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

Abstract

The oxygen nonstoichiometry of large oxygen‐deficient Ruddlesden–Popper oxides LaxSr3−xFe2O7−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−[LanormalSr·])/2. Meanwhile, La0.5Sr2.5Fe2O7−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, hO−hboldO∘, and the partial molar entropy of oxygen, sO−sboldO∘, calculated from the statistical thermodynamic calculation are in good agreement with those using the Gibbs–Helmholtz equation.

New two-layer Ruddlesden—Popper cathode materials for protonic ceramics fuel cells

“…The partial substitution of Ni for Fe in Sr 3 Fe 2 O 7-δ can increase the electrical conductivity and the oxygen permeability without structural transformation [23,24]. In our previous works, the introduction of La into Sr-site forming La x Sr 3-x Fe 2 O 7-δ can enhance thermo-chemical stability as well as large oxygen deficiency [25,26]. Moreover, another RP phase, La 3 Ni 2 O 7-δ (n = 2), showed large electrical conductivity, acceptable stability, and similar thermal expansion coefficient to electrolytes [27][28][29].…”

Phase Stability and Magnetic Properties of Compositionally Complex n = 2 Ruddlesden–Popper Perovskites

“…This finding is consistent with research on conventional n = 2 Ruddlesden–Popper perovskites that are known to accommodate oxygen vacancies at the 2 a position. 31 − 33 This feature has also been investigated in the La 0.54 Sr 2.46 Fe 2 O 7−δ system via powder diffraction and density functional theory methods. The results revealed a significantly lower oxygen vacancy formation energy for the 2 a position in comparison to that of the 4 e or 8 g site, though a small number of vacancies have been observed on the 8 g site at increased temperatures (800–900 °C).…”

New Two-layer Ruddlesden-popper Cathode Materials for Protonic Ceramics Fuel Cells

“…The partial substitution of Ni for Fe in Sr 3 Fe 2 O 7−δ can increase the electrical conductivity and the oxygen permeability without structural transformation [21][22]. In our previous works, the introduction of La into Sr-site forming La x Sr 3−x Fe 2 O 7−δ can enhance thermo-chemical stability as well as large oxygen de ciency [23][24]. Moreover, another RP phase, La 3 Ni 2 O 7−δ (n = 2), showed large electrical conductivity, acceptable stability and similar thermal expansion coe cient to electrolytes [25][26][27].…”