COMPARATIVE CRYSTAL-CHEMISTRY OF HEAVY-METAL FLUORIDES AND COMPLEX NIOBATES AND TANTALATES IN TERMS OF A NEW CRYSTALLINE STATE CONCEPT

In many crystal structures of Nb (Ta) fluorides and complex oxides ana lyzed earlier, cationic sublattices of F, I, and AA types were found ( face- and body-centered cubic and hexagonal monolayer sublattices, res pectively). The density of anion packing in the cationic sublattice (N -a/N-c - the ratio of the number of anions to that of cations) is clos e to two,. this means that the mean valence of a cation is about two i n fluorides and about four in oxides. The;re and other structural feat ures are explained by the tendency of the atomic systems of the struct ures toward fewer degrees of freedom. The review of the structures sug gests that there are stability regions of structures both in the field of cationic sublattice parameters and in the field of compositions.