Determination of the local structure in Ba sub 1 minus x K sub x BiO sub 3 by x-ray-absorption spectroscopy

The local structure around Bi atoms has been studied in Ba{sub 1{minus}{ital x}}K{sub {ital x}}BiO{sub 3} (0.0{lt}{ital x}{lt}0.4) via x-ray-absorption spectroscopy. The structure is determined in terms of coordination numbers and interatomic distances around the Bi atoms between 300 and 13 K from Bi {ital L}{sub 3} extended x-ray-absorption fine-structure data. The charges on Bi and O have been measured as a function of K doping using Bi {ital L}{sub 3}, and O {ital K}-edge x-ray-absorption near-edge-structure spectroscopy. It was found that the ordered breathing-mode-type distortion in BaBiO{sub 3} is replaced by a disordered distortion of the same type when K is substituted into the Ba site, which persists down to 13 K for {ital x}=0.2 and disappears for the {ital x}=0.4 composition. The overall Bi valence increases only slightly whereas O 2{ital p} orbitals are doped with holes with increasing K content. As the insulator-metal boundary is crossed for {ital x}=0.35 these holes become delocalized, and contribute to the metallic behavior of the Ba{sub 0.6}K{sub 0.4}BiO{sub 3} compound.