On the mechanism of superconductivity in BaBi(Pb)O3Ba(K)BiO3 systems: analysis of XAFS-spectroscopy data

Based on the XAFS-studies of BaPb1−xBixO3 (BPB) and Ba1−xKxBiO3 (BKB) systems carried out by the author from 1983 to 1996, the peculiarties of the local and the electronic structures of both compounds have been explained. It was shown that bismuth in BaBiO3 is situated in two different positions with valence states Bi3+ and Bi3+L2 (here L2 denotes the presence of two holes in the nearest oxygen environment), that causes subsequent difference in BiO bond strengths. Temperature dependent X-ray absorption investigations on the Bi L3 edge point out significant local structure anomalies around Bi atoms in BaBiO3 and BaPb0.25Bi0.75O3 at low temperatures. It was found that BiO, BiBa distances abnormally increase with temperature decreasing from ∼90 K. These anomalies are accompanied by abnormal behaviour of the Debye-Waller factor σ2 of the BiO bonds at low temperatures in contrast to the Einstein model. The nature of the above anomalies was explained in the frame of the model, taking into account the asymmetry of oxygen octahedra rotation modes which promotes understanding of the mechanism of superconductivity in BPB-BKB systems.