Energetics in the brownmillerite-perovskite pseudobinary Ca2Fe205-CaTi03

The energetics of intermediate phases in the Ca2Fe2C>5-CaTi03 pseudobinary system was determined by high temperature solution calorimetry. The results suggest that both the intermediate compounds Ca3Fe2Ti08 and Ca4Fe2Ti2On are entropically stabilized because their enthalpies of formation are positive with respect to a mixture of Ca2Fe205 and CaTi03. 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 Ca2Fe205, Ca3Fe2Ti08, and Ca4Fe2Ti2On 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.