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**Vanadium surface oxides on Pd(111): A structural analysis**

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C. Klein^{1}, G. Kresse^{2}, S. Surnev^{3}, F.P. Netzer^{3}, M. Schmid^{1}, P. Varga^{1}

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*^{1} Institut für Allgemeine Physik,
Technische Universität Wien, A 1040 Wien, Austria

^{2} Institut für Materialphysik und CMS, Universität Wien, A-1090 Wien, Austria

^{3} Institut für Experimentalphysik,
Karl-Franzens-Universität Graz, A-8010 Graz, Austria
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*Phys. Rev. B 68, 235416 (2003)*

Scanning tunneling microscopy (STM) studies of vanadium oxides grown on
Pd(111) show interesting structures especially in the low coverage region.
Evaporation of V in an oxygen background at elevated sample temperature
(250 °C) results in the formation of a non-periodic honeycomb-like structure
growing from the steps, which starts to transform into an ordered phase at
a vanadium coverage of approx. 0.2 monolayers (ML).
At 0.31 ML the entire surface is covered by this well-ordered open (4x4) structure.
Annealing this structure in H_{2} atmosphere transforms the phase into a V_{2}O_{3} surface oxide with (2x2) periodicity,
whose optimal coverage is reached at 0.5 ML vanadium.
Models for both ordered structures have been suggested before on the basis of
ab initio density functional theory (DFT) calculations and
molecular dynamics simulations and these models are now unambiguously confirmed
by quantitative low energy electron diffraction (LEED) analyses.
In the (4x4) phase, the V atoms are surrounded by four oxygen atoms in
an unusual tetrahedral coordination leading to a
V_{5}O_{14} stoichiometry.
This tetrahedral coordination allows the oxide to adopt
open loosely packed two-dimensional (2D) and 1D structures,
which are stabilized by the surface-oxide interface energy.
Furthermore, it is shown that state of the art DFT calculations can indeed
predict complex structures exactly as well as that
modern quantitative LEED is capable of dealing with very large unit cells.

Corresponding author: P. Varga (varga).

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