  Longitudinal optical phonons with oxygen character were measured in
La_0.7Sr_0.3MnO_3 by inelastic neutron scattering in the (1 0 0) cubic
direction and results were compared with shell model predictions. Measurements
were performed in several Brillouin zones, which enabled us to identify the
eigenvectors independent of the shell model. All major disagreements between
model predictions and experimental results are primarily due to the anomalous
downward dispersion of the bond-stretching vibration. The main new result is
that the rhombohedral distortion of the cubic lattice makes the bond-stretching
vibrations interact strongly with bond-bending modes folded into the cubic
Brillouin zone.
