VNb9O25−δ—Synthesis, electrical conducting behaviour and density functional theory (DFT) calculation

In order to investigate the influence of the oxygen partial pressure (p(O2)) on the electrical conductivity, VNb9O25 was prepared by thermal decomposition of freeze-dried oxalate precursors and by a solid state reaction of V2O5/Nb2O5 mixtures. The samples were characterised by X-ray diffraction, grain size analysis and scanning electron microscopy (SEM). The electrical conductivity of the n-type semiconductor VNb9O25−δ can be interpreted as an activated hopping process with a preferred localisation of charge carriers at V(IV) centres. The electronic structure of VNb9O25−δ was calculated within the framework of the local density approximation (LDA) to DFT. Partial reduction of V(V) centres causes localised vanadium states to appear inside the band gap. The calculated activation energy values are in good agreement with the experimental ones.