Chemical Substitutions in the Perovskite Structure

TABLE 11 Possible Substitution Studies in La2Cu04 According to the Ionic Sizes of Ions in Different Coordination Sites. 

The simplest quaternary derivative with the perovskite structure would be one in which two different transition metals might occupy the B-site position. This can be formulated as A(B,1 2B1 2)03, or preferably A2(B B0O6. These compounds can then crystallize with a doubled unit cell, if ordering occurs on the octahedral metal sites. Further compositional and structural adaptions could be obtained, as shown below, all possessing an overall 1 1 3 ratio of A B 0 atoms. In all the following examples and formulations, the proper stoichiometry will be maintained, and oxygen will be the principal anionic species. 

Additionally, the B metal ions can adopt different oxidation states, but maintaining the proper average oxidation state to effect charge balance. That could increase the total number of possibilities for new compositions to very large numbers, even when limited to the oxide 

The incorporation of Cu ions in the perovskite structure is known for only a few examples since this particular structure is normally stabilized by or requires a B atom in a high formal oxidation state such as Ti4+ in BaTiOs, or Rhs+ in LaRhOs. Further, since Cu can not be readily stabilized in its Cu(m) state, and is unknown in the tetravalent state, the simple formation of ternary compounds such as LaCuOg or BaCuOs is not expected. Even in the K2NiF4 structure, the stabilization of Cu4+ as in Ba2Cu04 is not expected, but the formation of a stable Cu(II) state is a distinct possibility, as in La2Cu04. Copper(II), however, has been introduced in the doubled-or tripled-perovskite structure. Examples of these, which include structural distortions from cubic symmetry, are listed  

In addition to these perovskite-related compounds, Raveau and Michel have prepared a series of Cu-containing phases which are structurally related to the perovskite structure. Certain of these derivatives have been discussed in a previous section. 

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