It is well recognized that perspective cathode materials for intermediate-temperature solid oxide fuel cells are cobalt-based perovskite oxides. The aim of this contribution is to study in details the formation of solid solutions between LaCoO₃ and LaNiO₃ perovskites. Two methods for the preparation of LaCo_1-yNi_yO₃ were used: (i) thermal decomposition of La-Co-Ni citrates obtained by freeze-drying of the corresponding solutions, and (ii) the method of Pechini. The distribution of Ni in LaCo_1-yNi_yO₃ solid solutions was examined by means of neutron diffraction and EPR spectroscopy.

Rhombohedrally distorted perovskites LaCo_1-yNi_yO₃ with 0 £ y £ 0.5 are obtained from citrate precursors at 600 ^oC. The oxidation state of (Co+Ni) is slightly higher than 3 for oxides prepared from freeze-dried citrates, while slightly reduced oxides are obtained by the method of Pechini. The substitution of Ni for Co leads to the increase in the unit cell parameters, which is in agreement with the different ionic sizes of Co³⁺ and Ni³⁺ ions. Oxides prepared from freeze-dried citrate precursors display plate-like aggregates with dimensions of about 5 mm, while the method of Pechini allows obtaining well separated particles. Inside the particle aggregates, hexagonal individual particles with sizes of about 70 nm were observed. In addition, HR-TEM analysis and neutron diffraction show the appearance of small spinel particles (with 10 nm sizes) deposited on the main LaCo_0.9Ni_0.1O₃ particles. The lattice parameter of impurity spinel phase is higher than that of pure Co₃O₄ spinel: a=8.40 – 8.45 Å as compared to a=8.08 Å, respectively.

The EPR spectra of LaCo_1-yNi_yO₃ obtained at 600 ^oC consist of a signal due to magnetic Ni clusters. These magnetic clusters show a broad size distribution, the mean size being increased with the Ni content. The distribution of the Ni ions in LaCo_1-yNi_yO₃ depends also on the preparation method. It seems that Pechini method promotes the formation of magnetic nickel clusters.

By increasing the annealing temperature up to 900 ^oC, the magnetic Ni clusters become invisible. In the same sequence, neutron diffraction analysis shows a random distribution of Co and Ni ions in LaCo_1-yNi_yO₃.

Authors are grateful to EC for a grant within the FAME project (FAME FP6-500159-1) and to the Centre of Competence MISSION (SSA, EC-INCO-CT-2005-016414).

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