We use resonance Raman scattering (incident photon energies between 1.8 and 4.13 eV), $\text{LDA}+\text{U}$ calculations, spectroscopic ellipsometry, and oblique IR reflectivity to characterize the strong electron-phonon interactions in the disordered perovskite ${\text{LaFe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_3$. When the photon energy coincides with a Cr to Fe Mott-Hubbard transfer gap around 2.4 eV the electron-phonon interaction is manifested by a Franck-Condon effect with exceptional first- and higher order scattering of a local oxygen breathing mode. At higher incident energies we observe a superposition of Franck-Condon scattering and Fröhlich interaction induced infrared active longitudinal optical two-phonon scattering activated mainly by O to Fe charge transfer. Our results establish ${\text{LaFe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_3$ as a model compound for research on electron-phonon interactions in strongly correlated complex systems and show that Franck-Condon scattering in complex solids is not limited to Jahn-Teller active compounds.

• Received 31 March 2009

DOI:https://doi.org/10.1103/PhysRevB.80.075103