The Eliashberg theory and the superconductors Ba(Pb,Bi)O-3 and (Ba,K)BiO3

Within the Eliashberg theory we present an analysis of the superconductivit y in the ceramic compounds BaPb1-xBixO3 and BaxK1-xBiO3. This analysis has been carried out calculating some of the thermodynamic properties, function al derivatives, and evaluating the isotope coefficient (alpha). These physi cal quantities are calculated by numerical solutions of the Eliashberg gap equations and the electron-phonon spectra alpha(2)(omega)F(omega) as determ ined by Shirai et al., using a realistic band-structure obtained in the tig ht-binding approximation. In the first oxide superconductor BaPb1-xBixO3, w e have studied several models for the alpha(2)(omega)F(omega) function look ing for better agreement with experimental data. In evaluating the isotope coefficient (alpha) in both compounds, we have used the Rainer and Culetto approach and the functional derivative for the critical temperature T-c. Wi th numerical calculations of some important BCS-ratios and the analysis of the isotope coefficient in these two bismuthate, we outline the importance of the electron-phonon coupling in both superconductors.