Molecular dynamics simulations of oxygen diffusion in GdBaCo2O5.5

International audienceThe mechanisms of oxygen diffusion in GdBaCo2O5.5 compound are investigated by molecular dynamics simulations. The results confirm that diffusion is mainly bidimensional with oxygen moving in the a,b plane while diffusion along the c axis is much more difficult. Between 1000 and 1600 K, the activation energy for diffusion is about 0.6 eV, close to experimental values. Going deeper inside the oxygen diffusion mechanism, we see that this diffusion occurs mainly in the cobalt planes while most of the oxygen vacancies are kept in the Gd planes. Analysis of oxygen motions show that Gd planes can be seen as source-sink for the oxygen vacancies rather than as fast pathways

Thermodynamics of hydration and oxidation in the proton conductor Gd-doped barium cerate from density functional theory calculations

International audienceHydration and oxidation of gadolinium-doped barium cerate, a system with highly promising properties when used as electrolyte for protonic ceramic fuel cells, are investigated by means of density functional calculations. The energy landscape of oxygen vacancies and interstitial protons in this strongly distorted orthorhombic perovskite is computed. Although the most stable sites for protons are found in the close vicinity of the dopant, the picture of a very complex energy landscape emerges, in which some sites far away from Gd are found more stable than other ones in its close vicinity, due to the highly distorted geometry of the hostmaterials. The fully hydrated phase can be approximated by a structure with 16 local minima. Both hydration (water incorporation) and oxidation (oxygen incorporation) are found to be exothermic processeswith reaction enthalpies of−1.34 eV/H2Omolecule and −0.70 eV/O atom, respectively. The hole polaron resulting from the exothermic incorporation of oxygen is found localized on oxygens around the dopant (small polaron) and carries a spin magnetic moment. Finally, the competition between hydration and oxidation is studied and discussed as a function of oxygen and water partial pressures

Densification par Spark Plasma Sintering (SPS) de matériaux d’électrolytes, difficilement densifiables, pour piles à combustible

Des matériaux tels que les apatites à base d’oxydes de lanthane et de silicium ou des pérovskites conductrices protoniques, potentiellement utilisables au sein de piles à combustible, présentent une grande résistance au frittage. Celle-ci limite d’autant plus leur utilisation au sein de piles à combustible, surtout s’ils doivent être employés comme électrolytes. Plusieurs stratégies peuvent être envisagées pour remédier à ce problème parmi lesquelles l’emploi de nouvelles méthodes de frittage ou le choix d’une méthode de synthèse efficace (permettant par exemple de diminuer la taille des grains ou de limiter celle des agrégats souvent rédhibitoires au moment du frittage). Nous présentons ici les résultats de frittage par Spark Plasma Sintering (appelé par la suite SPS) en les comparants à ceux obtenus par frittage conventionnel haute température. Les matériaux étudiés ont des compositions dérivées des phases La9,330,67Si6O26 et BaZr0,9Y0,1O2,950,05 pour lesquelles des problèmes de frittage ont été rencontrés. Nous insisterons sur les particularités des matériaux obtenus par SPS en termes de structure et microstructure et des conséquences sur les propriétés de transport anionique

Influence of synthesis route and composition on electrical properties of La9.33 + xSi6O26 + 3x/2 oxy-apatite compounds

Oxy-apatite materials La9.33 + xSi6O26 + 3x/2 are thought as zirconia-substitutes in Solid Oxide Fuel Cells due to their fast ionic conduction. However, the well-known difficulties related to their densification prevent them from being used as such. This paper presents strategies to obtain oxyapatite dense materials. First, freeze-drying has been optimized to obtain ultrafine and very homogeneous La9.33 + xSi6O26 + 3x/2 (0≤x≤0.67) nanopowders. From these powders, conventional and Spark Plasma Sintering (SPS) have been used leading to very dense samples obtained at temperatures rather lower than those previously reported. For instance, SPS has allowed to prepare fully dense and transparent ceramics from 1200 °C under 100 MPa. The microstructure and transport properties of such samples have been then evaluated as a function of sintering conditions and lanthanum content. It will be show that for lanthanum content higher than 9.60 per unit formula, the parasitic phase La2SiO5 appears leading to a egradation of conduction properties.We also show that grain boundaries and porosity (for conventionally-sintered materials) seem to have blocking effects on oxygen transport. The highest overall conductivity values at 700 °C, i.e. σ700 °C=7.33.10−3 S cm−1, were measured for La9.33Si6O26 material conventionally-sintered at 1500 °C which contains bigger grains' size by comparison with σ700 °C=4.77.10−3 S cm−1 for SPS-sintered materials at the same temperature but for few minutes. These values are associated with activation energies close to 0.83–0.91 eV, regardless of sintering condition, which are commonly encountered for anionic conductivity into such materials

