Stability of NdBaCo2−xMnxO5+δ (x = 0, 0.5) layered perovskites under humid conditions investigated by high-temperature in situ neutron powder diffraction

The double perovskites NdBaCo2−xMnxO5+δ (x = 0 and 0.5) were investigated using in situ high temperature neutron powder diffraction in dry argon and wet atmospheres (40% D2O/argon and 40% D2O/air) in order to assess their stability as cathodes in proton conducting fuel cells. The x = 0 oxide loses oxygen on heating in dry argon at T > 400 °C and exhibits an oxygen vacancy order–disorder transition as evidenced by the orthorhombic Pmmm to tetragonal P4/mmm transition. Refinement of site occupancy factors suggests that the oxygen vacancies mainly form in the Nd layers and to a lesser extent at the equatorial positions of the transition metal polyhedra; at 800 °C, δ ∼ 0. When the gas was changed to wet argon at 800 °C and the sample cooled to 260 °C, no structural modification or change in the oxygen content was detected and no impurity phases formed, highlighting the excellent stability of the sample in wet atmospheres. On switching the gas to wet air at 260 °C, thermal analysis and neutron powder diffraction data together reveal that the sample intercalates mainly oxygen rather than proton defects within a two-phase process involving two orthorhombic phases, reflecting the symmetry of the reduced and oxidised materials. On heating, the sample transforms at T ≥ 600 °C to a single tetragonal phase whose symmetry is retained up to 800 °C and on subsequent cooling. The x = 0.5 material prepared in argon adopted a tetragonal P4/mmm structure at RT with δ ∼ 0. Its symmetry remains tetragonal on heating/cooling in wet argon. On changing the gas to wet air at 260 °C, it takes up oxygen via a two-phase process involving two tetragonal phases. Since fast oxidation is the main process that fills the oxygen vacancies of these double perovskites in wet air, a large oxygen deficiency seems to be not the only requirement for effective proton incorporation in this family of materials with basic characteristics

High temperature structural stability, electrical properties and chemical reactivity of NdBaCo2-xMnxO5+δ (0 ≤ x ≤ 2) for use as cathodes in solid oxide fuel cells

© the Partner Organisations 2014.The effects of Mn substitution for Co on the crystal chemistry, oxygen content, thermal expansion and electrical conductivity of the NdBaCo2-xMnxO5+δ perovskites (0 ≤ x ≤ 2) have been investigated. The NdBaCo2-xMnxO5+δ samples exhibit structural changes with increasing Mn contents from orthorhombic (x = 0) to tetragonal (0.5 ≤ x ≤ 1) then to cubic (1.5 ≤ x ≤ 2.0) symmetry. All the samples lose oxygen when heated in air at T > 400 °C although the degree of oxygen loss and kinetics of oxygen exchange between the gas phase and oxide decrease with increasing Mn contents. The thermal expansion coefficients evaluated from ex situ XRD and electrical resistivity decrease with increasing Mn substitution and the values for the x = 1.5 and 2.0 compositions match with those of the Ce0.8Gd0.2O1.95 (GDC) and La0.8Sr0.2Ga0.8Mg0.2O2.8 (LSGM) electrolytes. With electrical conductivity values of >100 S cm-1 at 800 °C and good chemical stability with GDC and LSGM, the Mn-substituted perovskites are promising cathode materials for SOFCs. This journal i

Redox behavior of the SOFC electrode candidate NdBaMn2O5+d investigated by high-temperature in situ neutron diffraction: first real-time characterisation of an LnBaMn2O5.5 intermediate phase

The structural behavior of the tetragonal NdBaMn₂O₅ phase, a member of the family of A-site ordered layered manganites that have been recently suggested as possible mixed ionic and electronic conductors, has been investigated by means of in situ neutron powder diffraction (NPD). Considering applications in energy production and storage devices and use of NdBaMn2O5+₈ as electrode in symmetrical cells, the study was carried out in relevant atmosphere conditions, i.e. dilute hydrogen (wet and dry) and dry air in the temperature range 25- 800 °C. Neutron data under flowing hydrogen allowed monitoring of the structural phase transition from the charge-ordered to the charge disordered state as a function of temperature. Slow reduction of the fullyoxidised phase, NdBaMn₂O₆, previously formed from quick oxidation of the pristine material, enabled real-time observation of the intermediate NdBaMn₂O₅.₅ phase and its crystal characterization up to 700 °C in the course of its conversion to NdBaMn₂O₅. Oxygen vacancy ordering within the Nd layers of NdBaMn₂O₅.₅ correlated to antiferrodistortive orbital ordering of the Jahn-Teller Mn³⁺ion in the square pyramids and octahedra results in large thermal expansion and relatively slow anisotropic oxygen diffusion occurring in the NdO layer. The four heating/cooling cycles evidenced no oxygen miscibility between the three distinct phases detected in the NdBaMn₂O₅+₈ system with ~ 0, 0.5 and 1and clearly demonstrated that reversible oxygen nintercalation/deintercalation underpins the phase stability of the LnBaMn₂O₅+₈ materials to redox cycling and to wet atmosphere in high temperature electrochemical devices

