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The standard Gibbs energies of the reaction of EuFeO3 and Fu3Fe5O12 were determined to be −63300±200 cal and −297800±600 cal respectively at 1200°C by means of following reactions: Fe+1/2 Eu2O3+3/4 O2=EuFeO3 and 5 Fe+3/2 Eu2O3+15/4 O2=Eu3Fe5O12, based on the phase equilibria in the Fe–Fe2O3–Eu2O3 system.

The standard Gibbs energy of formation of \barCFeO3(\barC=La, Nd, Sm, Eu, Gd, Tb, and Dy) from metallic iron, \barC2O3, and oxygen has been determined at temperatures from 1473 to 1673 K. Based on the free-energy data, the heat of reaction and the entropy change resulting from the reaction have been calculated. The values of the heat of reaction of LaFeO3, NdFeO3, SmFeO3, EuFeO3, and GdFeO3 (the first group) were identical, −107 kcal/mol, at the present temperature interval. However, the values of the heat of reaction of TbFeO3 and DyFeO3 (the second group) decreased with an increase in the temperature. The entropy change of each reaction in the first group was constant, independent of the temperature, and each value decreased in the sequence from LaFeO3 to GdFeO3. The entropy change of the second group decreased with an increase in the temperature. The change of the standard Gibbs energy was intimately related to the ionic radii of \barC-ions, and the relationship between the Gibbs energy and the toleran...