•M. Merz¹, M. Meven¹, N. Nücker², S. Schuppler², S. Ustinovich³, A. Soldatov³, S. Shiryaev³, S. Barilo³, D. Arena⁴, J. Dvorak⁴, S. Stadler⁴ und Y. Idzerda^4
1Institut für Kristallographie, RWTH-Aachen
²Forschungszentrum Karlsruhe, IFP, Karlsruhe
³Academy of Sciences, Minsk
⁴Naval Research Laboratory, Washington

With O 1s near-edge x-ray absorption spectroscopy, the unoccupied electronic structure of Ba_1-xK_xBiO₃ and (Ba,K)₃Bi₂O₇ single crystals as well as of highly-ordered Ba_1-xK_xBiO₃ films has been investigated using the surface-insensitive fluorescence yield. Clear indications for both, a charge-density wave derived band as well as a conduction band are found. The data support a picture, where the intrinsically doped holes in BaBiO₃ predominantly reside on the oxygen orbitals of the compressed Bi^V+-O octahedra, leading to the well-known static charge disproportionation and the alternation of enlarged and compressed octahedra. Upon substitution of Ba by K, the intensity attributed to the unoccupied states resulting from the charge-density wave is effectively reduced. Concomitantly, the spectral weight of the conduction band develops and shows a marked increase with growing K content. For higher potassium concentrations (x ³ 0.5), on the other hand, a small amount of holes still remains in the O 2p states of the charge-density wave, thereby pointing to a local, and most probably dynamical, breathing mode of the BiO₆ octahedra.