Heterobimetallic Ba–Co aminopolycarboxylate complexes as precursors for BaCoO3-δ oxides; towards a one-stage-deposition of cobaltite films

Abstract

Aminopolycarboxylate (APC) ligands have been selected to prepare seven new heterobimetallic barium-cobalt complexes as molecular precursors for BaCoO_3-δ oxide materials. The use of APC = nitrilotriacetate (nta³⁻), ethylenediaminetetraacetate (edta⁴⁻), cyclohexane-1,2-diaminetetraacetate (cdta⁴⁻) and diethylenetriaminepentaacetate (dtpa⁵⁻) polydentate chelating agents enables the possible incorporation of Ba and Co^II or Co^III in a 1 : 1 ratio. The crystal structure of the new BaCo^II(nta)CH₃COO·2H₂O and BaCo(cdta)·5H₂O have been refined from single crystal X-ray diffraction data. Our choice for the barium-cobalt system is based on the existence of a number of distinct BaCoO_3-δ hexagonal perovskite polytypes, depending on the oxygen stoichiometry. It yields an easy probing of the cobalt oxidation state after thermal treatments. Temperature controlled X-ray diffraction (TCXRD) technique has been used to characterize the phases in competition upon thermal cycles of the precursors. The influences of the nature of ligands, initial cobalt oxidation state, temperature and working atmosphere on the process of formation of the mixed-oxide have been investigated. All the APC-precursors show appearance of a mixture of crystallized barium carbonate and Co₃O₄ in various ratio during the degradation process under air or flowing oxygen at 350–450 °C. Further thermal treatment under flowing oxygen leads to pure submicronic 2H-BaCoO_3-δ above 650 °C in the case of cdta complexes, i.e. in softer conditions compared to the solid state route performed under high oxygen pressure. No essential influence of the cobalt valence in the precuror complexes was observed regarding the target BaCoO_3-δ oxygen non stoichiometry. Finally, with a view to access porous electrodes for SOFCs, we demonstrated the possibility of one-stage-deposition of BaCoO_3-δ layers on dense YSZ substrates, with a relatively good adherence, via the high temperature degradation of these complexes.