Oxygen Desorption and Perovskite Stability

As seen from Fig. 10.11, the value of (3-8) in Lai xSrxCo03 falls off with decreasing oxygen activity much more rapidly than for the other compounds shown. The general trend at which the perovskites become nonstoichiometric follows that of the relative redox stability of the late transition metal ions occupying the B-site, i.e. Cr Fe Mn Co . The reductive nonstoichiometry of the cobaltites increases further by partial B-site substitution with copper and nickel. 

The reductive (and oxidative) nonstoichiometry and the stability in reducing oxygen atmospheres of perovskite-type oxides was reviewed by Tejuca et al. [174]. Data from temperature programmed reduction (TPR) measurements indicate that 

The stability of the undoped perovskites LaBOs., at 1000°C, expressed in terms of Pq decreases in the order LaCrOa (10 ° atm) LaFeOs (10 atm) LaMn03 5 (10 atm) LaCoOs (10 atm), noting that the cited value for LaCrOs corresponds with the lowest limit in a thermogravimetric study by Nakamura et al. [176]. The same trend was foxmd by means of TPR [174]. 

Tabata et al. [177] and Seyama [178] both described significant differences in the chemical composition of the surface, due to Sr segregation, compared with the bulk composition in a series of powders Lai. Sr CoOs. This indicates a behaviour of the surface different from that of the bulk in these compoimds. Not only can this account for a number of observations made in the total oxidation of CO and CH4, as discussed by the authors, but it is also considered to be an important factor when one tries to correlate the composition of a perovskite with its activity in surface oxygen exchange. 

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Localized vs. Delocalized Electrons Oxygen Desorption and Perovskite Stability Equations for Oxygen Transport Electronic Conductivity... 

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