Surface recrystallization - an underestimated phenomenon affecting oxygen exchange activity

The rate of oxygen surface exchange on (La0.6Sr0.4)(0.99)CoO3- (LSC) and (La0.6Sr0.4)(0.98)FeO3- (LSF) was investigated by means of Electrical Conductivity Relaxation (ECR) in oxidizing atmospheres (0.2-0.1 bar pO(2)) from 650 degrees C to 900 degrees C. We observed reversible and reproducible changes in materials' performance as a consequence of thermal treatment, manifested through drastic changes in surface exchange coefficient (k(chem)) and the related activation energy with thermal history. The effect is found to be more pronounced in case of LSF, where k(chem) at 650 degrees C and 0.1 bar pO(2) can vary from 3 x 10(-4) to 1.1 x 10(-5) cm s(-1), while bulk properties such as electrical conductivity and the chemical diffusion coefficient of oxygen remain constant. The changes are related to a surface-confined phenomenon and the transition from one state to another is found to follow classical models of nucleation and growth processes. The findings demonstrate the importance of thermal history for studies of oxygen exchange kinetics in perovskite materials and offer an explanation for some of the discrepancies found in the literature.