Structure and properties of nonstoichiometric mixed perovskites A 3B′ 1+ xB″ 2− xO 9− δ

Abstract

Nonstoichiometric mixed perovskites A 3B′ 1+ x B″ 2− x O 9− δ , e.g. Ba 3Ca 1.18Nb 1.82O 9− 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 SrCu 0.4Nb 0.6O 2.9 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) Å 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 SrCu 0.4Nb 0.6O 2.9 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 °C. Using manganese instead of copper, a phase that is redox stable was prepared. SrMn 0.4Nb 0.6O 3− 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) Å 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 Sr 2Mn 0.8Nb 1.2O 6− δ . The material maintains perovskite structure in 5% H 2 although thermal expansion was observed on reduction. The conductivity of Sr 2Mn 0.8Nb 1.2O 6 is 0.36 S/cm in air at 900 °C. Conductivity decreases in 5% H 2 indicates p-type conduction at low pO 2.