Lattice softening in superconducting compositions of Ba(K)BiO₃

Temperature dependent x-ray absorption spectra investigation were measured for Ba_1-xK_xBiO₃ (Bi L₃-edge) with x = 0.0, 0.25, 0.4, 0.5 and for BaPbO₃ (Pb L₃-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. Temperature dependence of the Debye-Waller factor of Bi-Bi bond strongly differs from 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.