Oxygen nonstoichiometry of Sr(Co,Fe)O3−δ-based perovskites: I. Coulometric titration of SrCo0.85Fe0.10Cr0.05O3−δ by the two-electrode technique

The p(O2)–T–δ diagram of perovskite-type SrCo0.85Fe0.10Cr0.05O3−δ was determined by the coulometric titration technique in the temperature range 770–1250 K at oxygen partial pressures from 8 10−10 to 0.5 atm. Stability of the cubic perovskite phase of SrCo0.85Fe0.10Cr0.05O3−δ, existing down to the oxygen pressures of 10−3–10−5 atm, was found to be slightly higher than that of SrCo0.80Fe0.20O3−δ, probably due to stabilization of oxygen octahedra neighboring Cr4+ cations. When the oxygen nonstoichiometry of the Cr-containing perovskite decreases from 0.47 to 0.38, the partial molar enthalpy and entropy for overall oxygen incorporation reaction vary in the ranges −165 to −60 kJ mol−1 and 90 to 150 J mol−1 K−1, respectively. Within the stability limits of the single perovskite phase, the p(O2)–T–δ diagram can be adequately described by equilibrium processes of oxygen incorporation, cobalt disproportionation and interaction of cobalt and iron cations, with the thermodynamic functions independent of defect concentrations. Increasing grain size in SrCo0.85Fe0.10Cr0.05O3−δ ceramics from submicron size to 100–200 μm has no effect on the oxygen thermodynamics. The two-electrode coulometric titration technique, based on the alternate use of electrodes for oxygen pumping and e.m.f. measurements, is described and verified by studying oxygen nonstoichiometry of La0.3Sr0.7CoO3−δ and PrOx.