Max-Planck-Institut für Festkörperforschung, Heisenbergstr.1, D-70569 Stuttgart, Germany

We report density-functional calculations based on the local-density approximation (LDA) of the properties of doped barium bismuthates. Using the linear-response approach developed in the framework of the linear muffin-tin-orbital method the phonon spectrum of /the Ba_0.6K_0.4BiO₃ system is calculated and is compared with the results of the neutron-diffraction measurements. The effect of doping in the calculation is modeled by the virtual crystal and mass approximations. The electron-phonon coupling constant lambda is then evaluated for a grid of phonon wave vectors using the change in the potential due to phonon distortion found self-consistently. A large coupling of the electrons to the bond-stretching oxygen vibrations and especially to the breathinglike vibrations is established. Also, a strongly anharmonic potential well is found for the tilting-like motions of the oxygen octahedra. This mode is not coupled to the electrons to linear order in the displacements; therefore an anharmonic contribution to lambda is estimated using the frozen-phonon method. Our total (harmonic plus anharmonic) lambda is found to be 0.34. This is too small to explain high-temperature superconductivity in Ba_0.6K_0.4BiO₃ within the standard mechanism. Finally, based on standard LDA and LDA+U like calculations, a number of properties of pure BaBiO₃ such as tilting of the octahedra, breathing distortion, charge disproportionation, and semiconducting energy gap value is evaluated and discussed in connection with the negative-U extended Hubbard model frequently applied to this compound.

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Physical Review B, 57 14453-69, 1998.