X-ray absorption of Ba1-xKxBiO3 and BaPb1-yBiyO3:: Competition between bipolaronic and charge-density wave states

Bulk-sensitive O 1s near-edge X-ray absorption spectroscopy on Ba1-xKxBiO3 and BaPb1-yBiyO3 single crystals shows for undoped BaBiO3 a distinct charge-density wave derived band slightly above the Fermi level; upon K doping it is effectively reduced and competes with bipolaronic hole states whose spectral weight increases with K content. For Pb doping, on the other hand, a wide Pb-O band is formed near E-F while the intensity of the bipolaronic hole band remains unaffected. The data suggest a crucial role of hole bipolarons for bismuthate superconductivity.