Correlation of the local and the macroscopic properties of high-temperature superconductors: EXAFS data analysis

Based on temperature-dependent EXAFS studies of Ba_1−xK_xBiO₃ superconducting oxide, a model of the relationship between the local crystal and the local electronic structures has been proposed. Oxygen ions were found to vibrate in a double-well potential and their vibrations are correlated with the local electron pair transfer due to the dynamical exchange BiO₆ ↔ Bi²O₆, where ² denotes the hole pair in the antibonding Bi 6s O2p_σ* orbital of the octahedral complex. The similarity of the EXAFS spectra features of Ba_1−xK_xBiO₃ and La_2−xSr_xCuO₄ allowed us to propose a similar model for Cu-based superconducting oxides. The model, in a common approach, explains the antiferromagnetic state, metal–insulator phase transition and the appearance of superconductivity at Sr²⁺ doping of La₂CuO₄, and eliminates discrepancy between the neutron-scattering and EXAFS data.