Oxygen diffusion and surface exchange in La1-xSrxFe0.8Cr0.2O3-δ (x=0.2, 0.4 and 0.6)

Oxygen tracer diffusion (D*) and surface exchange rate constant (k*) have been measured, using isotopic exchange and depth profiling by secondary ion mass spectrometry (SIMS), in La1-xSrxFe0.8Cr0.2O3-delta (x=0.2, 0.4 and 0.6). Measurements were made as a function of temperature (700 - 1000 degreesC) and oxygen partial pressure (0.21 - 10(-21) atm) in dry oxygen, water vapour and water vapour/hydrogen/nitrogen mixtures. At high oxygen activity, D* was found to increase with increasing temperature and Sr content. The activation energies for D* in air are 2.13 eV (x=0.2), 1.53 eV (x=0.4) and 1.21 eV (x=0.6). As the oxygen activity decreases, D* increases as expected qualitatively from the increase in oxygen vacancy concentration. Under strongly reducing conditions, the measured values of D* at 1000 degreesC range from congruent to 10(-8) cm(2) s(-1) for x=0.2 to congruent to 10(-7) cm(2) s(-1) for x=0.4 and 0.6. The activation energies determined at constant H2O/H-2 ratio are 1.21 eV (x=0.2), 1.59 eV (x=0.4) and 0.82 eV (x=0.6). The surface exchange rate constant of oxygen for the H2O molecule is similar in magnitude to that for the O-2 molecule and both increase with increasing Sr concentration. (C) 2004 Elsevier B.V. All rights reserved.