SPECIFIC FEATURES OF RARE-EARTH POLYHEDRA IN ALKALI LANTHANIDE NITRATES. A.G.Vigdorchik, Institute of Crystallography, Russian Acad. of Sci., Leninsky pr. 59, Moscow, 117333, Russia

This research was performed in the framework of a systematic study undertaken to explain the structure and properties of compounds of alkali rare-earth nitrates with common formula of (n-3m)A[(m)Ln(n)(NO₃)(k)(H₂O)](l)(H₂O), where A= alkali cations; n,m,k,l=stoichiometric coeff. The analysis of crystal-chemical characteristics of Ln coordination polyhedra was carried out on the basis of our own experimental data (about 50 crystal compounds) and the data reported in literature. Their shape, symmetry and other properties were analyzed. In all compounds reviewed, the coordination numbers(CN) of the Ln atoms are 10 or 12. CN=12 is characteristic only of the elements of the cerium subgroup. Three types of coordination of the Ln-atoms by nitrate group were found: monodentate, bidentate and bridging bidentate. While reviewing structural data on the compounds in question, we face an exceedingly limited assortment of coordination polyhedra for high coordination numbers - there are only three types: icosahedron, tetracapped trigonal prism, and bicapped tetragonal antiprism. All 12-vertex polyhedra appear as slightly distorted icosahedra. An atom of Ln tends to form a symmetrical coordination arrangement even in the case when it occupies a position without any symmetry elements. It should also be noticed that complexes with the different composition and stoichiometries may correspond to identical coordination polyhedra.