Oxygen nonstoichiometry and thermodynamic quantities in the Ruddlesden-Popper oxides La_xSr_{3 - x}Fe₂O_{7 - δ}

Oxygen nonstoichiometry in the Ruddlesden-Popper type oxides La_xSr_{3 - x}Fe₂O_{7 - δ} (x = 0, 0.25 and 0.5) was determined by means of the high temperature gravimetry and the coulometric titration. The measurements were carried out in the temperature range between 773 and 1073 K and the P(O₂) range between 10^-40 and 1 bar. The P(O₂) dependencies of the oxygen nonstoichiometry exhibited typical plateaus at δ ∼ 1.0 for Sr₃Fe₂O_{7 - δ}, δ ∼ 0.875 for La_0.25Sr_2.75Fe₂O_{7 - δ} and δ ∼ 0.75 for La_0.5Sr_2.5Fe₂O_{7 - δ}, where the average oxidation state of iron is 3 +. In the La_xSr_{3 - x}Fe₂O_{7 - δ} series, La_0.5Sr_2.5Fe₂O_{7 - δ} showed the smallest oxygen nonstoichiometry and was the most thermochemically stable compound against on P(O₂), temperature, and the La content. Thermodynamic quantities of the partial molar enthalpy of oxygen, h_O - h_O^°, and the partial molar entropy of oxygen, s_O - s_O^°, were calculated from measured nonstoichiometric data. The behavior of oxygen nonstoichiometry and thermodynamic quantities indicates that the ideal-solution-like defect formation is established (La,Sr)₃Fe₂O₇ in the oxygen deficient region.