Ru-Pyrochlores: Compositional Tuning for Electrochemical Stability as Cathode Materials for IT-SOFCs

International audienceUsing XRD, impedance spectroscopy, and XPS, it was shown that both Bi2Ru2O7 and Pb2Ru2O6.5, generally announced as attractive electrode materials for IT-SOFCs, chemically react with CGO. In addition, a never-mentioned time-instability under electrochemical measurements with YZS as electrolyte mediates the polarization tests. For Pb2Ru2O6.5, long time experiments (stabilization time > 900 h, previously to complementary electrochemical tests), yields strong increasing of the resistance polarization. Concerning Bi2Ru2O7, a partial transformation into Bi3Ru3O11 is observed after impedance measurements. The investigation of the Bi2−xMxRu2O7−δ (M = Sr, Pb) solid solutions was performed with the aim to improve the catalytic/electric properties. For low x values, it shows a time and chemical stabilization of the electrode/electrolyte cells, able to preserve high electrode performances of these materials. For higher substitution, the chemical/electrochemical instability reappears. According to our polarization measurements versus oxygen pressure and temperature in the so-called “stabilized” samples, the electrode performance appears limited by the diffusion of adsorbed oxygen on the triple point boundary. It is fully compatible with the metallic behavior checked in all the series

Study of the structural, magnetic, and electrical properties of the 5H Hexagonal-Type Perovskite BaMn0.2Co0.8O2.80

A combination of X-ray, neutron and electron diffraction, and high-resolution electron microscopy have been used to establish the 5H structural type of a new hexagonal-type perovskite BaMn0.2Co0.8O2.80. The structure can be described as a (cc′chh) 5H hexagonal polytype with ordered oxygen vacancies where the cubic c′ layer corresponds to a composition of [BaO2] as opposed to [BaO3]. The resulting layer structure consists of [MnCo2O12] blocks of three sharing faces octahedra linked by corners to two unconnected [CoO4] tetrahedra. Electron Energy Switch order Loss Spectroscopy shows Mn to be present only as Mn(+IV) and therefore Co is present as mixed +III/+IV. Mn(+IV) and Co(+III) ions are distributed over the face sharing octahedral sites whereas Co(+IV) ions are located on the tetrahedral sites. The magnetic behavior is more complex than is observed for BaCoO2.80 (a ferromagnet with Tc = 47 K) and can be described by a Stoner–Wohlfarth model of random-anisotropic, noninteracting monodomain ferromagnetic clusters. The ferromagnetic clusters occur below 35 K and are assigned to groups of Co ions in octahedral and/or tetrahedral sites; however, incorporation of Mn ions in the octahedral sites disrupts the transition into long-range three-dimensional ferromagnetic order. Impedance Spectroscopy data reveals semiconducting grain conductivity at room temperature (1 × 10−2 S cm−1); however, subambient data reveal an unusual temperature dependence with a smooth changeover from a thermally activated process (0.07 eV) in the range 40–300 K to a low-temperature state below 40 K with a near-zero activation energy. The data cannot be described by conventional Arrhenius or variable-range hopping conduction models and the conduction mechanism(s) remain unresolved. Several possible suggestions for the conductivity behavior are made, including Anderson localization, anisotropic conduction associated with the 5H crystal structure or some complex correlated mechanism between the magnetic and electronic transport properties. The electrical microstructure of BaMn0.2Co0.8O2.8 ceramics consist of semiconducting grains and constrictive grain boundaries and therefore exhibit internal barrier layer capacitor (IBLC) behavior, with a high and temperature-stable apparent permittivity of 10 000 (at 10 kHz) above 100 K

Spin-Flop Transition and Magnetocaloric Effect through Disconnected Magnetic Blocks in CoIII^{III} /CoIV^{IV} Oxybromides

The magnetic properties of the layered Banþ1ConO3n-1Br (n=5 and 6) oxybromides have been determined by means of magnetization measurements and neutron diffraction under variable magnetic field. For n=6, the magnetic phase diagram has been built on the basis of several features, including the notions of short-range ordering, spin reorientation, and spin-flop transitions. In those compounds, the competition between magnetic exchanges arise from the existence of [Ba2O2Br]- double-layers that separate perovskite 2D-blocks. The latter are dominated by ferromagnetic (FM) intraexchanges while the interblock exchanges are antiferromagnetic (AFM). Interestingly, a perturbation created by an external magnetic field could be on the same order of magnitude than the interblock exchange leading to a complex set of spin reorientations versus H and T. From the point of view of magneto-crystalline anisotropy, theAFMsystem with moments parallel to the c-axis turns into a FM system with moments aligned in the perpendicular (a,b) planes. The magnetic entropy is distributed within at least three phenomena, spread out in a wide range of temperature. Here, the influence of the magneto-crystalline anisotropy on the magnetocaloric effect is unambiguously shown

Crystal chemistry and selected physical properties of inorganic fluorides and oxide-fluorides

importance in the development of many new technologies, andare impacting various key points of modern life, that is, energyproduction and storage, microelectronics and photonics,catalysis, automotive, building, etc. Many research fields andapplications are indeed concerned by a better knowledge of therelationships occurring between the structure of suchcompounds and some pertinent physical properties. ThisReview deals with the structural chemistry of solid-stateinorganic fluorides and oxide-fluorides, mostly transitionmetal-based, including rare-earth elements. Such a Review hasnot been published for a long time.1 Articles that recentlyappeared on inorganic fluorinated compounds were mostlyfocused on material science characteristics: morphology, surfacefunctionalization, nanostructuration of the materials andapplications, rather than on the description of characteristicstructural features.2−5 Detailed reviews focused on rare earthbasedinorganic fluorides have also appeared some yearsago..