The se two adhesion energies for the (110) substrate are by a factor of two to three larger than over other possible adsorption sites on perfect(110) or ( 100) surfaces. We compare our atomistic calculations for one to three Ag planes with those obtained by the shell model for 10 Ag planes and the Image Interaction Mod el addressing the case of thick metal layers. Qualitatively, our ab initio results agree well with many features of these models. The main charge redi stributions are well in line with those expected from the Image Model. Ther e is also broad agreement in regard to orders of magnitude of energies. (C) 1999 Elsevier Science B.V. All rights reserved.