Phonon densities of states of insulating ${\mathrm{BaBiO}}_3$ and superconducting ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_3$ are determined by neutron-scattering experiments. The main peaks in ${\mathrm{BaBiO}}_3$ occur at 35, 43, 63, and 71 meV. In ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_3$, the phonon spectrum softens and is comprised of two bands around 30 and 60 meV. Molecular-dynamics results, which are in good agreement with the experiment, reveal that the observed phonon peaks are due to oxygen vibrations. The softening of oxygen phonon modes around 30 and 60 meV is due to electron-phonon coupling, indicating that ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_3$ is a normal BCS superconductor.

• Received 21 March 1989

DOI:https://doi.org/10.1103/PhysRevLett.62.2628