Electron-phonon coupling and properties of doped BaBiO3

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 Ba0.6K0.4BiO3 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 vector's 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 breathing like vibrations is established. Also, a strongly anharmonic potential well, is found for the tiltinglike 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 Ba0.6K0.4BiO3 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.