PHONON-DISPERSION CURVES OF BA0.6K0.4BIO3

The lattice dynamics of Ba0.6K0.4BiO3 was investigated by inelastic ne utron scattering on a superconducting single crystal (T-c=26 K (midpoi nt)). The results were analysed on the basis of a shell model includin g free carrier screening and a term to account for a breathing deforma bility of the BiO6 octahedra. At low frequencies the dispersion curves are very similar to those observed in BaPb0.75Bi0.25O3. However, the screening effects indicate that Ba0.6K0.4BiO3 is more ionic in nature than BaPb0.7Bi0.25O3. The frequencies of the highest LO branches assoc iated with Bi-O bond stretching vibrations decrease on passing from th e Gamma-point to the zone boundary.