Hard X-ray core level photoemission of vanadium oxides

Abstract

We have studied vanadium oxides corresponding to formally 3 d 0 ( V 2 O 5 ), 3 d 1 ( VO 2 ), and 3 d 2 ( V 2 O 3 ) configurations and compare them with mixed valent Na 0.33 V 2 O 5 , 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 Na 0.33 V 2 O 5 and three-dimensional V 2 O 3 undergo metal–insulator (MI) transition at T MI = 135 and 160 K, respectively. The observed V 2 p core level spectra of V 2 O 5 and VO 2 show a single peak with chemical shifts corresponding to the V 5 + and V 4 + , while two components ( V 5 + and V 4 + ) are observed in Na 0.33 V 2 O 5 . Metallic V 2 O 3 shows a bulk character additional peak at low binding energy in the V 2 p core levels. We also study the V 1 s 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 1 s spectra across the MI transition in V 2 O 3 suggests an additional screening channel available in the Mott–Hubbard correlated metal phase.