Electronic structure and the metal-semiconductor transition in BaPb1-xBixO3 studied by photoemission and x-ray-absorption spectroscopy.

Abstract

We have studied the electronic structure of ${\mathrm{BaPb}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Bi}}_{\mathit{x}}$${\mathrm{O}}_{3}$ and its changes across the composition-dependent metal-semiconductor transition using photoemission and O 1s x-ray-absorption spectroscopy. For the parent insulator ${\mathrm{BaBiO}}_{3}$, the peak-to-peak splitting of the Bi 6s band is found to be large, whereas, the minimum gap is much smaller, qualitatively consistent with the large (\ensuremath{\sim}2 eV) optical gap of the direct type and the smaller (\ensuremath{\sim}0.5 eV) transport gap of indirect type. Pb substitution for Bi induces new states of Pb 6s character outside the band gap of ${\mathrm{BaBiO}}_{3}$ and does not induce in-gap spectral weight unlike in cuprate superconductors; the splitting of the Bi 6s band is not significantly reduced by Pb substitution throughout the semiconducting region. Substituted Pb remains tetravalent in the semiconducting phase and does not supply the Bi-O network with extra holes, which explains the stability of the semiconducting phase up to the Pb content of \ensuremath{\sim}65%. Nevertheless, the band gap collapses in the metallic region (x0.35), where a clear Fermi edge is observed. The shifts of the core-level and valence-band peaks with x suggest that a small (\ensuremath{\sim}0.3 eV) rigid-band shift of the Fermi level occurs in the metallic region. Comparison of the photoemission, optical absorption, and transport data on ${\mathrm{BaBiO}}_{3}$ strongly suggests that polaronic levels are created within the band gap of ${\mathrm{BaBiO}}_{3}$ and accommodate thermally excited charge carriers.