Phase equilibrium and electrical conductivity of SrCo0.8Fe 0.2O3-δ

Abstract

The phase diagram of the SrCo0.8Fe0.2O 3-δ compound has been determined at high temperatures (823≤T≤1223K) and in the oxygen partial pressure range (10 -5≤pO2≤1atm) by thermogravimetric measurements of the equilibrium pO2, high temperature X-ray diffraction and electrical conductivity measurements. The cubic perovskite phase SrCo 0.8Fe0.2O3-δ is stable in a broad range of oxygen content, while the orthorhombic brownmillerite phase SrCo 0.8Fe0.2O2.5 stabilizes within a small range around 3-δ=2.5 at temperatures below 1073K. Equilibrium pO2 measurements under isothermal conditions show chemical hysteresis at the perovskite to brownmillerite transition. The hysteresis loop decreases its amplitude in pO2 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 hO2oxide and sO2oxide 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 0.28eV as the oxygen content increases from 2.4 to 2.6. These results are interpreted in the frame of the small polaron model. © 2004 Elsevier Inc. All rights reserved.