Oxyfluorides, anion substituted

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. 

Substitution Variants. The fluorite-type structure is maintained in principle when alkaline earth elements are replaced partially by rare-earth elements. Charge compensation is achieved by occupation of additional interstitial anionic sites.The coordination of the metal atoms may increase from 8 to 9 or even 10 by this. Another way of charge balance is the partial replacement of fluorine by oxygen to form oxyfluorides. Since the possible interstitial positions provide pathways for anion disorder and movement, this class of materials shows fluoride ionic conductivity. 

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