Multiple-order Raman scattering from rare-earth manganites: Oxygen isotope and rare-earth substitution effects

The paper presents results of a detail study of polarized multiple-order Raman spectra of several, isostructural to LaMn O3, RMn O3 compounds (R=Pr,Eu,Dy,Ho,Y) and oxygen isotope substitution effects. A comparison of the spectra of EuMn O316 and isotopically substituted EuMn O318 provides strong evidence that the multiple-order Raman spectra are of pure phonon rather than of orbiton or mixed orbiton-phonon origin. Based on polarization properties and in close comparison with the results of lattice-dynamics calculations, we argue that the prominent structure at 650 cm-1, replicated in the multiple-order scattering, is of phonon density-of-states origin. We also present a group-theoretical analysis of the selection rules for the two-phonon Raman spectra of orthorhombic manganites. The polarized two-phonon Raman spectra of RMn O3 are consistent with a second-order scattering process involving only Brillouin-zone boundary phonons.