A-site order–disorder in the NdBaMn2O5+d SOFC electrode material monitored in situ by neutron diffraction under hydrogen flow

The A-site disordered perovskite manganite SOFC electrode material, Nd0.5Ba0.5MnO3, has been obtained by heating the A-site-ordered and vacancy ordered layered double perovskite, NdBaMn2O5, in air at 1300 °C for 5 h. Combined Transmission electron microscopy (TEM) images and Neutron powder diffraction (NPD) analysis at 25 °C revealed that Nd0.5Ba0.5MnO3 has a pseudotetragonal unit cell with orthorhombic symmetry (space group Imma, √2 ap × 2 ap × √2 ap) at 20 °C with the cell dimensions a = 5.503(1) Å, b = 7.7962(4) Å, c = 5.502(1) Å, in contrast to Pm-3m or Cmcm that have been previously stated from X-ay diffraction studies. The in situ neutron diffraction study carried out on Nd0.5Ba0.5MnO3 in hydrogen flow up to T~ 900 °C, allows monitoring the A-site cation disorder-order structural phase transition of this representative member of potential SOFC anode materials between air sintering conditions and hydrogen working conditions. Oxygen loss form Nd0.5Ba0.5MnO3 proceeds with retention of A-site disorder until the oxygen content reaches the Nd0.5Ba0.5MnO2.5 composition at 600 °C. The phase transition to layered NdBaMn2O5 with localization of the oxygen vacancies in the Nd layer proceeds at 800 °C with retention of the oxygen content. Impedance spectroscopy measurements for the A-site ordered electrode material, NdBaMn2O6, screen printed on a Ce0.9Gd0.1O2-δ (CGO) electrolyte showed promising electrochemical performance with polarization resistance of 1.09 Ω cm2 at 700 °C in air without any optimization

Self-erasable inkless imprinting using a dual emitting hybrid organic-inorganic material

International audienceSmart emissive materials that can react to external stimuli in a reversible way are challenging to develop and have been the subject of considerable interest. Here, we present a printable hybrid material that can withstand numerous writing-erasing cycles. This material is based on a poly(methyl methacrylate) host matrix embedding by copolymerization of a red NIR phosphorescent metal cluster and a blue green 3-oxindole emitter. Irradiation of the homogeneous and stable hybrid films changes the emission color from white to deep red because of the oxygen perturbed energy transfer from the organic dye to the metal cluster. Because of the low PMMA gas permeability, encrypted data lifetime can be tuned from minutes to days, can be self-erased, and be rewritten at will. This material represents a key stepping stone for anticounterfeiting, optoelectronic, data recording, and many other technologies © 2019 Elsevier Lt

Reduction of Sr2MnO4 investigated by high temperature in situ neutron powder diffraction under hydrogen flow.

International audienceThis experiment emphasizes the first example of two-phase sequential Rietveld refinements throughout a solid/gas chemical reaction monitored by Neutron Powder Diffraction (NPD) at high temperature. The reduction of the n = 1 Ruddlesden-Popper (RP) oxide Sr(2)MnO(4) heated under a flow of 5% H(2)-He has been investigated throughout two heating/cooling cycles involving isothermal heating at 500 and 550 °C. Oxygen loss proceeds above T ∼ 470 °C and increases with temperature and time. When the oxygen deintercalated from the "MnO(2)" equatorial layers of the structure results in the Sr(2)MnO(3.69(2)) composition, the RP phase undergoes a first order I4/mmm → P2(1)/c, tetragonal to monoclinic phase transition as observed from time-resolved in situ NPD. The phase transition proceeds at 500 °C but is incomplete; the weight ratio of the P2(1)/c phase reaches ∼41% after 130 min of isothermal heating. The fraction of the monoclinic phase increases with increasing temperature and the phase transition is complete after 80 min of isothermal heating at 550 °C. The composition of the reduced material refined to Sr(2)MnO(3.55(1)) and does not vary on extended heating at 550 °C and subsequent cooling to room temperature (RT). The symmetry of Sr(2)MnO(3.55(1)) is monoclinic at 550 °C and therefore consistent with the RT structure determined previously for the Sr(2)MnO(3.64) composition obtained from ex situ reduction. Consequently, the stresses due to phase changes on heating/cooling in reducing atmosphere may be minimized. The rate constants for the reduction of Sr(2)MnO(4.00) determined from the evolution of weight ratio of the tetragonal and monoclinic phase in the time-resolved isothermal NPD data collected on the isotherms at 500 and 550 °C are k(500) = 0.110 × 10(-2) and k(550) = 0.516 × 10(-2) min(-1) giving an activation energy of ∼163 kJ mol(-1) for the oxygen deintercalation reaction