Optical absorption spectra and the nature of conduction carriers of hole-doped Sr_1+xLa_1-xFeO₄

The optical absorption as calculated from the diffuse reflectivity in the range of 0.5-5eV and the temperature dependence of d.c. electrical conductivity of Sr_1+xLa_1-xFeO₄ (0 ≦ x ≦ 0.3) with a K₂NiF₄ structure, were measured. The results obtained were analyzed based on the small polaron model, and discussed with reference to the changes in the physical properties and the electronic structure previously reported. The Mott type and charge-transfer type excitations were observed at ∼2.2 and ∼2.7eV, respectively, in the optical absorption spectrum of the undoped SrLaFeO₄. An additional broad absorption band below 1 eV was observed, and its intensity was directly proportional to the d.c. electrical conductivity at room temperature. Thus the absorption band below 1 eV was assigned to the optically induced polaron hopping. Based on the small polaron theory, the hopping energies of small polarons from 0.14 to 0.24 eV were obtained. These values were close to the activation energies of d.c. electrical conductivity obtained from the σT^3/2 vs 1/T plots. The results, in which the Fermi-level was fixed in the middle of the energy band gap of the photoemission spectroscopy previously reported could not be explained by the rigid-band model, but were consistent with the small polaron model.