Synthesis, structural and electrical characterizations of Sr2Fe1−xMxNbO6 (M = Zn and Cu) with double perovskite structure

Sr2Fe1−xZnxNbO6−x/2 (0 ≤ x ≤ 0.5) and Sr2Fe1−xCuxNbO6−x/2 (0.01 ≤ x ≤ 0.05) with the double perovskite structure have been synthesized. The crystal structures at room temperature were determined from Rietveld refinements of X-ray powder diffraction data. The plots of the imaginary parts of the impedance spectrum, Z″, and the electric modulus, M″, versus log (frequency), possess maxima for both curves separated by less than a half decade in frequency with associated capacities of 2 nF. The enhancement of the overall conductivity of Sr2Fe1−xMxNbO6−x/2 (M = Cu and Zn) is observed, as increases from 2.48 (3) × 10−4 S/cm for Sr2FeNbO6 to 3.82 (5) × 10−3 S/cm for Sr2Fe0.8Zn0.2NbO5.9 at 673 K. Sr2Fe0.8Zn0.2NbO5.9 is chemically stable under the oxygen partial pressure from 1 atm to 10−22 atm at 873 K. The p and n-type electronic conductions are dominant under oxidizing and reducing conditions, respectively, suggesting a small-polaron hopping mechanism of electronic conduction. However, for Sr2Fe0.99Cu0.01NbO5.995, the conductivity is almost constant in an oxygen partial pressure range of 1-10−15 atm at 873 K, which indicates that the oxide ion conduction plays a key role in electrical transport.