Temperature dependent x-ray absorption spectra investigation were measured
for Ba1-xKxBiO3 (Bi L-3-edge) with x = 0.0, 0.25, 0.4, 0.5 and for BaPbO3 (
Pb L-3-edge). It was found that at low temperatures the Debye-Waller factor
of the square diagonal Bi-Bi bond has the maximum value near the insulator
-metal phase transition for the compound with x = 0.25 and x = 0.4. Tempera
ture dependence of the Debye-Waller factor of Bi-Bi bond strongly differs f
rom the Einstein model curve that well describes the harmonic systems (for
example BaPbO3). This behavior is consistent with the strong anharmonicity
of the Bi-O shell due to the double-well vibration potential reported by us
earlier. Presented results point to the essential lattice softening of the
superconducting compositions, which is important for the understanding of
superconductivity mechanism in perovskite type oxides.