ANHARMONICITY AND SUPERCONDUCTIVITY IN BA0.6K0.4BIO3

The temperature dependence of the x-ray absorption spectra above the L -3 absorption edge of bismuth in the superconducting oxide Ba0.6K0.4Bi O3 are investigated. It is found that the local structure is different from the simple cubic structure indicated by x-ray and neutron-diffra ction data. It is shown that the oxygen atoms move in an anharmonic do uble-well potential arising as a result of the existence of two nonequ ivalent types of octahedral environments of bismuth. Vibrations in suc h potential modulate the Bi-O bond lengths at the low frequency of the rotational (''tilting'' type) mode of the oxygen octahedra and thus g ive rise to a strong electron-phonon interaction, which explains the q uite high superconducting transition temperatures T-c similar to 30 K. (C) 1998 American Institute of Physics.