Nonstoichiometry re-equilibration

When the oxygen achvity is changed in the surrounding of a binary oxide AOj s that has previously been equilibrated with the surrounding, the nonstoichiometry of the oxide changes towards a new equilibrium value. The overall kinetics of this nonstoichiometry re-equilibration process typically consists of the surface reaction step 

For an infinite bar of AOi g with the square cross-section of 2a x 2a, for example, the overall re-equilibration kinetics is normally described in terms of the two kinetic parameters - the surface reaction rate constant k, and chemical diffusivity D as [12,27]  

and 8(cx3) are the mean (at time f), initial (at f = 0) and final (as f — CX3) value of oxygen nonstoichiometry, respectively. By monitoring the temporal variation of the nonstoichiometry 8(f) by either thermogravimetry or a 8-sensitive property (e.g., electrical conductivity), it is possible to determine the two kinetic parameters. With regards to binary systems, it is believed that the relaxation kinetics may be well understood. Chemical diffusion, in particular, has long been understood in the light of chemical diffusion theory [28], or in the light ofthe ambipolar diffusion theory [29]. 

With regards to the complex oxides, however, it is fair to say that the nonstoichiometry relaxation kinetics and chemical diffusion are, as yet, less well understood. In 

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Nonstoichiometry Re-Equilibration 473 initial and boundary conditions, which are given in Figure 10.20 as ... 

Deviations from stoichiometry and chemical diffusion in metal-deficit samples were studied at 973 to 1273K, for S activities of 0.1 to lOOOOPa, by using a microthermogravimetric technique. It was found that the non-stoichiometry was a function of temperature and the equilibrium S pressure thus indicating that the cation vacancies did not interact, were fully ionized and were randomly distributed throughout the crystal lattice. Re-equilibration-rate measurements of nonstoichiometry showed that the vacancy diffusivity (a direct measure of defect mobility) did not depend upon their concentration and could be described by ... 

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