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) Angstrom, c = 7.9658(1) Angstrom, V = 250.51(1) Angstrom(3) according to electron and X-ray diffraction. The material is redox stable and maintains its structure in a reducing atmosphere. After reducing in 5% H-2 at 900 degreesC for 6 h, Sr2FeNbO6 still exhibits a tetragonal structure with space group I4/m (87), a = 5.6051(1) Angstrom, c = 7.9644(1) Angstrom, V = 250.22(1) Angstrom(3), 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 degreesC and the total weight loss is about 0.2 wt % from room temperature to 950 degreesC in 5% H-2. The morphology of this material does not significantly change on reduction according to SEM observation. The conductivities of this material in air and 5% H-2 were 3.13 x 10(-2) and 2.39 S/cm, respectively at 900 degreesC. The apparent conduction activation energy of Sr2FeNbO6 in air is 0.74 +/- 0.02 eV between 400 and 900 degreesC and in 5% H-2 is 0.28 +/- 0.02 eV between 140 and 560 degreesC and 0.58 +/- 0.02 eV between 560 and 900 degreesC, indicating it is a semiconductor in both atmospheres. The increase of de conductivity of Sr2FeNbO6 at low p(O-2) indicates n-type electronic conduction. The dc conductivity of Sr2FeNbO6 at low p(O-2) exhibits a p(O-2)(-1/6) dependence that is interpreted by a simple defect chemistry model.