CRYSTAL CHEMISTRY OF TANTALUM AND NIOBIUM FLUORIDE COMPOUNDS

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. 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 isomorphic substitutions in the cation sublattice. The ratio between the anionic and cationic radii leads to coordination numbers, the lowest of which is six, which correspond to an octahedral polyhedron of anions around a central cation. The structure of such compounds is discussed from the viewpoint of the ratio X:Me (number of anions surrounding one cation or central atom) in a descending order from 8 to 1.