Cationic-anionic sublattices, complex oxidation

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. 

One of the most important parameters that defines the structure and stability of inorganic crystals is their stoichiometry - the quantitative relationship between the anions and the cations [134]. Oxygen and fluorine ions, O2 and F, have very similar ionic radii of 1.36 and 1.33 A, respectively. The steric similarity enables isomorphic substitution of oxygen and fluorine ions in the anionic sub-lattice as well as the combination of complex fluoride, oxyfluoride and some oxide compounds in the same system. On the other hand, tantalum or niobium, which are the central atoms in the fluoride and oxyfluoride complexes, have identical ionic radii equal to 0.66 A. Several other cations of transition metals are also sterically similar or even identical to tantalum and niobium, which allows for certain isomorphic substitutions in the cation sublattice. 

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