MAIN STRUCTURAL TYPES OF CATION MATRICES OF BINARY NIOBATES AND TANTALATES WITH THE CATION-RATIO 1 2/

Of 27 typical structures of niobates and tantalates, five structures a re assigned to the F-type of cation sublattice (ABC-packing of trigona l 3(6) planes), eight structures are referred to the I-type (body-cent ered cation sublattice), and eight structures are considered to be the beta-K2UF6 type with a hollow hexagonal framework centered by a scant y-type cation. In two cases (Mg3Nb6O11 and Cu5Ta11O30), the cation fra mework consists of two types of different-scale nets that are commensu rate in a plane. In some structures (K2Ta4F4O9, ThTa2O7, and beta-K2UF 6 type), the ''splitting'' plane effect is noted for the first time: t he neighboring nonequivalent planes of the same family (d(hkl) approxi mate to 1.9 Angstrom) are filed by cations according to the complement principle, when the vacant sites of one plane lie over the occupied s ites of another and vice versa. These structures are treated as modifi ed hexagonal one-layered (AA) type. In the structure of La2TaO4Cl3, th e Cl- anion enters the cation matrix, i.e., occupies the vacant sites of its nets.