Electronic Structure of β-NaxV2O5 (x ≈ 0.33) Polycrystalline Films: Growth, Spectroscopy, and Theory

We present a detailed study of the microstructure and electronic structure of beta-NaxV2O5 (x approximate to 0.33) polycrystalline films, combining film growth, X-ray spectroscopies, and first-principles calculations. High-quality crystalline and stoichiometric V2O5 and beta-Na0.33V2O5 films were grown by a sol-gel process, spin-coating, and rapid thermal annealing. The V2O5 film, which exhibits a rough surface, is preferentially oriented in the (001) direction perpendicular to the surface, whereas the b-axis of beta-Na0.33V2O5 is oriented in the substrate plane. The beta-Na0.33V2O5 film consists of a nested layered structure composed of single-crystalline rods of a few hundred nanometers in diameter and a few micrometers in length. Photoemission and X-ray absorption measurements of beta-Na0.33V2O5 confirm the Na incorporation and the presence of mixed V5+ and V4+ species and weakly occupied V 3d states. At the V L-edge, X-ray absorption and resonant inelastic X-ray measurements suggest a larger crystal field for beta-Na0.33V2O5 compared with isoelectronic beta-Sr0.17V2O5. We observe the lowest local crystal-field dd* transition at an energy of similar to-1.6 +/- 0.1 eV for beta-Na0.33V2O5, which is substantially larger than beta-Sr0.17V2O5; this large difference is interpreted as arising from the stronger distortions to the VO6 octahedra in beta-Na0.33V2O5.