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 $123 {\mathrm{cm}}^{\ensuremath{-}1}$ and $236 {\mathrm{cm}}^{\ensuremath{-}1},$ 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 $R3\ifmmode\bar\else\textasciimacron\fi{}c$ structure, but are rather induced by the dynamic Jahn-Teller effect.