Giant phonon anomalies in the bond-stretching modes in doped BaBiO3:: comparison to cuprates manganites and nickelates

We report inelastic neutron scattering measurements on the phonon dispersion in Pb- and K-doped BaBiO3. The highest longitudinal branch of bond-stretching character exhibits a pronounced bending down and a loss of intensity towards the half-breathing mode at the (0.5 0 0)-zone-boundary in both systems. Along the [1 1 0]- and [1 1 1]-directions we observe even more anomalous effects in the K-doped compound. The LO bond-stretching branches split in these directions with the lower branch softening down to 10.5 THz at the R-point breathing mode. Compared to undoped BaBiO3 this effect represents a doping induced phonon renormalization by more than 40%. We further compare the anomalous features in the bond-stretching branches in doped BaBiO3 to those observed in other metallic perovskite-like cuprates, manganites and nickelates. (C) 2002 Elsevier Science B.V. All rights reserved.