Oxygen Nonstoichiometry and Ionic Conductivity of Sr₃Fe₂_-<i>_x</i>Sc<i>_x</i>O₇_-_δ

Abstract

The substitution of scandium for iron in the Ruddlesden−Popper Sr3Fe2-xScxO7-δ (x = 0−0.3) system increases tetragonal unit-cell volume and oxygen nonstoichiometry and decreases partial p- and n-type electronic conductivities studied in the oxygen partial pressure range from 1 × 10-20 to 0.7 atm at 973−1223 K. The solubility of Sc3+ corresponds to approximately x ≈ 0.35. The relatively low, temperature-activated hole mobility indicates a small-polaron mechanism of the electronic transport, as for undoped Sr3Fe2O7-δ. The atomistic computer simulations showed that major contribution to the ionic conductivity is provided by the oxygen sites surrounded by iron cations in the perovskite-type layers of Sr3(Fe,Sc)2O7-δ structure, whereas stable ScO6 octahedra are essentially excluded from the oxygen diffusion processes. Minimum migration energy, 0.9−1.4 eV, was found for nonlinear pathways formed by the apical O1 sites linking iron−oxygen polyhedra along the c-axis and equatorial O3 positions in the perovskite-type planes. The direct O3 → O3 jumps are characterized with higher energetic barrier, 1.5−2.2 eV. Because of the increasing concentration of vacant O3 sites induced by scandium doping, the apparent activation energy for oxygen−ionic transport decreases from about 2 eV, as observed for undoped strontium ferrite at 1123−1223 K, down to 0.95−1.15 eV for Sr3Fe2-xScxO7-δ (x = 0.2−0.3). As a result, the partial ionic conductivity of Sr3Fe1.7Sc0.3O7-δ at temperatures below 1000 K becomes higher than that in Sr3Fe2O7-δ.