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 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

Synthesis and structural characterisation of the new K₂NiF₄-type phases, A₂In_0.5Sb_0.5

In this paper we report the synthesis and structural characterisation of two new K2NiF4-type phases, Ba2In0.5Sb0.5O4 and Sr2In0.5Sb0.5O4. To our knowledge these are the first examples of K2NiF4 compounds of general formula A2MIII0.5M'V0.5O4 with both 3+ and 5+ cations in the octahedral sites. Ba2In0.5Sb0.5O4 is shown to have a tetragonal cell (space group I4/mmm, a=4.1651(1), c=13.299(1) Å) with an essentially disordered arrangement of In and Sb. In the case of Sr2In0.5Sb0.5O4, however, ordering of In and Sb is observed leading to an expanded unit cell (Pmcb, a=5.7592(1), b=5.7740(1), c=12.543(1) Å). The results therefore show that varying the size of the alkaline earth cation has a pronounced effect on the ordering of In and Sb within the structure.</p

Structural characterization of mixed conducting perovskites La(Ga,M)O3-δ ( M = Mn, Fe, Co, Ni)

Comparative analysis of the structure refinement results of perovskite-like LaGa0.5M0.5O3-δ (M = Mn, Fe, Co, Ni) and data on other LaGaO3-based phases, heavily doped with transition metal cations, shows that on doping the structural changes in these oxides follow common trends for the perovskite-type systems. The maximum ionic conductivity, observed in various perovskites when the tolerance factor values are approximately 0.96-0.97, was found to correlate with the transition from orthorhombic to rhombohedral structure and maximum lattice distortion. The perovskite unit cell distortion near the orthorhombic-rhombohedral phase boundary may hence play a positive role in the ionic transport processes. © 2002 Elsevier Science Ltd. All rights reserved