Perovskites cation transport

In the last decade, study of double perovskites has accelerated with the discovery of tunneling magnetoresistive (TMR) effects in Sr2FeMo06.i56N. Both the magnetism and transport in these oxides is strongly influenced by the cationic nature and the order/disorder of the ions at the B-site. Ferromagnetism and half-metallic properties are often... 

The main factor determining the possibility of high cationic conductivity in a crystalline phase is its structure type. This determines possible sites for potentially mobile cations, their coordination, type of bottlenecks (see Section 7.2.2), connectivity, and so on. Other factors (which are discussed in Sections 7.2.3—7.2.4) are also very important, but only valid within a given structure type. For example, the perovskite type, with its cubo-octahedral cation sites connected via square bottlenecks (Figure 7.4), is absolutely inappropriate for transport of large cations such as Na + or K +, and any substitutions may hardly change this situation to a radical degree. 

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). 

ABO3 perovskite-type oxides with transition-metal ions at the B-site have high ionic and electronic transport in the form of p or n semi-conductivity (mixed ionic and electronic conductivity), caused by different oxidation states of the transition-metal cation. For dense ceramic membranes, perovskite-type oxides with the following cations are preferred A = Ln (lanthanide ion), Ca, Sr, Ba B = Cr, Mn, Fe, Co, Ni, Cu. 

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