Charge disproportionation in (X0.6Sr0.4)0.99Fe0.8Co0.2O3−δ perovskites (X = La, Pr, Sm, Gd)

The change in crystal structure and the oxidation state in iron of iron-cobalt-based perovskites with different A-site cations is investigated by the use of powder XRD and Mössbauer spectroscopy. The perovskites investigated are (X0.6Sr0.4)0.99Fe0.8Co0.2O3−δ, where X is La, Pr, Sm or Gd. It is shown that the crystal structure changes from hexagonal to orthorhombic when the size of the A-site cation is lowered. At room temperature, the oxidation state of iron increases when the size of the A-site cation is lowered. This is explained by a crystal packing effect. At lower temperatures, charge disproportionation of Fe(IV) occurs to Fe(III) and Fe(V). The transition temperature depends on the type of the A-site cation. Some of the Fe is reduced to Fe(III) by transference of electrons from Co upon cooling below room temperature.