Enthalpy increments and redox thermodynamics of SrFeO3−δ

Enthalpy increments,(Delta ₂₉₈^T{H^0}), for highly nonstoichiometric SrFeO3−δ(δ = 0.18–0.41) were obtained between 373 and 1273 K in air using drop calorimetry. The analysis of the(Delta ₂₉₈^T{H^0}left(T ight))dependence at lower temperatures allowed evaluating the enthalpy of tetragonal to cubic({{I4}/{mmm}} ightarrow Pmar{3}m)phase transition at 560 K, 1.57 kJ/mol, and the Maier–Kelley function for(Delta ₂₉₈^T{H^0}left(T ight))of tetragonal SrFeO3−δ(space groupI4/mmm). Combined investigation of oxygen nonstoichiometry δ(T) dependence, measured by thermogravimetry, and higher-temperature(Delta ₂₉₈^T{H^0}left(T ight))of cubic SrFeO3−δ(space group(Pmar{3}m)) yielded the temperature-dependent reduction (oxygen release) enthalpy,(Delta H_{{ m{red}}}^{ m{0}}). Calorimetrically-determined(Delta H_{{ m{red}}}^{ m{0}})of SrFeO3−δincreases from 65 ± 7 kJ/mol O at 873–973 K to 84 ± 7 kJ/mol O at 1073–1273 K, which may indicate that the short-range vacancy ordering in SrFeO3−δis hampered at higher temperatures.