Raman Shift of RMnO₃ (R = La, Pr, Nd, Sm) Manganites

Raman-active phonons in orthorhombic perovskite-like RMnO₃ 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 perovskite-like compounds with the Pnma structure and to the results of lattice dynamical calculations. The observed Raman lines in the spectra of RMnO₃ were assigned to definite atomic motions. The remaining spectral weight can be explained by the presence of dynamic John--Teller distortions that lower the symmetry of the cubic perovskite.