Coordination polyhedra FeXn (X = F, Cl, Br, I) in the crystal structure

The Voronoi-Dirichlet polyhedra (VDP) and the method of intersecting spheres were used to carry out crystallochemical analysis of 300 compounds containing coordination polyhedra FeXn (X = F, Cl, Br, I). The Fe(III) and Fe(II) atoms were found to display a coordination number equal to 4, 5, or 6 with respect to the halogen atoms, thus forming tetrahedral, trigonal-bipyramidal, and octahedral complexes, respectively. An increase in the oxidation number of iron in FeX6 (X = F, Cl) or FeX4 (X = Cl, Br, I), as well as the FeX4 --> FeX6 transformation (X = Cl, Br, I) with a fixed valence state of the Fe atoms, is accompanied by a decrease in the VDP volume (by 1.3-2.2 Angstrom(3) and 2.2-4.1 Angstrom(3), respectively). It was shown that, although for the same X, the Fe-X distances change within the range of 0.1-0.5 Angstrom, the radii of the spheres whose volume is equal to the VDP volume of the Fe atoms change only by 0.01-0.06 Angstrom. This makes it possible to approximate the Fe atoms in the crystal structure by soft (deformable) spheres with a constant volume. Nonvalence Fe ... Fe interactions in the crystal structure are discussed.