ELECTRONIC-STRUCTURE AND THE METAL-SEMICONDUCTOR TRANSITION IN BAPB1-XBIXO3 STUDIED BY PHOTOEMISSION AND X-RAY-ABSORPTION SPECTROSCOPY

We have studied the electronic structure of BaPb1-xBixO3 and its chang es across the composition-dependent metal-semiconductor transition usi ng photoemission and O 1s x-ray-absorption spectroscopy. For the paren t insulator BaBiO3, the peak-to-peak splitting of the Bi 6s band is fo und to be large, whereas, the minimum gap is much smaller, qualitative ly consistent with the large (approximately 2 eV) optical gap of the d irect type and the smaller (approximately 0.5 eV) transport gap of ind irect type. Pb substitution for Bi induces new states of Pb 6s charact er outside the band gap of BaBiO3 and does not induce in-gap spectral weight unlike in cuprate superconductors; the splitting of the Bi 6s b and is not significantly reduced by Pb substitution throughout the sem iconducting region. Substituted Pb remains tetravalent in the semicond ucting phase and does not supply the Bi-O network with extra holes, wh ich explains the stability of the semiconducting phase up to the Pb co ntent of approximately 65%. Nevertheless, the band gap collapses in th e metallic region (x <0.35), where a clear Fermi edge is observed. The shifts of the core-level and valence-band peaks with x suggest that a small (approximately 0.3 eV) rigid-band shift of the Fermi level occu rs in the metallic region. Comparison of the photoemission, optical ab sorption, and transport data on BaBiO3 strongly suggests that polaroni c levels are created within the band gap of BaBiO3 and accommodate the rmally excited charge carriers.