Phase Equilibrium and Electrical Conductivity of SrCo0.8Fe0.2O3-d

GRUNBAUM, N.; MOGNI, L.; PRADO, F.; CANEIRO, A.

JOURNAL OF SOLID STATE CHEMISTRY

Elsevier

Año: 2004 vol. 177 p. 2350 - 2357

0022-4596

The phase diagram of the SrCo0.8Fe0.2O3-d compound has been determined at high temperatures (823 <  T < 1223 K) and in the oxygen partial pressure range (10(−5) < pO2 < 1 atm) by thermogravimetric measurements of the equilibrium pO2, high temperature X-ray diffraction and electrical conductivity measurements. The cubic perovskite phase SrCo0.8Fe0.2O3-d 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-d=2.5 at temperatures below 1073 K. 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 hO2 oxide and sO2 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.