Phase stability and oxygen non-stoichiometry of SrCo0.8Fe0.2O3-d measured by in-situ neutron diffraction

The phase stability, oxygen stoichiometry and expansion properties of SrCo0.8Fe0.2O3−δ (SCF) were determined by in situ neutron diffraction between 873 and 1173 K and oxygen partial pressures of 5×10−4 to 1 atm. At a pO2 of 1 atm, SCF adopts a cubic perovskite structure, space group Pm3¯m, across the whole temperature range investigated. At a pO2 of 10−1 atm, a two-phase region exists below 922 K, where the cubic perovskite phase coexists with a vacancy ordered brownmillerite phase, Sr2Co1.6Fe0.4O5, space group Icmm. A pure brownmillerite phase is present at pO2 of 10−2 and 5×10−4 atm below 1020 K. Above 1020 K, the brownmillerite phase transforms to cubic perovskite through a twophase region with no brownmillerite structure observed above 1064 K. Large distortion of the BO6 (B=Co, Fe) octahedra is present in the brownmillerite structure with apical bond lengths of 2.2974(4)Å and equatorial bond lengths of 1.9737(3) Å at 1021 K and a pO2 of 10−2 atm. SCF is highly oxygen deficient with a maximum oxygen stoichiometry, 3−δ, measured in this study of 2.58(2) at 873 K and a pO2 of 1 atm and a minimum of 2.33(2) at 1173 K and a pO2 of 5×10−4 atm. Significant differences in lattice volume and expansion behavior between the brownmillerite and cubic perovskite phases suggest potential difficulties in thermal cycling of SrCo0.8Fe0.2O3−δ membranes