Photoemission study of monoclinicBaBiO3

We present the results of photoemission studies of ‘‘valence-disordered’’ monoclinic ${\mathrm{BaBiO}}_3$ in the photon energy range 15–120 eV. The line shapes of the valence-band photoemission spectra and the Ba contributions to the valence band are similar to the line shapes of the total density of states and Ba partial density of states, respectively. The Bi 5d core levels show only one peak for each spin-orbit component. Oxygen resonance is observed, demonstrating the existence of empty O 2p states. These results support a more covalent rather than a simple ionic picture for the electronic states of ${\mathrm{BaBiO}}_3$. Our data suggest that the charge fluctuation between the two unequal Bi sites is small. We also discuss the origin of the semiconducting character of the ‘‘valence-disordered’’ ${\mathrm{BaBiO}}_3$.