Oxygen ionic conduction in brownmillerite CaAl0.5Fe0.5O2.5+δ

The oxygen permeability of CaAl0.5Fe0.5O2.5+δ brownmillerite membranes at 1123-1273 K was found to be limited by the bulk ionic conduction, with an activation energy of 170 kJ/mol. The ion transference numbers in air are in the range 2 × 10-3 to 5 × 10-3. The analysis of structural parameters showed that the ionic transport in the CaAl0.5Fe0.5O2.5+δ lattice is essentially along the c axis. The largest ion-migration channels are found in the perovskite-type layers formed by iron-oxygen octahedra, though diffusion in tetrahedral layers of the brownmillerite structure is also possible. Heating up to 700-800 K in air leads to losses of hyperstoichiometric oxygen, accompanied with a drastic expansion and, probably, partial disordering of the CaAl0.5Fe0.5O2.5+δ lattice. The average thermal expansion coefficients of CaAl0.5Fe0.5O2.5+δ ceramics in air are 16.7 × 10-6 and 12.6 × 10-6 K-1 at 370-850 and 930-1300 K, respectively

Mixed conductivity of zircon-type Ce1-xAxVO 4±δ (A = Ca, Sr)

Incorporation of alkaline-earth cations into the zircon-type lattice of Ce1-xAxVO4+δ (A = Ca, Sr; x = 0 - 0.2) was found to significantly increase the p-type electronic conductivity and to decrease the Seebeck coefficient, which becomes negative at x ≥ 0.1. The oxygen ionic conductivity is essentially unaffected by doping. The ion transference numbers of Cea-xAxVO4+δ in air, determined by the faradaic efficiency measurements, are in the range from 2 × 10-1 to 6 × 10-3 at 973-1223 K, increasing when temperature increases or alkaline-earth cation content decreases. The results on the partial conductivities and Seebeck coefficient suggest the presence of hyperstoichiometric oxygen, responsible for ionic transport, in the lattice of doped cerium vanadates. The activation energies for the electron-hole and ionic conduction both decrease on doping and vary in the ranges 39-45 kJ/mol and 87-112 kJ/mol, respectively

Oxygen transport in Ce0.8Gd0.2O2 - δ-based composite membranes

Gadolinia-doped ceria electrolyte Ce0.8Gd0.2O2 - δ (CGO) and perovskite-type mixed conductor La0.8Sr0.2Fe0.8Co0.2O3 - δ (LSFC), having compatible thermal expansion coefficients (TECs), were combined in dual-phase ceramic membranes for oxygen separation. Oxygen permeability of both LSFC and composite LSFC/CGO membranes at 970-1220 K was found to be limited by the bulk ambipolar conductivity. LSFC exhibits a relatively low ionic conductivity and high activation energy for ionic transport (∼ 200 kJ/mol) in comparison with doped ceria. As a result, oxygen permeation through LSFC/CGO composite membranes, containing similar volume fractions of the phases, is determined by the ionic transport in CGO. The permeation fluxes through LSFC/CGO and La0.7Sr0.3MnO3 - δ/Ce0.8Gd0.2O2 - δ (LSM/CGO) composites have comparable values. An increase in the p-type electronic conductivity of ceria in oxidizing conditions, which can be achieved by co-doping with variable-valence metal cations, such as Pr, leads to a greater permeability. The oxygen ionic conductivity of the composites consisting of CGO and perovskite oxides depends strongly of processing conditions, decreasing with interdiffusion of the phase components, particularly lanthanum and strontium cations from the perovskite into the CGO phase

Spin state ordering and magnetic structures in the cobaltites YBaCo2O5 delta delta 0.50 and 0.44

The antiferromagnetic ferromagnetic phase transition in YBaCo2O5.50 and YBaCo2O5.44 cobaltites with different types of oxygen ion ordering in the [YO0.5 0.44] layers has been studied by neutron powder diffraction in combination with group theoretical analysis. As a result, the crystal and magnetic structures above and below the phase transformation temperature Ti were determined and successfully refined. In both cases, the proposed models involve a spin state ordering between diamagnetic t lt;sub gt;2g lt; sub gt; lt;sup gt;6 lt; sup gt;e lt;sub gt;g lt; sub gt; lt;sup gt;0 lt; sup gt;, S 0 and paramagnetic t lt;sub gt;2g lt; sub gt; lt;sup gt;4 lt; sup gt;e lt;sub gt;g lt; sub gt; lt;sup gt;2 lt; sup gt;, S 2 Co3 ions with octahedral coordination. Electronic ordering results in a nonzero spontaneous magnetic moment in the high temperature magnetic phases with isotropic negative exchange interactions. In the case of YBaCo2O5.5, the phase transformation does not change the Pmma 2ap 2ap 2ap symmetry of the crystal structure. The wave vectors of magnetic structures above and below Ti are k 0 and k c 2, respectively. In the case of YBaCo2O5.44 a crossover P4 nmm 3sqrt 2 ap 3sqrt 2 ap 2ap gt;I4 mmm 3sqrt 2 ap 3sqrt 2 ap 4ap was involved to solve the low temperature magnetic structure. The wave vectors in both high temperature and low temperature magnetic phases are k 0. Mechanisms of the phase transformation in both compositions are discussed in the light of obtained magnetic structures. The proposed spin configurations were compared with other models reported in literatur