Hard X-ray core level photoemission of vanadium oxides

We have studied vanadium oxides corresponding to formally 3d0 (V2O5), 3d1 (VO2), and 3d2 (V2O3) configurations and compare them with mixed valent Na0.33V2O5, using hard X-ray core level photoemission spectroscopy (PES), which enables to probe bulk electronic states due to large escape depth of high kinetic energy photoelectrons. The quasi-one-dimensional mixed-valence compound Na0.33V2O5 and three-dimensional V2O3 undergo metal-insulator (MI) transition at TMI = 135 and 160 K, respectively. The observed V 2p core level spectra of V2O5 and VO2 show a single peak with chemical shifts corresponding to the V5+ and V4+, while two components (V5+ and V4+) are observed in Na0.33V2O5. Metallic V2O3 shows a bulk character additional peak at low binding energy in the V 2p core levels. We also study the V 1s core level spectra of these compounds, which are possible only using hard X-rays and confirm the systematics in valence states of vanadium oxides. The temperature dependence of the low binding energy peak in V 1s spectra across the MI transition in V2O3 suggests an additional screening channel available in the Mott-Hubbard correlated metal phase.