Structure and properties of nonstoichiometric mixed perovskites A3B′1+xB2-xO9-δ

Nonstoichiometric mixed perovskites A(3)A(1 + x)'B2 - x''O9 - delta, e.g. Ba3Ca1.18Nb1.82O9 - delta, exhibit high proton and oxygen-ion conductivity. It is expected that mixed ionic and electronic conductors may be found in these compounds if the B-sites are partially substituted by a first row transition element. These mixed conductors may be potential anode materials for fuel cell applications. The structure of single phase SrCu0.4Nb0.6O2.9 was studied by both X-ray and neutron diffraction. It is tetragonal with space group P4/mmm (123), a=3.9608(4) Angstrom, c=3.9757(2) Angstrom, V=62.37(2) Angstrom(3) according to neutron diffraction. Rietveld refinement indicates that the oxygen vacancy tends to stay at O-1 (1c) site with O-2 (2e) fully occupied. AC impedance measurements indicate that electronic conduction is probably dominant in air. The DC conductivity of SrCu0.4Nb0.6O2.9 at pO(2) in the range of 10 (- 22)-10 (- 12) atm exhibits a p(O-2) - 1/4 dependence consistent with n-type electronic conduction. The material was unstable in 5% H-2 at elevated temperatures but stable in argon at 900 degreesC. Using manganese instead of copper, a phase that is redox stable was prepared. SrMn0.4Nb0.6O3 - delta exhibits an orthorhombic structure with space group Pbnm (62), 3 a = 5.6451(3) Angstrom, b = 5.6589(2) Angstrom, c = 7.9729(2) Angstrom, V = 254.69(7) Angstrom(3) according to X-ray diffraction. Such a unit cell indicates that it is a double perovskite and therefore the formula is better written as Sr2Mn0.8Nb1.2O6 - delta. The material maintains perovskite structure in 5% H-2 although thermal expansion was observed on reduction. The conductivity of Sr2Mn0.8Nb1.2O6 is 0.36 S/cm in air at 900 degreesC. Conductivity decreases in 5% H-2 indicates p-type conduction at low pO(2). (C) 2002 Elsevier Science B.V. All rights reserved.