Stability of reduced V2O5(001) surfaces -: art. no. 045422

The defect-free V2O5(001) surface and ordered structures involving oxygen vacancies have been studied for a wide range of defect concentrations, Theta (1/6less than or equal toThetaless than or equal to1 monolayer, ML), combining density functional theory and statistical thermodynamics. At Theta=1/4 ML the oxygen vacancy formation energy for the singly coordinated surface oxygen atoms (vanadyl oxygen, O-1) is by similar to1.7 eV/atom and similar to2.0 eV/atom lower than the corresponding values for two- and threefold coordinated surface oxygen atoms, respectively. Between 1/3 and 1/2 ML the alignment of vanadyl oxygen vacancies (O-vac(1)) along the [010] direction is by 70 meV/atom (Theta=1/3 ML) and 120 meV/atom (Theta=1/2 ML) more favorable than along the [100] direction, with the concentration-induced change of the vacancy formation energy for structures with vacancies aligned along the [100] direction being smaller than 20 meV/atom. The lowest vacancy formation energy of 1.87 eV/atom corresponds to the (1x1)-O-vac(1) (Theta=1/2 ML) phase with defects forming a trenchlike structure with rows along the [010] direction. Above 1/2 ML the vacancy formation energy increases up to 2.07 eV/atom (Theta=1 ML). The ease of formation of nonrandom vacancy structures with a favored alignment along the [010] direction is discussed in terms of special vacancy-induced lattice distortions. It is also argued that the trenches along the [010] direction provide preferred paths for continuous reduction of the surface starting from isolated defect sites. However, this missing-row structure would be stable only at very low oxygen partial pressures close to conditions for which V2O5 decomposes into VO2 and O-2.