First-principles calculations for VxOy grown on Pd(111)

An approach to access the stability of oxides growing on top of a metal sup port is presented. In combination with first-principles calculations, it al lows to predict the stable structures as a function of the thickness of the evaporated metal ad-layer and as a function of the oxygen pressure. The id eas are applied to thin vanadium oxide films growing on Pd(1 1 1). To inves tigate the stability of these oxide films, first-principles calculations fo r more than 50 thin films of VxOy on Pd were performed at varying stoichiom etry and coverage. The general principles determining the growth of thin va nadium oxide films on Pd(l 11) are discussed, and the experimental results are interpreted in the light of the first-principles calculations. At I ML vanadium coverage, a complicated succession of structures is predicted by t he calculations. At high oxygen pressure bulk like V2O3 phases are stable. At lower oxygen pressure, however, a surface stabilised (2 x 2) reconstruct ion with a formal stoichiometry Of V2O3 is predicted, and rectangular and h exagonal vanadium-dioxide phases are expected to grow. At very low oxygen p ressures, first the vanadium-di oxide phases and then the surface V2O3 phas e decompose and the liberated V atoms move subsurface. These predictions ar e in good general agreement with experiment. An important result of the stu dy is that the metal surface stabilises thin films which have no equivalent bulk phases. (C) 2001 Elsevier Science B.V. All rights reserved.