Phase stability and oxygen non-stoichiometry of SrCo 0.8Fe 0.2O 3− δ measured by in situ neutron diffraction

Abstract

The phase stability, oxygen stoichiometry and expansion properties of SrCo 0.8Fe 0.2O 3− 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 pO 2 of 1 atm, SCF adopts a cubic perovskite structure, space group Pm3¯ m, across the whole temperature range investigated. At a pO 2 of 10 − 1 atm, a two-phase region exists below 922 K, where the cubic perovskite phase coexists with a vacancy ordered brownmillerite phase, Sr 2Co 1.6Fe 0.4O 5, space group Icmm. A pure brownmillerite phase is present at pO 2 of 10 − 2 and 5 × 10 − 4 atm below 1020 K. Above 1020 K, the brownmillerite phase transforms to cubic perovskite through a two-phase region with no brownmillerite structure observed above 1064 K. Large distortion of the BO 6 ( 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 pO 2 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 pO 2 of 1 atm and a minimum of 2.33(2) at 1173 K and a pO 2 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 SrCo 0.8Fe 0.2O 3− δ membranes.