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The Raman and infrared phonons of isostructural rhombohedral LaMnO 3 and 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 LaAlO 3 and LaMnO 3 ͑at 123 cm Ϫ1 and 236 cm Ϫ1 , respectively͒ 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 LaMnO 3 are not proper modes of the rhombohedral R3 c structure, but are rather induced by the dynamic Jahn-Teller effect.

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The perovskite-like manganites R 1−x A x MnO 3 , where R is a trivalent rare earth or Y and A is a divalent alkaline earth element, are characterized by a strong interplay of magnetism, electric transport and crystallographic distortion. At doping levels 0.15 < x < 0.45 the materials exhibit colossal magnetoresistance near the concomitant ferromagnetic and insulator-metal transitions. At a fractional doping level, such as x = 0.5, the crystallographic and magnetic environment is strongly modified and charge ordering between Mn 3+ and Mn 4+ or phase separation takes place. In this work, the polarized Raman spectra of the orthorhombic and rhombohedral phases of parent RMnO 3 compound were analyzed in close comparison with results of lattice dynamic calculations. We argue that the strong high-wavenumber bands between 400 and 700 cm −1 , which dominate the Raman spectra of rhombohedral RMnO 3 and magnetoresistive La 1−x A x MnO 3 are not proper Raman modes for the R3c or Pnma structures. Rather, the bands are of phonon density-of-states origin and correspond to oxygen phonon branches activated by the non-coherent Jahn-Teller distortions of the Mn 3+ O 6 octahedra. The reduction of these bands upon doping of La 1−x A x MnO 3 and their disappearance in the ferromagnetic metallic phase support the model. The variation with temperature of the Raman spectra of La 0.5 Ca 0.5 MnO 3 is also discussed. The results give a strong indication for charge and orbital ordering and formation of superstructure at low temperatures.

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