The Raman-active phonons in orthorhombic perovskitelike ${\mathrm{YMnO}}_3$ and ${\mathrm{LaMnO}}_3$ were studied by measuring Raman spectra in various scattering configurations. The experimental Raman line wave numbers and the expected shapes for the phonon modes were compared to those reported for other perovskitelike compounds with the $\mathrm{Pnma}$ structure and to the results of lattice dynamical calculations. The observed Raman lines in the spectra of ${\mathrm{YMnO}}_3$ and ${\mathrm{LaMnO}}_3$ were assigned to definite atomic motions. The much larger linewidths and strong variation of the Raman spectra with increasing laser power in the case of ${\mathrm{LaMnO}}_3$ provide evidence for a structural instability that may result in a phase transition under laser annealing.

• Received 25 June 1997

DOI:https://doi.org/10.1103/PhysRevB.57.2872