A SUPERCELL LDA-LMTO-ASA CALCULATION OF COMPOSI-TION DEPENDENCE OF HOPFIELD CONSTANT AND T c IN Ba 1-x K x BiO 3

The electronic structure of (Ba 1-x K x )BiO 3 has been calculated by using a supercell LDA-LMTO-ASA method at some compositions. The calculation was performed by doubling unit-cell for x=0.25, 0.5 and 1.0 and by quadruple unit-cell for x = 0.25 and 0.75. The band parameters, total density of states TDOS (E F ), partial DOS PDOS (E F ) and self-consistent potential V t (r), obtained from LMTO calculation were combined with the experimental Debye temperature Θ D From this, the Hopfield constant η t , electron-phonon coupling constant λ and superconducting transition temperature T c and its x-dependence were evaluated by the McMillan theory and Gaspari-Gyorffy approximation. It was found that the λ≈1 and the maximum T c ≈ 10K which is a slowly varying function of composition x. Our results exhibit a significant difference between the traditional theory and experiment and imply a mechanism which would be related to the composition dependent structure transformation besides the BCS methanism.