Oxygen nonstoichiometry, the defect equilibrium model and thermodynamic quantities of the Ruddlesden–Popper oxide Sr₃Fe₂O_7−δ

Abstract

Oxygen nonstoichiometry of the Ruddlesden-Popper oxide Sr3Fe2O7-δ was measured at intermediate temperatures (773-1073 K) by coulometric titration and high temperature gravimetry. The oxygen nonstoichiometric behavior was analyzed using the defect equilibrium model with localized electrons. From the defect chemical analysis, estimated oxygen vacancy concentration at the O3 sites increases and at the O1 sites decreases with the increasing temperature. This characteristic behavior is considered to be caused by the redistribution of oxygen and vacancies between the O1 and O3 sites. 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 Gibbs-Helmholtz equation are in good agreement with those from the statistical thermodynamic calculation based on the defect equilibrium model, indicating that the proposed defect equilibrium model is reasonable.

Oxygen Nonstoichiometry and Thermodynamic Explanation of Large Oxygen‐Deficient Ruddlesden–Popper Oxides LaxSr3−xFe2O7−δ

“…8,[18][19][20] Weight-loss measurements during the reduction of the sample were also applied to determine the oxygen contents by the preceding researchers.…”

“…6(c)] increases with negative linear correlation, indicating high temperature is not helpful to the forward reaction of oxygen incorporation, resulting from the decrease of K OX and the increase of K f . 7,8 While temperature increases, both of V AEAE O1 and V AEAE O3 increase, the oxygen vacancy concentration at the O1 sites at high temperatures are lower than those at low temperature due to the increase of K I , leading to the happen of oxygen redistribution phenomenon. Meanwhile, the increasing ½Fe 0 Fe [in Fig.…”

“…[7][8][9] Due to the oxygen redistribution phenomenon, oxygen-ion easily migrates through the octahedral edges of O3-O1-O3 jumps with small energy barrier of 0.9-1.4 eV. [7][8][9] Due to the oxygen redistribution phenomenon, oxygen-ion easily migrates through the octahedral edges of O3-O1-O3 jumps with small energy barrier of 0.9-1.4 eV.…”

“…The oxygen redistribution phenomenon was also confirmed by theoretical calculations and experiments using single crystal or electrode specimens. [7][8][9] Due to the oxygen redistribution phenomenon, oxygen-ion easily migrates through the octahedral edges of O3-O1-O3 jumps with small energy barrier of 0.9-1.4 eV. Meanwhile, Sr 3 Fe 2 O 7Àd -type oxides own larger oxygen-ion diffusion coefficient, for example, 6.2 9 10 À6 cm 2 /s at 973 K, compared to simple cubic perovskite-type conductors.…”

Tin and iron co-doping strategy for developing active and stable oxygen reduction catalysts from SrCoO3−δ for operating below 800 °C

“…The oxygen nonstoichiometries of the materials at high temperatures were calculated from the weight loss of samples detected by a thermogravimetric analyzer (METTLER TGA/DSC 1/1600HT). During the test, all samples were exposed to air with identical annealing conditions since the oxygen vacancy concentration of a sample can be affected by the annealing temperature and the oxygen partial pressure [40]. The electrical conductivities of a series of samples from 300 to 900 C were measured via the DC four-probe method.…”