X-ray photoelectron spectroscopy investigation of perovskites La_1−x Sr _x FeO_3−y (0 ≤x < 1.0), prepared via a mechanochemical route

Abstract

Perovskite-structure oxides La_1−x Sr _x FeO_3−y (x = 0, 0.2, 0.6, 1) were synthesized by the mechanochemical method. In order to refine the stoichiometric composition and the charge state of ions, these samples were studied by X-ray photoelectron spectroscopy (XPS). An investigation of perovskites with x = 0, 0.2, and 0.6 in air at room temperature showed that these samples do not contain oxygen vacancies (y = 0), i.e., they are fully oxidized. Hence, to produce electrical neutrality, these samples should contain iron(4+) cations in an amount proportional to the degree of substitution (x) of strontium(2+) for lanthanum(3+). However, no Fe⁴⁺ cations were found in the oxides. All perovskites contain only Fe³⁺ cations, oxygen ions O²⁻ along with oxygen ions with reduced electron density (O⁻). These data provid evidence of the possible electron density redistribution from oxygen ions to iron cations. The fact that the oxides contain oxygen ions with reduced electron density suggests that weakly bound lattice oxygen in substituted perovskites is represented by O⁻ ions.