Infrared spectroscopic study of CaFe0.7Co0.3O3

Temperature-dependent infrared spectroscopy has been investigated for CaFe0.7Co0.3O3 which undergoes a ferromagnetic transition at TC≈177K. It is observed that the spectral weight is transferred from ∼4800–14 000cm−1 to ∼0–4800cm−1 as the temperature is lowered around TC. Such a large-range spectral weight transfer is attributed to the Hund's interaction. The phonons in CaFe0.7Co0.3O3 show minor asymmetric line shapes, implying relatively weak electron-phonon coupling compared with the parent compound CaFeO3. The optical conductivity also reveals a broad peak structure in the range of ∼700–1500cm−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 α∼0.4–0.5, implying that CaFe0.7Co0.3O3 falls into the weak-coupling regime.