BULK HELICOIDS IN CRYSTAL STRUCTURES OF RARE-EARTH AND SODIUM TETRAKISPIVALOYLTRI-FLUOROACETONATES, NaLn(PTA)₄. T.M. Polyanskaya, G.V. Romanenko, N.V. Podberezskaya, Institute of Inorganic Chemistry Sib. Branch RAS, Novosibirsk, Russia

We have carried out an X-ray study of the volatile NaLn(PTA)₄ (Gd-Lu) complexes. The parameters for Er are as follows: a=13.114, c=21.004 Å, sp. gr. P3₁, Z=3. In the structure, each PTA ligand performs a tridentate function, forming a six-membered chelate cycle with the Er atom, and, besides, coordinates the neighboring Na atom by the additional Na-O bond. The coordination polyhedron of each metal atom includes the O atoms of four PTA ligands. The Na atom bridges two non-equidistant Er atoms (3.833 and 3.893 Å). The M-O bond strengths are much the same (the mean Er-O and Na-O bond values 2.325 and 2.396 Å). Four vertices of the eight-vertex Na polyhedron are occupied by the F atoms (the mean Na-F 2.678 Å) of the same PTA ligands which are shared by Na and Er. The trans-edge-sharing eight-vertex polyhedra about Er and Na form the array with extended structure which runs toward a crystallographic 3₁ axis, but the Er and Na ions are not sited on the axis and the extended array has a 'helical' structure. The independent link of the array is equal to two polyhedra : the Na1 atom, nearest to Er, supposedly converts into a gaseous phase.