Tilted and nontilted Ag overlayer on a Ni(111) substrate: Structure and energetics

The first stages of silver growth on a nickel (111) surface, for coverages up to 2 monolayers, have been investigated by quenched molecular dynamics (QMD) simulations and low energy electron diffraction (LEED). After a thermal treatment, a collinear (7 x 7) reconstruction, with Ag(111)parallel to Ni(111) and Ag[1 (1) over bar0]parallel to Ni[1 (1) over bar0], observed at room temperature, is seen to change into a tilted reconstruction where the silver lattice is rotated by 2.4 +/- 0.4 degrees with respect to the substrate, the new cell dimensions being close to the (7 x 7) ones. In order to determine the geometry of this kind of tilted but still commensurate reconstruction, we have generated a large number of solutions with a rotation of the silver layer from 1 degrees to 40 degrees with respect to the substrate. The adsorption energies calculated by QMD simulations have shown that three of those tilted reconstructions, corresponding, respectively, to rotation angles equal to 1 degrees, 2.2 degrees, and 3.5 degrees, are energetically more stable than the collinear one. These results support not only the present LEED data but also the tilted superstructures observed by various authors using LEED or scanning tunneling microscopy.