Guilhem Dezanneau

Theoretical and experimental study of the structural, dynamical and dielectric properties of perovskite BaSnO3

The structural, dynamical and dielectric properties of the cubic phase of perovskite barium stannate BaSnO3, a potential candidate as protonic conductor for solid oxide fuel cells, have been investigated by the means of first-principles density functional calculations, and the structural and electrical properties have been explored at low temperature. From density functional perturbative calculations, the phonon modes, the Born effective charges and the dielectric tensor are derived and analyzed, at zero pressure. The phonon band-structure of the cubic phase does not exhibit unstable modes, in good agreement with x-ray diffraction, which shows that BaSnO3 remains perfectly cubic down to 10 K. The dielectric response in BaSnO3 as measured and calculated is lower than in titanate and zirconate perovskites.

Molecular dynamics simulations of oxygen diffusion in GdBaCo2O5.5

The 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.

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.

Oxygen diffusion mechanism in the mixed ion-electron conductor NdBaCo2O5+x

Double perovskite cobaltites were recently presented as promising cathode materials for solid oxide fuel cells. While an atomistic mechanism was proposed for oxygen diffusion in this family of materials, no direct experimental proof has been presented so far. We report here the first study that directly compares experimental and theoretical diffusion pathways of oxygen in an oxide, namely in the double cobaltite compound, NdBaCo2O5+x. Model-free experimental nuclear density maps are obtained from the maximum entropy method combined with Rietveld refinement against high resolution neutron diffraction data collected at 1173 K. They are then compared to theoretical maps resulting from classical molecular dynamics calculations. The analysis of 3D maps of atomic densities allows identifying unambiguously the pathways and the mechanisms involved in the oxide ion diffusion. It is shown that oxygen diffusion occurs along a complex trajectory between Nd- and Co-containing a,b planes. The study also reveals that Ba-containing planes act as a barrier for oxygen diffusion. The diffusion mechanism is also supported through the oxygen sites occupancy analysis that confirms the increase of oxygen vacancies in the cobalt-planes on heating. The use of such combined experimental and theoretical analysis should be considered as a very powerful approach for materials design.

Oxygen transport kinetics of the misfit layered oxide Ca3Co4O9+d

The oxygen transport kinetics of the misfit-layered cobaltite, Ca3Co4O9+δ, known for its thermoelectric properties, was investigated by combined application of 18O/16O isotope exchange and electrical conductivity relaxation techniques. Although oxygen diffusion is found to be two orders of magnitude lower than in well-investigated lanthanum nickelates, e.g., La2NiO4+δ, the mixed ionic–electronic conductor Ca3Co4O9+δ is found to exhibit fast surface exchange kinetics (k* = 1.6 × 10−7 cm s−1 at 700 °C to be compared to 1.3 × 10−7 cm s−1 for the nickelate), rendering it a promising electrode for application as an air electrode in solid oxide cells. In parallel, the chemical nature of the outermost surface of Ca3Co4O9+δ was characterized by means of Low Energy Ion Scattering (LEIS) spectroscopy. The absence of cobalt at the samples outermost surface suggests that the Ca2CoO3−δ rock salt layers in the structure may play a key role in the oxygen exchange mechanism.

How dopant size influences the protonic energy landscape in BaSn1−xMxO3−x/2 (M = Ga, Sc, In, Y, Gd, La)

The energy landscape of the protonic defect is investigated in acceptor-doped barium stannate using density-functional calculations. Several trivalent dopants are studied (Ga, Sc, In, Y, Gd, La), covering a wide range of ionic radii. All the dopants are found attractive with respect to the proton, with (negative) association energies varying from −0.40 to −0.07 eV. A radius rc1 is defined to separate the “small” dopants that induce tensile stress from the “large” ones that induce compressive stress in the host matrix (rc1 ≈ 0.72 A). The protonic energy surface exhibits a non-trivial evolution with ionic radius of the dopant: for low dopant radii, the most stable protonic site is the oxygen first-neighbor of the dopant, while for high dopant radii, the most stable position is obtained when the proton is bonded to an oxygen second-neighbor of the dopant. This evolution of the protonic energy surface with dopant ionic radius is smooth and the transition takes place between In and Y, i.e. for a critical radius rc2 between 0.80 and 0.90 A (rc2 > rc1 significantly). The dopant–proton association energy exhibits a minimal value ≈−0.07 eV (weakest attraction) at this transition, i.e. in the case of yttrium, for which the first-neighbor and second-neighbor positions are almost degenerate. Other dopants, smaller or larger, are more attractive to protons. The present study gives useful information about the modification of the trapping effect according to the dopant ionic size.

