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

Abstract

In this work the electronic structure of V₂O₅, reduced V₂O_5−x (V₁₆O₃₉) and sodium intercalated NaV₂O₅ 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 V₂O₅ 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 V₂O_4.8. These films have been intercalated by sodium using vacuum deposition with subsequent spontaneous intercalation (NaV₂O₅) 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 V₂O₅ single crystals have been performed. The experimental data for the freshly cleaved and only marginally reduced V₂O₅ single crystals and the NaV₂O₅ results are in good agreement with the calculated values. The ResPES results for V₂O_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 V₂O₅ and by performing a spin-polarized calculation of an antiferromagnetic ordered NaV₂O_5.