Refinement of the structure of potassium heptafluoroniobate, K₂NbF₇, from neutron-diffraction data

Abstract

The crystal structure of K/NbF7, determined originally from two-dimensional X-ray data, has been refined by the method of least-squares with three-dimensional data. The refinement included adjustment of the scattering factors of niobium and potassium relative to the fluorine scattering factor. For potassium the new value 0.370_+0.004 cm-a2 was found. Effects of extinction were minimized by omittingfrom the final refinement a number of the observations, as suggested by systematic analysis of the quality of agreement between calculated and observed F2 values as a function of observed intensity. Shifts of atomic positions by as much as 0.15 A occurred in the refinement, but the general description of the structure, including the description of the NbF~-ion, remains unchanged. The standard errors of the coordinates are about 0.001 A for the niobium atom and 0.002 to 0.003 ,~ for the fluorine atoms and the potassium ions. Analysis of the anisotropic thermal parameters shows that the NbF, 2-ion is a nearly ideal case for use of the 'riding' model in calculating bond-length corrections, and the Nb-F bond lengths have been corrected accordingly. Exceptionally large vibrational displacements of two of the fluorine atoms seem plausible in view of the packing in the crystal.

Crystal structure refinement of K3HfF7 and the disorder of the heptafluorohafnate complex

“…The position of the pentagonal bipyramidal ion with reasonable dimension has 24 equivalent orientations. For good comparison with values of the hypothetical (HfF 7 ) 3_ complex, the ordered structure of potassium heptafluoroniobate (Brown and Walker, 1966) with (NbF 7 ) 2-polyhedra, with Nb -F distances from 1.940 to 1.978 A and F -Nb -F angles from 73.9 to 97.7 ° and the F -F distances varying from 2.357 to 2.912 A, are given here. The quite similar dimensions are also shown by the calculated polyhedra volumes (VOLCAL, Hazen and Finger, 1982) of (NbF 7 ) 2~ with V P = 10.6 A 3 and (HfF 7 ) 3 ~ with V P = 12.3 A 3 .…”

The crystal structure of rubidium hexafluoroprotactinate(V), RbPaF6

“…Two edges are shared in each polyhedron to form chains with the K atoms between them. In K2NbF7 and K2TaF7 the heavy metal has the well-known seven coordination of a trigonal prism with one face capped (Hoard, 1939;Brown & Walker, 1966). Recently PaCI5 was found to crystallize with infinite chains of PaCI7 pentagonal bipyramids sharing apices (Dodge, Smith, Johnson & Elson, 1967), while NbC15 and TaCI5 form octahedrally coordinated dimeric units (Zalkin & Sands, 1958).…”

Crystal chemistry of thiourea complexes of lead(II) salts

“…For completeness we note that capped trigonal prismatic coordination has also been found in the NbF 2 ~ and TaF 2 ~~ ions, which do not have ns 2 lone pairs [see Brown and Walker (1966) for the structure of K 2 NbF 7 and Wells (1975, p. 398) for a general discussion].…”