The standard molar Gibbs energy of formations of BaFe₁₂O₁₉(s), BaFe₂O₄(s), Ba₂Fe₂O₅(s), Ba₃Fe₂O₆(s) and Ba₅Fe₂O₈(s) have been determined using solid-state electrochemical technique employing CaF₂(s) as an electrolyte. The reversible e.m.f. values have been measured in the temperature range from 970 to 1151K. The oxygen chemical potential corresponding to three phase equilibria involving technologically important compound BaFe₁₂O₁₉(s) has been determined using solid-state electrochemical technique employing CSZ as an electrolyte from 1048 to 1221K. The values of Δ_fG_m⁰(T) for the above ternary oxides are given by

Δ_fG_m⁰(BaFe₁₂O₁₉,s)/kJmol⁻¹(±0.6)=−5431.3+1.5317(T/K)(970≤T/K≤1151)

Δ_fG_m⁰(BaFe₂O₄,s)/kJmol⁻¹(±1.3)=−1461.4+0.3745(T/K)(970≤T/K≤1151)

Δ_fG_m⁰(Ba₂Fe₂O₅,s)/kJmol⁻1(±1.4)=−2038.3+0.4433(T/K)(970≤T/K≤1149)

Δ_fG_m⁰(Ba₃Fe₂O₆,s)/kJmol⁻¹(±1.5)=−2700.1+0.6090(T/K)(969≤T/K≤1150)

And

Δ_fG_m⁰(Ba₅Fe₂O₈,s)/kJmol⁻¹(±1.6)=−3984.1+0.9300(T/K)(973≤T/K≤1150)

The uncertainty estimates for ΔfGm0 includes the standard deviation in the e.m.f. and uncertainty in the data taken from the literature. An isothermal oxygen potential diagram for the system Ba–Fe–O was constructed at 1100K based on the thermodynamic data obtained in this study.