Abstract
Based on the XAFS-studies of BaPb 1− x Bi x O 3 (BPB) and Ba 1− x K x BiO 3 (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 BaBiO 3 is situated in two different positions with valence states Bi 3+ and Bi 3+ L 2 (here L 2 denotes the presence of two holes in the nearest oxygen environment), that causes subsequent difference in BiO bond strengths. Temperature dependent X-ray absorption investigations on the Bi L 3 edge point out significant local structure anomalies around Bi atoms in BaBiO 3 and BaPb 0.25Bi 0.75O 3 at low temperatures. It was found that BiO, BiBa distances abnormally increase with temperature decreasing from ∼90 K. These anomalies are accompanied by abnormal behaviour of the Debye-Waller factor σ 2 of the BiO 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.
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