J. Phys.: Condens. Matter 12 (2000) 3767–3786. Printed in the UK PII: S0953-8984(00)07507-X Extended x-ray absorption fine-structure indication of a double-well potential for oxygen vibration in Ba 1-x K x BiO 3 A P Menushenkov and K V Klementev Moscow State Engineering Physics Institute, 115409 Moscow, Russia Received 6 September 1999, in final form 16 February 2000 Abstract. X-ray absorption spectra of the oxide systems Ba 1-x K x BiO 3 and BaPbO 3 above the Bi and Pb L 3 absorption edges were investigated. It was shown that oxygen ions move in a double-well potential and their oscillations are correlated with the charge-carrier movement. The observed breathing-like oxygen vibration in the double-well potential with large amplitude and low frequency causes the strong electron–phonon coupling and high T c -values for doped BaBiO 3 . Based on the experimental data, a model of the relationship of the electronic and local crystal structures is proposed that is in good agreement with the results from transport measurements, and inelastic neutron and electron scattering, Raman scattering, and photoemission spectroscopy. In the framework of the model, the possible reasons for the superconductivity in perovskite-like oxides are discussed. 1. Introduction Although superconductivity was discovered in BaPb 1-x Bi x O 3 (BPBO) significantly earlier [1] than in cuprates, the question of the nature of the superconducting state in this oxide as well as in the cognate system Ba 1-x K x BiO 3 (BKBO) is still unanswered. The structures of the crystal lattices of copper oxide high-temperature superconductors (HTSCs) and bismuth-based oxides have some important common characteristics. Both oxide classes have perovskite-like lattices with CuO n (n = 4, 5, 6) or Bi(Pb)O 6 complexes joined at the common oxygen ions. In bismuthates, the intersection of octahedral complexes in the three crystallographic directions determines their three-dimensional cubic structure. The CuO n complexes are joined in CuO 2 planes, which produces the two-dimensional structure of copper oxides. Because of the strong hybridization of covalent Bi(Pb) 6s, Cu 3d–O 2p σ bonds, the above- mentioned complexes are the most tightly bound entities of the perovskite-like structure. There- fore such important peculiarities of perovskite structure as lattice instability with respect to the soft tilting mode of CuO n or BiO 6 complexes (see for a review [2]) and highly anisotropic thermal factors of oxygen-ion vibration [3], which indicate the large amplitude of rotation oscillations, are inherent to both classes of superconducting oxides and cause anharmonic vibrations of oxygen atoms that may be described as movement in a double-well potential [4,5]. These structural instabilities of the perovskite-like lattice can be related to the transition to a superconducting state [2, 4, 5]. The layered structure of copper oxide compounds, the presence of several non-equivalent copper positions, and there being a number of different Cu–O bonds complicate the local structure analysis. At the same time, the simplicity of the cubic three-dimensional structure 0953-8984/00/163767+20$30.00 © 2000 IOP Publishing Ltd 3767