A LEED I-V structural determination of the c(4x2) Ni3O4/Pd(100) monolayer phase:: an ordered array of Ni vacancies

The structure of the interface-stabilized c(4 x 2) Ni3O4/Pd(1 0 0) monolayer has been determined by using Low Energy Electron Diffraction (LEED) I-Vexperiments. Tensor LEED simulations converged to a Rp factor of 0.22 with a structural model based on the NiO(l 00) surface, where all the atoms of the oxide phase are coplanar. In order to obtain the correct c(4 x 2) periodicity of the phase, a Ni vacancy array has been introduced into the model. In details, oxygen atoms sit on top of palladium atoms, while nickel atoms occupy the fourfold hollow sites. The lattice constant of NiO was contracted in order to match the substrate periodicity, and only a very small buckling of the Ni atoms (0.04 Angstrom) was introduced. The most relevant change with respect to a rigid NiO(1 0 0) structure is the in-plane displacement of oxygen atoms by about 0.2 A along directions almost coinciding with the (0 10) substrate azimuths, away from the vacant Ni site. The proposed structure is perfectly compatible with the STM images preliminarly reported in the literature. (C) 2004 Elsevier B.V. All rights reserved.