Theoretical and experimental determination of the electronic structure of V₂O₅, reduced V₂O_5−xand sodium intercalated NaV₂O₅

Abstract

In this work the electronic structure of V(2)O(5), reduced V(2)O(5-x) (V(16)O(39)) and sodium intercalated NaV(2)O(5) has been studied by both theoretical and experimental methods. Theoretical band structure calculations have been performed using density functional methods (DFT). We have investigated the electron density distribution of the valence states, the total density of states (total DOS) and the partial valence band density of states (PVBDOS). Experimentally, amorphous V(2)O(5) thin films have been prepared by physical vapour deposition (PVD) on freshly cleaved highly oriented pyrolytic graphite (HOPG) substrates at room temperature with an initial oxygen understoichiometry of about 4%, resulting in a net stoichiometry of V(2)O(4.8). These films have been intercalated by sodium using vacuum deposition with subsequent spontaneous intercalation (NaV(2)O(5)) at room temperature. Resonant V3p-V3d photoelectron spectroscopy (ResPES) experiments have been performed to determine the PVBDOS focusing on the calculation of occupation numbers and the determination of effective oxidation state, reflecting ionicity and covalency of the V-O bonds. Using X-ray absorption near edge spectra (XANES) an attempt is made to visualize the changes in the unoccupied DOS due to sodium intercalation. For comparison measurements on nearly stoichiometric V(2)O(5) single crystals have been performed. The experimental data for the freshly cleaved and only marginally reduced V(2)O(5) single crystals and the NaV(2)O(5) results are in good agreement with the calculated values. The ResPES results for V(2)O(4.8) agree in principle with the calculations, but the trends in the change of the ionicity differ between experiment and theory. Experimentally we find partly occupied V 3d states above the oxygen 2p-like states and a band gap between these and the unoccupied states. In theory one finds this occupation scheme assuming oxygen vacancies in V(2)O(5) and by performing a spin-polarized calculation of an antiferromagnetic ordered NaV(2)O(5.).

Oxygen Defects at Reducible Oxide Surfaces: The Example of Ceria and Vanadia

“…With increasing temperature V 3+ species were gradually created. The latest results obtained by resonant photoelectron and X-ray absorption near edge structure spectroscopies are in accord with previous findings [80]. Note that if singly coordinated oxygen atoms are removed, the presence of V 4+ species indicates that the two electrons left in the system per defect created do not remain together at the V atom to which the removed oxygen was initially bonded, as discussed below (Sect.…”

“…Furthermore, Sauer and Döbler performed 2L clusters studies using PBE and B3LYP [22] and found a corresponding shrinkage from 3.21 to 1.86 Å and expansion from 1.58 to 1.69 Å with B3LYP. Interlayer bond formation upon creation of isolated vanadyl oxygen defects in the bulk was also reported [80,81,133]. Note that the atomic displacements due to isolated O 1 vacancies occur at all defect sites upon increasing the defect concentration, as shown by Ganduglia-Pirovano and Sauer [23].…”

“…These solutions may differ in total energy by, for instance, about 0.3 eV for CeO 2 bulk or surface containing an oxygen vacancy [76,78]. DFT+U has also been applied to oxygen defects at pure V 2 O 5 surfaces [79], and in the bulk [80,81]. Similarly to CeO 2 , different U values-for the V 3d states-have been used with GGAs and the electronic structure has been investigated.…”

“…V 2 O 5 (001) is easily reduced by low-energy particle bombardment and thermal annealing, and it is very sensitive to all spectroscopic and scanning probe techniques, as also discussed in Chap. 14 [79,80,[124][125][126][127][128][129] [134] suggested that the surface oxygen defects formed after exposure to atomic hydrogen are likely to be the bridging oxygen atoms.…”

Ab initioand photoemission study of correlation effects in SrRuO3thin films

“…After background subtraction the VB photoemission spectrum for SrRuO 3 can be calculated for a particular photon energy using the following approximate expression (DOS approximation): [40][41][42] …”