Oxygen nonstoichiometry, chemical expansion, mixed conductivity, and anodic behavior of Mo-substituted Sr3Fe2O7-δ

The incorporation of molybdenum in the Ruddlesden-Popper type Sr3Fe2-xMoxO7-δ (x = 0–0.1) decreases oxygen deficiency, thermal expansion and electron-hole transport, and increases n-type electronic conductivity in reducing atmospheres. The oxygen ionic conduction remains essentially unaffected by doping. The equilibrium p(O2)–T–δ diagram of Sr3Fe1.9Mo0.1O7-δ, collected in oxygen partial pressure ranges from 10− 20 to 0.7 atm at 973–1223 K, can be adequately described by a defect model accounting for the energetic nonequivalence of apical O1 and equatorial O3 sites in the layered structure, in combination with iron disproportionation and stable octahedral coordination of Mo6+ and Mo5+ cations. The calculated enthalpy of anion exchange between the O1 and O3 positions, 0.49–0.51 eV, is in agreement with the values predicted by the atomistic computer simulation technique. The high-temperature X-ray diffraction studies showed a strongly anisotropic expansion of the Ruddlesden-Popper lattice on reduction, leading to very low chemical strains favorable for electrochemical applications. At 298–1223 K and oxygen pressures from 10− 8 to 0.21 atm, the linear thermal expansion coefficient of Sr3Fe1.9Mo0.1O7-δ varies in the narrow range (12.9–14.2) × 10− 6 K− 1. The relatively low level of n-type electronic conductivity leads, however, to a poor performance of porous Sr3Fe1.9Mo0.1O7-δ anodes in contact with lanthanum gallate-based solid electrolyte under reducing conditions.