ELECTRON-PHONON COUPLING AND PROPERTIES OF DOPED BABIO3

We report density-functional calculations based on the local-density a pproximation (LDA) of the properties of doped barium bismuthates. Usin g the linear-response approach developed in the framework of the linea r muffin-tin-orbital method the phonon spectrum of the Ba0.6K0.4BiO3 s ystem is calculated and is compared with the results of the neutron-di ffraction measurements. The effect of doping in the calculation is mod eled by the virtual crystal and mass approximations. The electron-phon on coupling constant lambda is then evaluated for a grid of phonon wav e 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 vib rations 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 f ound to be 0.34. This is too small to explain high-temperature superco nductivity in Ba0.6K0.4BiO3 within the standard;mechanism. Finally, ba sed on standard LDA and LDA + U like calculations, a number of propert ies of pure BaBiO3 such as-tilting of the octahedra, breathing-distort ion, charge disproportionation; and semiconducting energy gap value is evaluated and discussed in connection with the negative-U extended Hu bbard model frequently applied to this compound.