Oxygen Nonstoichiometry and Thermodynamic Explanation of Large Oxygen-Deficient Ruddlesden–Popper Oxides La_xSr_3−xFe₂O_7−δ

The oxygen nonstoichiometry of large oxygen-deficient Ruddlesden–Popper oxides La_xSr_3−xFe₂O_7−δ (LSFO7-x) (x = 0, 0.25, 0.5) was measured by the high-temperature gravimetry and the coulometric titration. In the composition series, the P(O₂) dependencies exhibited typical plateaus at δ = (2−[La_Sr])/2. Meanwhile, La_0.5Sr_2.5Fe₂O_7−δ showed the smallest oxygen nonstoichiometry and was the most thermochemically stable compound against P(O₂), temperature, and the La content. Based on the defect equilibrium model and the statistical thermodynamic calculation derived oxygen nonstoichiometric data, the substitution of La for Sr-site can promote the forward reaction of oxygen incorporation, the backward reaction of the disproportionation of the charge carriers, and oxygen redistribution between the O1 and O3 sites, resulting in the reduction of oxygen-deficient and the lower decomposition P(O₂). The obtained thermodynamic quantities of the partial molar enthalpy of oxygen, h_O – h°_O, and the partial molar entropy of oxygen, s_O – s°_O, calculated from the statistical thermodynamic calculation are in good agreement with those using the Gibbs–Helmholtz equation.