Kinetics and Mechanisms of Oxygen Surface Exchange on La0.6Sr0.4FeO3-delta Thin Films

The thermodynamic properties as well as oxygen exchange kinetics were examined on mixed ionic and electronic conducting (La0.6Sr0.4)(0.99)FeO3(-delta) (LSF64) thin films deposited on MgO single crystals. It is found that thin films and bulk material have the same oxygen stoichiometry for a given temperature and oxygen partial pressure [i.e., the incorporation reaction has the same reaction enthalpy (Delta H-0=-105 KJ/mol) and entropy (Delta S-0=-75.5 J/mol/K) as found for bulk material]. The thin film shows smaller apparent electrical conductivity than reported for bulk. This is due to imperfections in the film, which is not totally dense and contains closed porosity. Electrical conductivity relaxation was used to determine the surface exchange coefficient and its dependence on the temperature and oxygen partial pressure. Relaxation curves showed a good fit to a simple exponential decay. The vacancy surface exchange coefficient (k(V)) determined from K-chem shows a slope (log k(V) vs log P-O2) between 0.51 and 0.85. It is further found that k(V) is proportional to the product of the oxygen partial pressure and the vacancy concentration (k(V)proportional to P-O2 delta(infinity)). Different reaction mechanisms that can account for the observed P-O2 and delta-dependence of k(V) are analyzed. It is proposed that the vacancies are the active sites of adsorption of molecular oxygen and that the rate determining step for the exchange reaction is splitting of the adsorbed oxygen.