The infrared reflectance spectra of two ${\mathrm{LaMnO}}_3$ crystals (pure and doped) have been measured at various temperatures between 300 and 10 K. The infrared active phonons of the two samples have been studied in detail and have been compared to recent lattice dynamics calculations. In the case of the undoped ${\mathrm{LaMnO}}_3,$ the number of infrared active phonons agrees with the predictions of group theory. In contrast the phonon structures of the doped sample are broader and do not allow to identify all the expected lines.

• Received 8 December 1999

DOI:https://doi.org/10.1103/PhysRevB.61.11255