1 Department of Micro- and Nanotechnology, Technical University of Denmark2 Department of Physics, Technical University of Denmark3 Risø National Laboratory for Sustainable Energy, Technical University of Denmark4 DTU Danchip, Technical University of Denmark5 Technical University of Denmark6 Department of Energy Conversion and Storage, Technical University of Denmark
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 Mossbauer spectroscopy. The perovskites investigated are (X0.6Sr0.4)(0.99)Fe0.8Co0.2O3-delta, 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.
Solid State Ionics, 2005, Vol 176, Issue 17-18, p. 1555-1561