Nonstoichiometry, defects and thermodynamic properties of YFeO3, YFe2O4 and Y3Fe5O12

Oxygen nonstoichiometry of three ternary oxides, YFeO3 − δ, YFe2O4 − α and Y3Fe5O12 − θ, in the system Y–Fe–O was investigated as a function of oxygen partial pressure by thermogravimetry at high temperature. The defects responsible for nonstoichiometry were identified as oxygen vacancies for YFeO3 − δ and YFe2O4 − α, although the manner of variation of nonstoichiometric parameter with oxygen partial pressure for these two oxides is quite different. Cation interstitials are the predominant defects in Y3Fe5O12 − θ. Gibbs energies of formation of the three nonstoichiometric oxides were determined using solid-state electrochemical cells in the temperature range from 975 to 1475 K. YFe2O4 − α was found to be stable only above 1391 K. Gibbs energies of formation of the three stoichiometric compounds from their component binary oxides were obtained by combining information from solid state cells with results of thermogravimetric analysis using the Gibbs–Duhem relation. The results can be summarized as:(1/2)Y2O3 + (1/2)Fe2O3 → YFeO3; ΔGf(ox)O(± 250) (J/mol) = − 17, 126 − 8.263 T(1/2)Y2O3 + FeO + (1/2)Fe2O3 → YFe2O4; ΔGf(ox)O(± 260) (J/mol) = − 10, 352 − 13.24 T(3/2)Y2O3 + (5/2)Fe2O3 → Y3Fe5O12; ΔGf(ox)O(± 780) (J/mol) = − 56, 647 − 31.091 T.

► Variation of nonstoichiometric parameters with oxygen partial pressure.
► Identification of defects responsible for nonstoichiometry.
► A new cell design for accurate measurement of oxygen chemical potentials.
► Thermodynamic data for both nonstoichiometric and stoichiometric ternary oxides.
► Interrelations between defects, nonstoichiometry and thermodynamics.