The standard molar Gibbs free energies of formation of TbFeO₃(s) and Tb₃Fe₅O₁₂(s) have been determined using solid-state electrochemical cell employing different solid electrolytes. The reversible emfs of the following solid-state electrochemical cells have been measured in the temperature range 1050 ≤ T / K ≤ 1250.
Cell (I) : (–)Pt / {TbFeO₃(s) + Tb₂O₃(s)+ Fe(s)} // YDT/CSZ // {Fe(s)+
                Fe_0.95O(s)} / Pt(+)
Cell (II) : (–)Pt / {Fe(s) + Fe_0.95O(s)} // CSZ // {TbFeO₃(s)+
                 Tb₃Fe₅O₁₂(s) + Fe₃O₄(s)} / Pt(+)
The oxygen chemical potentials corresponding to the three-phase equilibria involving the ternary oxides have been computed from the emf data. The standard molar Gibbs free energies of formation of solid TbFeO₃ and Tb₃Fe₅O₁₂
calculated by the least-squares regression analysis of the data obtained in the present study are given by
{Δ_fG°_m(TbFeO₃,s) / (kJ · mol^–1) ± 3.2} = - 1357.5 + 0.2531 · (T / K);
    (1050 ≤ T / K ≤ 1548);
and
{Δ_fG°_m(Tb₃Fe₅O₁₂,s) / (kJ · mol^–1) ± 3.5} = - 4901.7 + 0.9997 · (T / K);
     (1050 ≤ T / K ≤ 1250).
The uncertainty estimates for Δ_fG°_m include the standard deviation in the emf and uncertainty in the data taken from the literature. Based on the thermodynamic information, oxygen potential diagram and chemical potential diagrams were computed for the system Tb–Fe–O at
T = 1250 K.