Defect Structure and Electrical Conductivity of the Ruddlesden−Popper Phases Sr₃FeMO_6+δ (M = Co, Ni)

In this work, we report a systematic study of the oxygen nonstoichiometry, high-temperature thermodynamics, and transport properties of the perovskite-related mixed conductor Sr 3 FeMO 6+ δ (M = Co, Ni). Thermogravimetry was used to determine the oxygen content change (6+δ) as a function of temperature ( T ) and equilibrium oxygen partial pressure ( p O 2 ) within the range 1 × 10 - 5 atm < p O 2 < 1 atm and 400 °C ≤ T ≤ 1000 °C. From the experimental values of the oxygen chemical potential (μ O 2 ), we determined both the partial molar enthalpy ( h O 2 ) and the partial molar entropy ( s O 2 ) for the composition range 5.9 < 6+δ < 6.6. Thermodynamic data were fitted using a straightforward defect model. The model was deduced from the mass action law assuming oxygen-vacancy formation and involving different fractions of localized and delocalized charge carriers in iron sites and the metal transition M3d−O2p band, respectively. Electrical resistivity measurements as a function of p O 2 at constant temperature were performed in the 650 °C ≤ T ≤ 1000 °C temperature range for Sr 3 FeCoO 6+ δ and Sr 3 FeNiO 6+ δ compounds. The activation energy values for the electrical-transport process at constant oxygen content values were obtained from the combination of electrical conductivity and thermogravimetry data. The electrical conductivity data are discussed within the frame of large polaron behavior in agreement with the thermodynamic data.