Phase equilibrium and electrical conductivity of SrCo 0.8Fe 0.2O 3− δ

Abstract

The phase diagram of the SrCo 0.8Fe 0.2O 3− δ compound has been determined at high temperatures (823⩽ T⩽1223 K) and in the oxygen partial pressure range (10 −5⩽ pO 2⩽1 atm) by thermogravimetric measurements of the equilibrium pO 2, high temperature X-ray diffraction and electrical conductivity measurements. The cubic perovskite phase SrCo 0.8Fe 0.2O 3− δ is stable in a broad range of oxygen content, while the orthorhombic brownmillerite phase SrCo 0.8Fe 0.2O 2.5 stabilizes within a small range around 3− δ=2.5 at temperatures below 1073 K. Equilibrium pO 2 measurements under isothermal conditions show chemical hysteresis at the perovskite to brownmillerite transition. The hysteresis loop decreases its amplitude in pO 2 with decreasing temperature. This behavior is discussed considering the evolution from coherent intergrowth interfaces with elastic strain energy to incoherent interfaces without elastic strain energy as T decreases. The thermodynamic quantities h O 2 oxide and s O 2 oxide for the perovskite phase decrease when increasing the oxygen defects concentration. The electrical conductivity ( σ) of the cubic phase exhibits a thermally activated behavior at high temperature. The variation of σ with the oxygen content is non-linear and the activation energy varies from 0.4 to 0.28 eV as the oxygen content increases from 2.4 to 2.6. These results are interpreted in the frame of the small polaron model.