Infrared spectroscopic study of CaFe0.7Co0.3O3

Temperature-dependent infrared spectroscopy has been investigated for ${\mathrm{CaFe}}_{0.7}{\mathrm{Co}}_{0.3}{\mathrm{O}}_{3}$ which undergoes a ferromagnetic transition at ${T}_{C}\ensuremath{\approx}177\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. It is observed that the spectral weight is transferred from $\ensuremath{\sim}4800--14 000{\mathrm{cm}}^{\ensuremath{-}1}$ to $\ensuremath{\sim}0$--$4800{\mathrm{cm}}^{\ensuremath{-}1}$ as the temperature is lowered around ${T}_{C}$. Such a large-range spectral weight transfer is attributed to the Hund's interaction. The phonons in ${\mathrm{CaFe}}_{0.7}{\mathrm{Co}}_{0.3}{\mathrm{O}}_{3}$ show minor asymmetric line shapes, implying relatively weak electron-phonon coupling compared with the parent compound ${\mathrm{CaFeO}}_{3}$. The optical conductivity also reveals a broad peak structure in the range of $\ensuremath{\sim}700--1500{\mathrm{cm}}^{\ensuremath{-}1}$. Fit by the model of single-polaron absorption, the broad peak is interpreted by the excitation of polarons. From the fitting parameters of the polaron peak, we estimate the electron-phonon coupling constant $\ensuremath{\alpha}\ensuremath{\sim}0.4--0.5$, implying that ${\mathrm{CaFe}}_{0.7}{\mathrm{Co}}_{0.3}{\mathrm{O}}_{3}$ falls into the weak-coupling regime.