ICOSAHEDRAL CATIONIC FRAMEWORK IN THE STRUCTURES OF MR2F7 FLUORIDES

An icosahedral cationic framework formed by BiCs6Bi6 icosahedra has be en revealed in the CsBi2F7 structure. It is shown that a similar frame work formed by ZnMg6Zn6 polyhedra is present in the structure of the M gZn2 Laves phase. It is established that the cationic motifs in the Cs Bi2F7 and beta-KEr2F7 structure types are similar. It is assumed that an analogous motif should also exist in the RbEr2F7 structure. The cat ionic motifs in MR2F7 (R = Y, Ln) are also considered. On the example of KLn(2)F(7) fluorides, it is shown that the type of cationic framewo rk depends on the ratio r(M)/r(R)(3+) of ionic radii.