Preparation of transparent oxyapatite ceramics by combined use of freeze-drying and spark-plasma sintering

Lanthanum silicate oxyapatites, ion-conducting materials presenting a strong aversion against densification, have been obtained in the form of dense transparent ceramics, by combining the beneficial use of freeze-drying and spark plasma sintering methods

Impact of uniaxial strain and doping on oxygen diffusion in CeO2

Doped ceria is an important electrolyte for solid oxide fuel cell applications. Molecular dynamics simulations have been used to investigate the impact of uniaxial strain along the directions and rare-earth doping (Yb, Er, Ho, Dy, Gd, Sm, Nd, and La) on oxygen diffusion. We introduce a new potential model that is able to describe the thermal expansion and elastic properties of ceria to give excellent agreement with experimental data. We calculate the activation energy of oxygen migration in the temperature range 900-1900K for both unstrained and rare-earth doped ceria systems under tensile strain. Uniaxial strain has a considerable effect in lowering the activation energies of oxygen migration. A more pronounced increase in oxygen diffusivities is predicted at the lower end of the temperature range for all the dopants considered

Simulations of oxygen diffusion mechanism in Nd1-xAExBaInO4-x/2(AE=Ca,Sr,Ba) through molecular dynamics

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Oxygen incorporation in acceptor-doped perovskites

International audienceOxygen is experimentally known to be incorporated in acceptor-doped perovskites at high temperatures, leading to a hole conductivity proportional to p 1/4 O2 and increasing with temperature [ 1 2O2 + V ** O → OXO + 2h *]. Either this high-temperature incorporation is thermodynamically favored by temperature, suggesting an endothermic process ( H0 > 0), or it is exothermic. In the latter case, since it is obviously associated with a S0 < 0, the process should be favorable only at low temperatures, except if kinetically blocked. To examine this phenomenon, the reaction ofO2 incorporation into the acceptor-doped perovskites BaSnO3 and BaZrO3, doped by trivalent dopants (Ga, Sc, In, Y), according toBaSn/Zr1−xMxO3−x/2 + x/4O2 → BaSn/Zr1−xMxO3, is studied by density-functional calculations for a high dopant concentration (x = 0.25). In this process, the charged vacancies V ** O resulting from the charge compensation produced by doping, are filled with oxygen atoms, yielding a metallic compound with holes. It is found to be exothermic in all cases, showing that these acceptor-doped perovskites are able to incorporate oxygen at low temperatures, whereas the reaction is unfavorable above a given temperature, whose value is discussed. At any rate, it is suggested that the process is kinetically blocked at low temperatures due to very slow thermally activated vacancy diffusion. A thermochemical approach is presented that tentatively explains why the hole conductivity increases with temperature at high temperatures, although the hole concentration decreases, yielding a model compatible with experimental observations and theoretical calculations

Reply to the ‘Comment on “Proton transport in barium stannate: classical, semi-classical and quantum regime”’

International audienceWe respond to the erroneous criticisms about our modeling of proton transport in barium stannate [G. Geneste et al., Phys. Chem. Chem. Phys., 2015, 17, 19104]. In this previous work, we described, on the basis of density-functional calculations, proton transport in the classical and semi-classical regimes, and provided arguments in favor of an adiabatic picture for proton transfer at low temperature. We re-explain here our article (with more detail and precision), the content of which has been distorted in the Comment, and reiterate our arguments in this reply. We refute all criticisms. They are completely wrong in the context of our article. Even though a few of them are based on considerations probably true in some metals, they make no sense here since they do not correspond to the content of our work. It has not been understood in the Comment that two competitive configurations, associated with radically different transfer mechanisms, have been studied in our work. It has also not been understood in the Comment that the adiabatic regime described for transfer occurs in the protonic ground state, in a very-low barrier configuration with the protonic ground state energy larger than the barrier. Serious confusion has been made in the Comment with the case of H in metals like Nb or Ta, leading to the introduction of the notion of (protonic) “excited-state proton transfer”, relevant for H in some metals, but (i) that does not correspond to the (ground state) adiabatic transfers here described, and (ii) that does not correspond to what is commonly described as the “adiabatic limit for proton transfer” in the scientific literature. We emphasize, accordingly, the large differences between proton transfer in the present oxide and hydrogen jumps in metals like Nb or Ta, and the similarities between proton transfer in the present oxide and in acid–base solutions. We finally describe a scenario for proton transfer in the present oxide regardless of the temperature regime