Oxygen nonstoichiometry, crystal and defect structure of the double perovskite GdBaCo 1.8Fe 0.2O 6 - δ

Abstract

Crystal structure of the double perovskite GdBaCo 1.8Fe 0.2O 6 - δ was investigated by means of in situ X-ray diffraction at temperatures between 25 and 800°C in air. Pmmm-P4/mmm phase transition was found to occur in GdBaCo 1.8Fe 0.2O 6 - δ at 515°C. The results of oxygen nonstoichiometry, δ, measured as a function of oxygen partial pressure, p O2, in temperature range 700 ≤ T,°C ≤ 1025 by means of thermogravimetric technique are presented for this double perovskite. Partial molar enthalpy and entropy of oxygen in the GdBaCo 1.8Fe 0.2O 6 - δ structure were calculated. Both thermodynamic properties were shown to increase dramatically in the vicinity of the oxygen nonstoichiometry value equal to 1. The p O2 dependences of oxygen nonstoichiometry were found to have inflections when the oxygen content of GdBaCo 1.8Fe 0.2O 6 - δ is equal to 5.0 exactly. The analysis of the defect structure of the double perovskite GdBaCo 1.8Fe 0.2O 6 - δ was carried out using the model based on the simple cubic perovskite GdCoO 3 as a reference state. Equilibrium constants of the appropriate defects reactions were, therefore, determined. As a consequence, concentrations of all defect species defined within the framework of the model proposed were calculated as functions of temperature and oxygen nonstoichiometry. Oxygen vacancies were shown to be formed during p O2 diminution in gas environment in the layers of GdBaCo 1.8Fe 0.2O 6 - δ crystal lattice where they are ordered until oxygen nonstoichiometry of the oxide becomes equal to unity afterwards oxygen vacancies are formed randomly in oxygen polyhedrons. © 2012 Elsevier B.V. All rights reserved.