Characterization of doped CMR-manganite perovskites by Raman spectrometry

Colossal magnetoresistive manganites of the solid solution series (La_1-xPr_x)_0.7Ca_0.3MnO₃ show first order Raman active phonon bands with A_g symmetry near 80 cm^-1, a doublet structure between 220 cm^-1-290 cm^-1 and a further band near 439 cm^-1. Similar bands were found in ceramics and films of similar composition and with a thickness of down to 60 nm. The bands are superimposed by a strong background scattering with broad maxima near 490 cm^-1 and 610 cm^-1. The bands between 220 cm^-1-290 cm^-1 and 439 cm^-1 were unambiguously identified as oxygen bands by isotope exchange experiments. The background maxima were related to second-order Raman phonon scattering in the same way. The variations of the double band structure between 220 cm^-1-290 cm^-1 can be quantitatively correlated with the orthorhombic distortion of the lattice, and provide a measure for the average ionic size of the A cation site, or the tolerance factor which describes the distortion of the octahedral network and determines the electronic properties of the system. Shifts of the Raman phonon bands related to an oxygen isotope exchange process can be quantitatively explained by the isotope mass ratio. Colossal oxygen isotope effects found in (La_1-xPr_x)_0.7Ca_0.3MnO₃ ceramics and films can be quantitatively related to the changing isotope composition. Significant contributions from lattice effects or impurities can be excluded from the Raman data in this case.