Oxygen nonstoichiometry and defect structure of the double perovskite GdBaCo 2O 6 − δ

Abstract

The results of oxygen nonstoichiometry, δ, measured by means of coulometric technique as a function of oxygen partial pressure, p O 2, in temperature range 900 ≤ T °C ≤ 1050 are presented for GdBaCo 2O 6 − δ with double perovskite structure. Partial molar enthalpy and entropy of oxygen in GdBaCo 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 O 2 dependences of oxygen nonstoichiometry and the δ dependences of the partial molar properties were found to have inflections when the oxygen content of GdBaCo 2O 6 − δ is equal to 5.0 exactly. The modeling of the defect structure of the double perovskite GdBaCo 2O 6 − δ was carried out by considering different reference states. Only the model based on the cubic perovskite GdCoO 3 as a reference state was shown to fit the experimental data on oxygen nonstoichiometry of GdBaCo 2O 6 − δ good enough. Equilibrium constants of the appropriate defects reactions were, therefore, determined. Concentrations of all defect species defined within the framework of this model were calculated as functions of temperature and oxygen nonstoichiometry. Oxygen vacancies were shown to be formed during p O 2 diminution in gas environment in the layers of GdBaCo 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.