Structural and Electrical Properties of the Perovskite Oxide Sr2FeNbO6

The mixed perovskite Sr2FeNbO6 has been shown to have a tetragonal structure with space group I4/m (87), a = 5.6078(1) A, c = 7.9658(1) A, V = 250.51(1) A3 according to electron and X-ray diffraction. The material is redox stable and maintains its structure in a reducing atmosphere. After reducing in 5% H2 at 900 C for 6 h, Sr2FeNbO6 still exhibits a tetragonal structure with space group I4/m (87), a = 5.6051(1) A, c = 7.9644(1) A, V = 250.22(1) A3, i.e., with a slightly higher degree of tetragonality. A lattice volume contraction of 0.12% was observed during the reduction, which may be attributed to the loss of lattice oxygen. TGA analysis indicates that Sr2FeNbO6 starts to lose oxygen at 300 C and the total weight loss is about 0.2 wt % from room temperature to 950 C in 5% H2. The morphology of this material does not significantly change on reduction according to SEM observation. The conductivities of this material in air and 5% H2 were 3.13 × 10-2 and 2.39 S/cm, respectively at 900 C. The apparent conduction activation energy of Sr2FeNbO6 in air is 0.74 (plus_minus) 0.02 eV between 400 and 900 C and in 5% H2 is 0.28 (plus_minus) 0.02 eV between 140 and 560 C and 0.58 (plus_minus) 0.02 eV between 560 and 900 C, indicating it is a semiconductor in both atmospheres. The increase of dc conductivity of Sr2FeNbO6 at low p(O2) indicates n-type electronic conduction. The dc conductivity of Sr2FeNbO6 at low p(O2) exhibits a p(O2)-1/6 dependence that is interpreted by a simple defect chemistry model.