Raman spectroscopy of CaMnO3:: Mode assignment and relationship between Raman line intensities and structural distortions -: art. no. 184301

Polarized Raman spectra of oriented finely twinned CaMnO3 thin films and nonpolarized Raman spectra of CaMnO3 ceramics were studied at room temperature using several excitation laser wavelengths. The selection rules for the polarized Raman spectra, obtained from samples consisting of finely twinned orthorhombic domains, were calculated and the symmetry of all observed Raman lines was determined. The relationship between the intensity of the Raman lines and the distortions in the ABO(3) perovskites with GdFeO3-type structure is discussed. These distortions can be described as superposition of four simple basic distortions: two MnO6 octahedral tilts, Jahn-Teller deformation of Mn3+ O-6 octahedra, and shift of the A ions from their sites in the ideal perovskite. Twenty of the 24 Raman-allowed modes in the real GdFeO3-type structure have counterparts in only one of the four simpler structures, obtained by a single basic distortion. The assignment of the Raman lines of CaMnO3 to definite atomic vibrations, most of them activated by a single basic distortion, was made in close comparison with the results of lattice dynamical calculations and the Raman spectra of isostructural LaMnO3 and CaGeO3.