PRINCIPLES OF STRUCTURAL FORMATION FOR INORGANIC COMPOUNDS WITH HEAVY ATOMS. S.V. Borisov, Institute of Inorganic Chemistry, Novosibirsk, Russia

We have considered the most abundant structure types of heavy-metal fluorides (about 50 types), binary and more complex Nb (Ta) oxides with the commonly occuring cation stoichiometry 1:1, 1:2, 1:3 (about 40 types), and as many structure types of tungstates and molybdates. For the most cases, it was found that: (1) the heavy atoms (cations) are located in the vicinity of crystallographic planes with d(hkl) in the 2,5-3,5A range forming the dense nets close to the ideal 3⁶, 4⁴, 3⁶+3³4² nets; (2) the points of intersection of these planes form a sublattice whose nodes are occupied by the cations, and sometimes by the large Cl^-; (3) the geometry of the cation sublattices is close to that of one of three types, i.e., the F type (the ABC close packing) - about 45% of all the structures, the I type (a body - centered cubic sublattice) - 30%, and the AA type (a hexagonal one-layer sublattice, in which the basis plane is split into two planes whose filled nodes complement each other) - 25%; (4) a set of the close-filled cation planes, specific to each sublattice type (with the special geometry of the cation nets), is retained even with the sublattice parameters deviating from the ideal values. The stable tendency toward an ordering of heavy and related atoms is indicative of decreasing a number of atomic degrees of freedom in the structure and can be explained in terms of a novel crystalline state concept. According to this concept, a crystal is considered as an atomic system controlled by selfconsistent sets of standing elastic waves related to crystallographic planes (Borisov S.V., J.Struct.Chem., 1992, V.33, N6, pp.871-877). The data on structure considerations have been published in J.Struct. Chem. in 1994 and 1995; a summury of our results will be published ibid in 1996. We have the unique set of the CAS-PAN programme (Bliznyuk N.A., Borisov S.V., J.Struct.Chem., 1992, V.33, N2, pp.284-304).