Grain-boundary resistivity versus grain size distribution in three-dimensional polycrystals

We have studied the accuracy of the bricklayer model in the evaluation of the grain-boundary resistivity associated to a real three-dimensional (3D) polycrystal. An impedance network electrically equivalent to the 3D structure has been solved by direct calculation. In order to examine the progressive evolution from the bricklayer model to a completely disordered system, the ideal polycrystal was modified in a controlled way, according to a Voronoi diagram approach. From this, the grain-boundary resistivity of a polycrystal, as deduced from the 3D bricklayer model, is lower than the real value. The upper limit of the error made is around 15% in relation to the real one.

Hydration, oxidation, and reduction of GdBaCo2O5.5 from first-principles

Structural, magnetic and chemical properties of GdBaCo2O5.5 regarding hydration, oxidation and reduction are studied by density-functional calculations in the DFT + U formalism. Besides the orthorhombic Pmmm structure, a lower energy, tilted, distorted structure is found. The hydrated configurations obtained by water dissociation in the oxygen vacancies of the GdO0.5 plane exhibit strong distortions with respect to the Pmmm structure. Simulation of protonic defects provides the energy landscape of incorporated protons, which preferentially bind to oxygens of the CoO2 planes, suggesting their possible bidimensional diffusion in this plane. We also studied oxygen incorporation (oxidation) in the oxygen vacancies of the GdO0.5 planes, and oxygen removal (reduction) from BaO, CoO2 and GdO0.5 planes. The oxidized compound, GdBaCo2O5.75, is rather p-type metallic, while the reduced compound, GdBaCo2O5.25, remains an insulator, due to electronic localization (Co3+ + e− → Co2+). Taking Pmmm as the reference, both hydration at high water concentration (one H2O per 38-atom supercell) and oxidation are found exothermic. However, if the distorted structure is chosen as the reference, these reactions become endothermic, at least at the high water/oxygen concentration studied. Reduction is, in both cases, endothermic. Nevertheless, negative formation energies of the protonic defects suggest the possibility of hydration at lower water concentration.

Influence of isotropic and biaxial strain on proton conduction in Y-doped BaZrO3: a reactive molecular dynamics study

Strain has been proposed as a potential tool to increase the oxygen ion conduction in oxides. Here we study by means of molecular dynamics simulations the influence of isotropic and biaxial strain on the proton conductivity of yttrium-doped barium zirconate to examine whether a similar influence occurs for hydrogen diffusion. Compressive isotropic pressure is indeed shown to favour proton diffusion by diminishing the oxygen–oxygen distance without affecting the symmetry. For moderate biaxial strain, a similar effect is observed i.e. a slight increase of proton conductivity occurs under compressive strain. High biaxial compressive/negative strain leads to a decrease in proton diffusion by inducing a symmetry breaking that results in a strong localisation of protons away from the B cations. The results are discussed and compared with previous DFT calculations and experimental results.

Oxide ion and proton transport in Gd-doped barium cerate: a combined first-principles and kinetic Monte Carlo study

Oxide ion and proton transport properties in the fuel cell electrolyte Gd-doped BaCeO3 are investigated by first-principles density-functional calculations and kinetic Monte Carlo simulations. Behind the apparent complexity of the energy landscape (related to the low-symmetry of the system), general tendencies governing the energy barriers can be extracted. In particular, the set of barriers is tested with respect to the Bell–Evans–Polanyi (BEP) principle that relates the activation energy Ea of a series of similar chemical reactions to their reaction enthalpies ΔH. This rule is found poorly satisfied in the case of oxide ion migration, but much better satisfied for proton hopping mechanisms. Protonic reorientations, by contrast, do not obey the BEP rule. Kinetic Monte Carlo simulations give insight into the macroscopic transport properties. We observe that dopants act as traps for oxygen vacancies and slow down their motion, whereas their effect on the protonic diffusion coefficient is more complex. This is related to the fact that a two-state picture roughly applies to the oxygen vacancy energy landscape, while it does not apply to the protonic one. Oxide ion migration exhibits strong anisotropy, the motion of the oxygen vacancies being favored in the equatorial planes, while protonic diffusion is found more isotropic.