Transition metal oxides double perovskites

Ordered Perovskite-type Compounds, A2(BB )06 Systems Cubic Fmim A feature of the perovskite structure is that, with the proper substitutions, many types of ordered structures can readily be formed. This can be accomplished by the substitution of two suitable metal ions (with different oxidation states) in the octahedral sites of the structure. In this case the unit ceil is doubled along the three cubic axes to generate an 0.8 A unit ceil (Figure 15). Partial substitution of different transition metal ions in the octahedral sites is also possible the general formulation for these compounds would be A2(B2 xB x)06. The parentheses in this formulation enclose atoms occupying the octahedral sites in the structure. 

The majority of new materials for SOFCs are perovskite stmctured oxides of general form ABO3.5 [23]. The ideal perovskite structure is a cubic close-packed ABO3 structure where the B-site cation sits within the octahedral interstices. Fig. 3.5. This stmcture is very flexible toward cation composition and tolerates large substitution fractions on either cation site. The Goldschmidt factor, a ratio of A, B, and O ionic radii, is often utilized to predict if a metal oxide will crystallize into the perovskite structure [24]. The A site of the commonly utilized perovskites is typically occupied by La, Ca, Sr, or Ba. The B site is typically a transition metal. Other stmctures investigated include double perovskites, apatites, and fluorites. 

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