Raman phonons and Raman Jahn–Teller bands in perovskite‐like manganites

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 R 3 c 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.