In order to evaluate promising directions in the development of mixed-conducting membrane materials for oxygen separation and partial oxidation of natural gas, a series of ferritebased ceramics were studied, including La 1-x Sr x Fe 1-y Ga y O 3-δ (x = 0.5 -0.8; y = 0 -0.4), La 1-x Sr x Fe 1-y Al y O 3-δ (x = 0.7 -1.0; y = 0 -0.5), La 0.3 Sr 0.7 Fe 0.7-x Al 0.3 Cr x O 3-δ (x = 0.1 -0.2), (Sr 2 Fe 3 ) 1-x (SrCo) x O z (x = 0 -0.8), CaFe 0.5 Al 0.5 O 2.5+δ and Ln 3-x Ca x Fe 5 O 12-δ (Ln = Gd, Y; x = 0 -0.5). The maximum oxygen permeation is observed for perovskite-type solid solutions with high oxygen deficiency, which exhibit, however, excessive thermal and chemically induced expansion. As for cobaltite-and gallate-based mixed conductors, the increase in ionic transport is accompanied with increasing limiting role of the surface exchange processes. The stability of perovskite-related ferrites in reducing atmospheres, which is comparable to that of LaFeO 3-δ and iron oxide, may be moderately increased or decreased by donor-or acceptor-type doping, respectively. In addition, the substitution of iron with cations having a more stable oxidation state, such as Ga 3+ , Al 3+ or Cr 3+/4+ , partly prevents the lattice expansion induced by oxygen nonstoichiometry variations, although the solubility of these dopants in the ferrite lattice is limited.

Defect Formation and Transport in La0.95Ni0.5Ti0.5O3-δ. -The ceramic title material is characterized by SEM/EDS, XRD, XPS, dilatometry, conductivity, Seebeck coefficient measurements, and atomistic simulations. At room temperature the title compound crystallizes in the orthorhombic space group Pbnm. The total conductivity of the compound is predominantly p-type electronic. The ionic conductivity is low and has an extremely high activation energy. Defect association, particularly the formation of ternary clusters involving Ni 3+ and La and O vacancies, and strong distortions in the oxygen sublattice around defect clusters appear to be responsible for the low ionic and electronic conductivities. -(YAKOVLEV, S. O.; KHARTON*, V. V.; NAUMOVICH, E. N.; ZEKONYTE, J.; ZAPOROJTCHENKO, V.; KOVALEVSKY, A. V.; YAREMCHENKO, A. A.; FRADE, J. R.; Solid State Sci. 8 (2006) 11, 1302-1311; Dep. Ceram. Glass Eng., Univ. Aveiro, P-3810 Aveiro, Port.; Eng.) -W. Pewestorf 07-014

The p(O2)-T- diagrams of La2Ni1-xMxO4+ (M=Co and Cu, x= 0-0.20), determined by the coulometric titration technique at 923-1223 K in the oxygen partial pressure range 10-4 to 0.6 atm, can be adequately described by equilibrium processes of oxygen intercalation into the rock-salt type layers and hole localization on B-site cations forming 3+ oxidation states. For the hole activity, a non-ideal solution model taking into account the repulsion of p-type electronic charge carriers can be used. The electrostatic repulsion excludes occupation of nearest neighboring sites and leads to splitting of the energy levels for more distant sites. The affinity of Ni and Cu cations with respect to the hole localization is similar and cannot be statistically separated analyzing the oxygen nonstoichiometry data only. On the contrary, cobalt cations tend to remain in the trivalent state and Co3+ should be treated as a separate type of charged point defect. Oxygen vacancies formed in the perovskite-like layers due to intrinsic Frenkel disorder have no essential effect on the oxygen thermodynamics. As expected, the thermodynamic functions governing the intercalation-related processes are independent of defect concentrations.

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Dense ceramic anodes of perovskite-type La1-x-ySrxCo1-zAlzO3-δ ( x = 0.45-0.70; y = 0- 0.05; z = 0-0.20) and K2NiF4-type La2Ni1-xMexO4+δ (Me = Co, Cu; x = 0-0.20), synthesized by the glycine-nitrate technique, were assessed for oxygen evolution in alkaline media. The lowest overpotentials are observed for (La0.3Sr0.7)0.97CoO3-δ, which exhibits a significant oxygen deficiency in combination with high conductivity associated with the A-site cation nonstoichiometry compensation mechanism via Co4+ formation. Perovskite-type cobaltite anodes are essentially stable in alkaline solutions, whilst La2NiO4-based electrodes exhibit degradation at the potentials where the oxygen evolution occurs, probably due to the electrochemical oxygen intercalation in the lattice.