Iron stereochemistry in oxygen-containing compounds

Abstract

The Voronoi-Dirichlet polyhedra (VDP) and the method of intersecting spheres were used to analyze the coordination of 1598 sorts of Fe atoms in the structure of 985 oxygen-containing compounds. The iron atoms whose oxidation number varies from 1 to 6 can coordinate 2 to 8 oxygen atoms in the crystal structure, giving rise to the FeOn coordination polyhedra shaped like square antiprisms (n = 8), octahedra or trigonal prisms (n = 6), square pyramids or trigonal bipyramids (n = 5), tetrahedra or squares (n = 4), triangles (n = 3), or dumbbells (n = 2). The effect of the valence state and the coordination number of iron on the VDP parameters was studied. A general linear correlation between the solid angles of the VDP faces corresponding to Fe-O bonds and the corresponding interatomic distances, which vary over a broad range (1.63–2.77 A), was established. It is shown in relation to the A2Fe2O5 oxides (A = