Energetics in the brownmillerite-perovskite pseudobinary Ca₂Fe₂O₅-CaTiO₃

The energetics of intermediate phases in the Ca₂Fe₂O₅-CaTiO₃ pseudobinary system was determined by high temperature solution calorimetry. The results suggest that both the intermediate compounds Ca₃Fe₂TiO₈ and Ca₄Fe₂Ti₂O₁₁ are entropically stabilized because their enthalpies of formation are positive with respect to a mixture of Ca₂Fe₂O₅ and CaTiO₃. At high temperatures, the configurational entropy of the random distribution of Fe and Ti ions on the octahedral sites appears sufficient to stabilize the intermediate phases. The enthalpies of formation from the oxides at 1073 K of Ca₂Fe₂O₅, Ca₃Fe₂TiO₈, and Ca₄Fe₂Ti₂O₁₁ are −45.0 ± 3.8, −123.7 ± 8.0, and −192.7 ± 11.2 kJ/mol, respectively. Their enthalpies of formation from the elements at 298 K are −2139.8 ± 4.4, −3798.4 ± 8.6, and −5447.3 ± 12.2 kJ/mol, respectively.