Aurélie Rolle

Red-ox behaviour in the La0.6Sr0.4CoO3±δ-CeO2 system

The compositions in the (100 − x)La0.6Sr0.4CoO3±δ-xCeO2 (LSCC) system with 5 ≤ x ≤ 76 are two-phase at room temperature. They consist of the modified perovskite with rhombohedral symmetry (Rc) and modified ceria with fluorite structure (Fmm). The cross-dissolution of La, Sr, Co and Ce cations between the initial La0.6Sr0.4CoO3±δ (LSC) and CeO2 takes place and results in the modification of the initial phases. This is particularly important for the modified ceria. The lattice parameter of the modified ceria increases due to the dissolution of La and Sr cations with larger ionic radii, thereby changing noticeably the oxygen sublattice in the fluorite structure. Above 300 °C LSCCx composites are three-phase due to the reversible change in the symmetry from rhombohedral (Rc) to cubic (Pmm) within the perovskite phase. Red-ox behaviour of the LSCC composites has been explored under air and argon atmospheres in terms of evolution of the chemical composition at the grains surface and phase interfaces, formation of oxygen vacancies and thermochemistry of this process. Reversible red-ox behaviour was observed in LSCCx with x = 8–37 most probably due to an observed high surface concentration of Co cations, that can be easily involved in the reduction/re-oxidation cycle. The increase in the surface concentration of Ce4+ cations together with the decrease in surface concentration of Co cations seems to result in the differences in the reduction and oxidation behaviour under air in LSCCx with x = 57–76. Formation of oxygen vacancies in LSC, LSCC02 and LSCCx with x = 5–76 in air was not accompanied by any distinct thermal events. This process becomes more endothermic with further increase in oxygen nonstoichiometry (δ) above certain values: δ > 0.08 in LSC, δ > 0.13 in LSCC02, and LSCC with x = 5–76. The LSCCx with x = 5–37 and with x = 57–76 show slightly different reduction behaviour under a(o2) = 7.4 × 10−5. In the composites with a relatively low CeO2 content, the extent of the reduction is proportional to the Co content in a composition, whereas the reduction of the LSCCx with x = 57–76 was more significant than expected. The changes in the enthalpy of oxygen vacancy formation and the kinetics of reduction have been discussed.

Remarkable transition from rocksalt/perovskite layered structure to fluorite/rocksalt layered structure in rapidly cooled Ln2CuO4

Lanthanide cuprates of formula Ln2CuO4 exist in two principal forms, T and T′ which are renowned for their exhibition at low temperatures of hole and electronic types of superconductivity, respectively. These structures differ primarily in the arrangement of oxygen between the perovskite layers and also in nature of the copper oxygen planes. The Cu-O distance in the T structure (~1.90 Å) is much shorter than the T′ (1.97Å), reflecting a transition between partial Cu+ and partial Cu3+ character. In seeking to find compositions that bridge these two structure/electron carrier types, we observed the transition from a T structure to a T′ type structure, resulting in the metastable form T″ with slightly larger volume but similar character to T′. This transition from T to T″ is associated with 5% increase in a and a 5% decrease in c parameters of the tetragonal unit cells, which results in disintegration of ceramic bodies.

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

Aminopolycarboxylate (APC) ligands have been selected to prepare seven new heterobimetallic barium-cobalt complexes as molecular precursors for BaCoO3-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 CoII or CoIII in a 1 : 1 ratio. The crystal structure of the new BaCoII(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 BaCoO3-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-BaCoO3-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 BaCoO3-δ oxygen non stoichiometry. Finally, with a view to access porous electrodes for SOFCs, we demonstrated the possibility of one-stage-deposition of BaCoO3-δ layers on dense YSZ substrates, with a relatively good adherence, via the high temperature degradation of these complexes.

Effect of dopant on the thermal and electrical behavior of nanostructured ceria materials

Nanopowders with compositions Ce<inf>0.8</inf>Gd<inf>0.2</inf>O<inf>2−δ</inf> (CG), Ce<inf>0.8</inf>Sm<inf>0.2</inf>O<inf>2−δ</inf> (CS), and Ce<inf>0.9</inf>Eu<inf>0.1</inf>O<inf>2−δ</inf> (CE) have been synthesized by a modified Pechini method. X-ray diffraction (XRD) revealed that all powders calcined at 550°C were single phase, with the cubic fluorite-type structure. The good sintering properties of the synthesized nanopowders allowed us to obtain dense ceramics (>97% theoretical density). The ionic conductivities of doped ceria ceramics were investigated as a function of temperature by using AC impedance spectroscopy in the temperature range 300–800°C. The best conductivity was evidenced for the Ce<inf>0.8</inf>Sm<inf>0.2</inf>O<inf>2−δ</inf> sample.

Oxide Ion Conduction in Oxygen Rich Doped Ba2In2O5+δ Brownmillerite

In order to stabilize the fast oxide ion conducting properties of the high temperature form of Ba 2 In 2 O 5 at lower temperature, oxygen rich doped Ba 2 In 2 O 5+δ brownmillerite were prepared by partial substitution of Ba and In with higher valence cation. Vanadium, niobium, tantalum, molybdenum and tungsten were introduced on indium site and bismuth on barium site. Solid solutions were evidenced in all the cases. They were characterized by high temperature X-ray diffraction and impedance spectroscopy. A neutron diffraction study was carried out and allowed to extract preferential oxygen pathways in these materials.