Electronic structure of β-Na_xV₂O₅ (x ≈ 0.33) polycrystalline films: Growth, spectroscopy, and theory

We present a detailed study of the microstructure and electronic structure of β-Na_xV₂O₅ (x ≈ 0.33) polycrystalline films, combining film growth, X-ray spectroscopies, and first-principles calculations. High-quality crystalline and stoichiometric V₂O ₅ and β-Na_0.33V₂O₅ 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 β-Na_0.33V₂O₅ is oriented in the substrate plane. The β-Na_0.33V₂O ₅ 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 β-Na_0.33V₂O₅ 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 β-Na _0.33V₂O₅ compared with isoelectronic β-Sr_0.17V₂O₅. We observe the lowest local crystal-field dd* transition at an energy of ∼-1.6 ± 0.1 eV for β-Na_0.33V₂O₅, which is substantially larger than β-Sr_0.17V₂O₅; this large difference is interpreted as arising from the stronger distortions to the VO₆ octahedra in β-Na_0.33V₂O₅.