DETERMINATION OF OXYGEN NONSTOICHIOMETRY AND DIFFUSIVITY IN MIXED CONDUCTING OXIDES BY OXYGEN COULOMETRIC TITRATION .1. CHEMICAL DIFFUSION IN LA0.8SR0.2COO3-DELTA

Oxygen coulometric titration has been applied to measure chemical diff usion in La0.8Sr0.2CoO3-delta between 700 and 1000 degrees C. The tran sient current response to a potentiostatic step has been transformed f rom the time domain to the frequency domain. The equivalent circuit us ed to fit the resulting impedance data contains the element that descr ibes the finite-length diffusion of oxygen into the sample specimen. O ther elements included are the gas-phase capacitance and the sum of th e gas-phase diffusion resistance and that associated with the limited surface exchange kinetics of the sample. The chemical diffusion coeffi cient of perovskite La0.8Sr0.2CoO3-delta oxygen partial pressure. Its value can be represented by (D) over cap (cm(2)/s)=5.91 x exp [(-135 k J/mol)/RT], and turns out to be practically independent of oxygen part ial pressure in the range 10(-2) -0.209 bar.