Anion substituted perovskites

Ionic conductivity is the transport of cations and/or anions across the perovskite under the influence of an electric field. As with diffusion, for ionic conductivity of cations and anions in perovskites to occur the structure must either contain open regions or a significant population of vacancies on the appropriate sublattice to allow ionic movement. Substitution is again widely used to create vacancies in perovskites with approximately cubic structures so as to increase conductivity. A further requirement, for strictly ionic conductivity, is the absence of cations with a variable valence. In cases where variable valence cations are present, electronic conductivity may also occur and in such cases will invariably dominate, in magnitude, the ionic conductivity (Sections 5.4 and 5.5). 

Partial substitution of A and B ions is allowed, yielding a plethora of compounds while preserving the perovskite structure. This brings about deficiencies of cations at the A-or B-sites or of oxygen anions (e.g. defective perovskites). Introduction of abnormal valency causes a change in electric properties, while the presence of oxide ion vacancies increases the mobility of oxide ions and, therefore, the ionic conductivity. Thus, perovskites have found wide apphcation as electronic and catalytic materials. 

Besides the ionic radii requirements, the other condition to be fulfilled is electroneutrality, i.e., that the sum of charges of A and B ions equals the total charge of X anions. This is attained in the case of oxides by means of charge distribution of the form Al+B5+03, A2+B4+03, or A3+B3+03. Moreover, partial substitution of A and B ions giving rise to complex oxides is possible while keeping the perovskite structure. Figure 3, elaborated from some comprehensive compilations of data on the structure and properties of this type of compound (4,15,17,21,22), shows that almost all the stable elements have been included in the perovskite framework, many of them in both the A and B positions. In what follows we will... 

Tsai et a/. also approached the problem of increasing O2 flux and stability. Their approach was to balance the substitutions on the A and B sites of the ABO3 perovskite. Stability is strongly influenced by a stable BO3 skeletal sublattice, and the choice of Fe with a mild Co substitution gives stability with reasonable oxygen anion conductivity. Then different amounts and types of aliovalent cations (Ca, Sr ", Ba " ) were partially substituted for La in a LaFeo.8Coo.203 s perovskite framework to attain higher electron conductivity. 

The substitution of the alkaline earth for lanthanum in different amounts has great impact on the electron-conducting properties. This is principally because strontium (and calcium or barium) has a valence of 2+ whereas lanthanum exists in the 3-i- form. Thus, as strontium is substituted for lanthanum, there arises a need to equilibrate the charge of the cations and oxygen anions in the perovskite. 

This relationship was first exploited by Goldschmidt, in 1926, who suggested that it could be used to predict the likelihood that a pair of ions would form a perovskite stmcture phase. When this was initially proposed, very few crystal structures had been determined and so ionic radii were used as a substitute for measured bond lengths. For this purpose, it is assumed that for a stable structure to form the cations, just touch the surrounding anions (Goldschmidt s mle), then ... 

Many ternary oxides with compositions of A B O3, A B O3, A B O3, A +B + 03, and an abundance of compounds with more complex compositions, are crystallized in perovskite structure. The perovskite structure is very flexible, allowing not only the substitution of different cations in positions A and B over a wide range of compositions Ai xA xBi xBx03, but also the introduction of vacancies or substitutions on the anion sublattice. It is for this reason that about 90% of the metallic elements of the Periodic Table are known to be stable in a perovskite-type oxide structure. 

The perovskite families are characterized by the great flexibility of their crystal structures so as to accommodate wide cation substitutions or anion vacancies, and... 

Although perovskites have not yet found application as commercial catalysts, they remain excellent models for the study of catalytic reactions. Moreover, they also represent promising candidates for polyfunctional catalysts, because they are characterized by a great flexibility of the crystal structure to accommodate cation substitutions, or to provide anion vacancies, both of which are extremely usefiil. For example, a combination of two different ions at the B-site in the system... 

The fluorite-type oxides are not the only possible 0 ion conductors. Perovskite-t e oxides wit anion acancies can be formed by substituting M for M or M for M. The ionic... 

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