Oxygen exchange and diffusion coefficients of strontium-doped lanthanum ferrites by electrical conductivity relaxation

Electrical conductivity relaxation experiments were performed on thin specimens of La1-xSr2FeO3-delta (x = 0.1, 0.4) at oxygen partial pressures p(o2) = 10(-5) - 1 bar in the temperature range 923 to 1223 K. The transient response of the electrical conductivity after a sudden change of the ambient oxygen partial pressure was analyzed in the frequency domain. The latter procedure allowed diffusion-limited and surface exchange-limited kinetics of re-equilibration to be distinguished. The response of specimens with thicknesses of 350 to 460 mu m indicated diffusion-controlled kinetics at p(o2) > 0.03 bar. The chemical diffusion coefficients, (D) over tilde, were found invariant with oxygen pressure. At 1073 K the absolute values were (D) over tilde = 6.5 x 10(-6) cm(2) s(-1) for x = 0.1 and (D) over tilde = 1.1 x 10(-5) cm(2) s(-1) for x = 0.4, with activation energies of about 80 kJ/mol. The equilibration process was governed by surface exchange at p(o2) < 0.01 bar. The surface exchange coefficient, k(o), was proportional to p(o2)(n), where n = 0.65 to 0.85. This pressure dependency was interpreted in terms of a slow surface process involving an oxygen molecule and a surface oxygen vacancy, and causes the observed sharp transition from diffusion- to exchange-controlled kinetics. The activation energy of k(o) was estimated at 110 to 135 kJ/mol.