Defect Structure and Electrical Conductivity of the Ruddlesden-Popper Phases Sr3FeMO6+? (M ) Co, Ni)

L. MOGNI; F. PRADO; A. CANEIRO

CHEMISTRY OF MATERIALS

AMER CHEMICAL SOC

Año: 2006 vol. 18 p. 4163 - 4170

0897-4756

In this work, we report a systematic study of the oxygen nonstoichiometry, high-temperature thermodynamics,and transport properties of the perovskite-related mixed conductor Sr3FeMO6+? (M ) Co, Ni).Thermogravimetry was used to determine the oxygen content change (6+?) as a function of temperature(T) and equilibrium oxygen partial pressure (pO2) within the range 1 10-5 atm < pO2 < 1 atm and400 °C e T e 1000 °C. From the experimental values of the oxygen chemical potential (ÌO2), we determinedboth the partial molar enthalpy (hO2) and the partial molar entropy (sO2) for the composition range5.9 < 6+? < 6.6. Thermodynamic data were fitted using a straightforward defect model. The modelwas deduced from the mass action law assuming oxygen-vacancy formation and involving differentfractions of localized and delocalized charge carriers in iron sites and the metal transition M3d-O2pband, respectively. Electrical resistivity measurements as a function of pO2 at constant temperature wereperformed in the 650 °C e T e 1000 °C temperature range for Sr3FeCoO6+? and Sr3FeNiO6+? compounds.The activation energy values for the electrical-transport process at constant oxygen content values wereobtained from the combination of electrical conductivity and thermogravimetry data. The electricalconductivity data are discussed within the frame of large polaron behavior in agreement with thethermodynamic data.