Comparative study of optical phonons in the rhombohedrally distorted perovskitesLaAlO3andLaMnO3

The Raman and infrared phonons of isostructural rhombohedral ${\mathrm{LaMnO}}_3$ and ${\mathrm{LaAlO}}_3$ are studied at room temperature. The experimental spectra are compared with the prediction of lattice-dynamical calculations and the lines observed are assigned to definite atomic vibrations. It is shown that the Raman mode of $A_{1g}$ symmetry in ${\mathrm{LaAlO}}_3$ and ${\mathrm{LaMnO}}_3$ (at $123\hspace{0.5em}{\mathrm{cm}}^{-1}$ and $236\hspace{0.5em}{\mathrm{cm}}^{-1},$ respectively) involves atomic motions that cause the rhombohedral distortion, i.e., it is a “soft” mode, and its position could be used as a measure of the degree of the distortion. It is also argued that the broad Raman bands in the high-frequency range of ${\mathrm{LaMnO}}_3$ are not proper modes of the rhombohedral $R3\mathrm{}¯c$ structure, but are rather induced by the dynamic Jahn-Teller effect.