Phase equilibrium and electrical conductivity of SrCo0.8Fe0.2O3-delta

摘要

The phase diagram of the SrCo0.8Fe0.2O3-delta Compound has been determined at high temperatures (823 less than or equal to T less than or equal to 1223 K) and in the oxygen partial pressure range (10(-5) less than or equal to pO(2) less than or equal to 1 atm) by thermogravimetric measurements of the equilibrium pO(2), high temperature X-ray diffraction and electrical conductivity measurements. The cubic perovskite phase SrCo0.8Fe0.2O3-delta is stable in a broad range of oxygen content, while the orthorhombic brownmillerite phase SrCo0.8Fe0.2O2.5 stabilizes within a small range around 3-delta = 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(O2)(oxide) and s(O2)(oxide) for the perovskite phase decrease when increasing the oxygen defects concentration. The electrical conductivity (sigma) of the cubic phase exhibits a thermally activated behavior at high temperature. The variation of sigma 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. (C) 2004 Elsevier Inc. All rights reserved.