Local structure anomalies of the BaBi(Pb)O₃ system at low temperatures: An X-ray absorption study

Temperature dependent X-ray absorption investigations were made on the Bi L₃ edge in BaPb_1-xBi_xO₃ with x=0.0, 0.75 and 1.0. Essential local structure anomalies around Bi atoms were observed in BaBiO₃ and BaPb_0.25Bi_0.75O₃ at low temperatures. It is found that Bi-O, Bi-Ba distances abnormally increase with the temperature decreasing from 90 K. These anomalies are accompanied by an abnormal behaviour of the Debye-Waller factor σ² of the Bi-O bonds at low temperatures in contrary to the Einstein model. The lengths and σ² of the Pb-O and Pb-Ba bonds of BaPbO₃ show an ordinary increase with temperature. The anomalies are shown to be due to the rotation distortion asymmetry of the oxygen octahedra owing to the difference in the strength of Bi-O bonds. The experimental results are discussed in the frame of the dynamic fluctuation rotation mode.