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

Abstract

The paper presents results of a detail study of polarized multiple-order Raman spectra of several, isostructural to $\mathrm{La}\mathrm{Mn}{\mathrm{O}}_{3}$, $R\mathrm{Mn}{\mathrm{O}}_{3}$ compounds $(R=\mathrm{Pr},\mathrm{Eu},\mathrm{Dy},\mathrm{Ho},\mathrm{Y})$ and oxygen isotope substitution effects. A comparison of the spectra of $\mathrm{Eu}\mathrm{Mn}\phantom{\rule{0.2em}{0ex}}^{16}\mathrm{O}_{3}$ and isotopically substituted $\mathrm{Eu}\mathrm{Mn}\phantom{\rule{0.2em}{0ex}}^{18}\mathrm{O}_{3}$ 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 $\ensuremath{\approx}650\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, 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 $R\mathrm{Mn}{\mathrm{O}}_{3}$ are consistent with a second-order scattering process involving only Brillouin-zone boundary phonons.