The perovskite-like manganites R1-xAxMnO3, 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 Mn3+ and Mn4+ or phase separation takes place. In this work, the polarized Raman spectra of the orthorhombic and rhombohedral phases of parent RMnO3 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 RMnO3 and magnetoresistive La1-xAxMnO3 are not proper Raman modes for the R3̄c 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 Mn3+O6 octahedra. The reduction of these bands upon doping of La1-xAxMnO3 and their disappearance in the ferromagnetic metallic phase support the model. The variation with temperature of the Raman spectra of La0.5Ca0.5MnO3 is also discussed. The results give a strong indication for charge and orbital ordering and formation of superstructure at low temperatures.
ASJC Scopus subject areas
- Materials Science(all)