Electronic structure of Ba1-xKxBiO3 and (Ba,K)3Bi2O7

M. Merz1, M. Meven1, N. Nücker2, S. Schuppler2, S. Ustinovich3, A. Soldatov3, S. Shiryaev3, S. Barilo3, D. Arena4, J. Dvorak4, S. Stadler4 und Y. Idzerda4
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 Ba1-xKxBiO3 and (Ba,K)3Bi2O7 single crystals as well as of highly-ordered Ba1-xKxBiO3 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 BiV+-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.