4s Molecular Orbitals and Strongly Correlated 3d States in TiO_x and VO_x

We have investigated the electronic structure of the rocksalt TiO_x and VO_x systems using polarization-dependent hard X-ray photoelectron spectroscopy. The ability to disentangle the various subshell contributions allowed us to unveil the presence of cation 4s molecular orbitals centered around anion vacancies. We observed a close relationship between the presence of these molecular orbitals and the conductive properties of the oxide material, exemplifying the importance of the 4s degrees of freedom for low-valent transition metal compounds. The experimentally vanishing intensity at the Fermi level classifies TiO_x and VO_x as bad metals and strongly suggests that stoichiometric and defect-free TiO and VO are Mott-Hubbard insulators.