Scanning tunneling microscopy 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 $(250\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$ results in the formation of a nonperiodic honeycomb-like structure growing from the steps, which starts to transform into an ordered phase at a vanadium coverage of $\ensuremath{\approx}0.2\mathrm{ML}$ (monolayer). At 0.31 ML the entire surface is covered by this well-ordered open $(4\ifmmode\times\else\texttimes\fi{}4)$ structure. Annealing this structure in ${\mathrm{H}}_{2}$ atmosphere transforms the phase into a ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ surface oxide with $(2\ifmmode\times\else\texttimes\fi{}2)$ 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 $(4\ifmmode\times\else\texttimes\fi{}4)$ phase, the V atoms are surrounded by four oxygen atoms in an unusual tetrahedral coordination leading to a ${\mathrm{V}}_{5}{\mathrm{O}}_{14}$ 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.
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