ON THE MECHANISM OF SUPERCONDUCTIVITY IN BABI(PB)O-3-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 peculiarities of the local and the electronic structures of both compounds have bee n explained. It was shown that bismuth in BaBiO3 is situated in two di fferent positions with valence states Bi3+ and Bi3+(L) under bar(2) (h ere (L) under bar(2) denotes the presence of two holes in the nearest oxygen environment), that causes subsequent difference in Bi-O bond st rengths. Temperature dependent X-ray absorption investigations on the BiL3 edge point out significant local structure anomalies around Bi at oms in BaBiO3 and BaPb0.25Bi0.75O3 at low temperatures. It was found t hat Bi-O, Bi-Ba distances abnormally increase with temperature decreas ing from similar to 90 K. These anomalies are accompanied by abnormal behaviour of the Debye-Waller factor sigma(2) of the Bi-O bonds at low temperatures in contrast to the Einstein model. The nature of the abo ve anomalies was explained in the frame of the model, taking into acco unt the asymmetry of oxygen octahedra rotation modes which promotes un derstanding of the mechanism of superconductivity in BPB-BKB systems.