Electron-phonon interactions in perovskites containing Fe and Cr studied by Raman scattering using oxygen-isotope and cation substitution

We use temperature-dependent inelastic light scattering to study the origin of the strong multiphonon scattering of a local oxygen breathing mode present in the mixed $B$-site orthorhombic (space group $Pnma$) perovskite ${\text{LaFe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_{3}$ but absent in isostructural ${\text{LaFeO}}_{3}$ and ${\text{LaCrO}}_{3}$. It is seen that the multiphonon scattering is critically sensitive to the presence of both Fe and Cr ions on the $B$ site. These results support our interpretation that the multiphonon scattering is activated by local electron-phonon interactions according to the Franck-Condon picture following an Fe-Cr charge transfer. Further, $^{18}\text{O}$ substitution is performed on the $x=0$, 0.04, and 0.5 compounds and clearly shows that all modes appearing above the first-order phonon-scattering region in these compounds originate from higher-order oxygen stretching vibrations. In particular this is the case for the strong second-order scattering dominating the scattering response in ${\text{LaFeO}}_{3}$. Accordingly we propose that these modes are generated by infrared-active longitudinal optical (IR LO) two-phonon and combination scattering activated by Fr\"ohlich interaction. For $x=0.02$ and 0.04 the characteristic IR LO two-phonon and Franck-Condon multiphonon-scattering profiles mix. We also study the influence of isovalent cation substitution and Sr doping in $A{\text{Fe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_{3}$ ($A=\text{La}$, Nd, and Gd) and ${\text{La}}_{1\ensuremath{-}y}{\text{Sr}}_{y}{\text{Fe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ ($y=0$, 0.16, and 0.5) on the strong electron-phonon coupling present in ${\text{LaFe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_{3}$. The Franck-Condon effect in ${\text{LaFe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_{3}$, is not significantly affected by isovalent $A$-site substitution, despite the increasing orthorhombic distortion associated with decreasing $A$-site ionic radii. On the contrary, aliovalent Sr doping causes a rapid decrease in the Franck-Condon scattering. This shows that the strong electron-phonon coupling in these compounds is highly sensitive to local lattice and electronic decoherence but insensitive to global lattice distortions. Finally, a preliminary assignment of the ${A}_{g}$ and ${B}_{2g}$ phonon modes in $A{\text{Fe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_{3}$ ($A=\text{La}$, Nd, and Gd) is made based on the present observations and published results for ${\text{LaCrO}}_{3}$ and $A{\text{MnO}}_{3}$. The modes associated with oxygen octahedral tilt and bending vibrations are heavily influenced by the magnitude of the orthorhombic distortion.

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