Structure and properties of nonstoichiometric mixed perovskites A3B′1+xB″2−xO9−δ

Nonstoichiometric mixed perovskites A3B′1+xB″2−xO9−δ, e.g. Ba3Ca1.18Nb1.82O9−δ, 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) Å, c=3.9757(2) Å, V=62.37(2) Å3 according to neutron diffraction. Rietveld refinement indicates that the oxygen vacancy tends to stay at O1 (1c) site with O2 (2e) fully occupied. AC impedance measurements indicate that electronic conduction is probably dominant in air. The DC conductivity of SrCu0.4Nb0.6O2.9 at pO2 in the range of 10−22–10−12 atm exhibits a p(O2) −1/4 dependence consistent with n-type electronic conduction. The material was unstable in 5% H2 at elevated temperatures but stable in argon at 900 °C. Using manganese instead of copper, a phase that is redox stable was prepared. SrMn0.4Nb0.6O3−δ exhibits an orthorhombic structure with space group Pbnm (62), a=5.6451(3) Å, b=5.6589(2) Å, c=7.9729(2) Å, V=254.69(7) Å3 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−δ. The material maintains perovskite structure in 5% H2 although thermal expansion was observed on reduction. The conductivity of Sr2Mn0.8Nb1.2O6 is 0.36 S/cm in air at 900 °C. Conductivity decreases in 5% H2 indicates p-type conduction at low pO2.