Defect Chemistry and Electrical Properties of ( La0.8Ca0.2 ) 0.95FeO3 − δ

Abstract

A-site-doped lanthanum ferrite perovskites are considered to be promising cathode materials for the lower temperature operation of solid oxide fuel cells due to their high catalytic activity, mixed electronic and ionic conductivities at reduced temperature, and adjustable thermal-expansion coefficient. The defect chemistry and electrical properties of were studied using thermogravimetry and dc electrical conductivity measurements at temperatures in the oxygen partial pressure range of . A point-defect model was developed, and the mass-action coefficients for the oxygen-exchange reaction at various temperatures were determined by fitting thermogravimetry data into the model. Equilibrium defect concentrations were calculated as a function of temperature and oxygen partial pressure based on the defect modeling results. Electrical conductivity measurements indicated that is a p-type conductor. At low temperature, conductivity increased with temperature, which reflected thermally activated conduction behavior. At high temperatures, conductivity decreased with temperature because the conductivity was mainly affected by a decrease in the hole concentration due to the progressive formation of oxygen vacancies at the expense of holes. Hole mobility under various conditions was determined using electric conductivity data and equilibrium hole concentrations, and the relationship between the mobility and temperature indicated a nonadiabatic, small polaron hopping conduction mechanism with activation energies of . The effects of temperature and oxygen partial pressure on the defect structure and electronic conduction were discussed in detail.