Electronic states of BaBiO_{3- delta} and K-doping effects studied by photoelectron spectroscopy

Changes in the electronic states for Ba_1-xK_xBiO _{3- delta} ( delta =0 and 0.5) due to various surface treatments (fracturing, scraping and ion sputtering) have been studied by X-ray photoelectron spectroscopy. It is found that marked spectral changes occur as a result of scraping and sputtering; the core levels are broadened and the intensity at the top of the valence bands is reduced. These results are discussed in terms of the reduction of elements and damage in the surface region. The above results cause the authors to conclude that fracturing these samples is suitable as a cleaning procedure for this system. The Bi 4f, Ba 4d and O 1s core levels have very low binding energies and are shifted by 0.2-0.4 eV to lower binding energies on 50% substitution of K for Ba. The Bi 4f line seems to contain a single chemical state for BaBiO_{3- delta}, and more than one chemical state for Ba_0.5K_0.5BiO _{3- delta}. The top of the valence band is found to be 0.5 eV below the Fermi level for BaBiO_{3- delta}, while a clear Fermi edge appears for the K-doped material in the valence band spectra from both X-ray and ultraviolet photoelectron spectroscopy. The electronic states of BaBiO_{3- delta} and K doping effects are discussed and compared with band calculations.