Infrared reflectivity and electronic states in perovskite-type oxides La1-xSrxFeO3and La1-xSrxCoO3

Optical phonon spectra are investigated by infrared reflectivity measurements for La 1-x Sr x FeO 3 (x=0, 0.1, 0.25 and 0.4) and La 1-x Sr x CoO 3 (x=0, 0.02 and 0.1). Using the phonon parameters determined by Kramers-Kronig and oscillator analysis, the effective charges and the electron-phonon coupling constants are evaluated for each phonon mode. On the basis of these results, the chemical bonding and the electronic band structure of these materials are discussed. It is found that the bonding in LaCoO 3 is covalent and that it is not affected by the Sr substitution. However, the bonding in LaFeO 3 is rather ionic and the Sr substitution increases the degree of the covalency in this material.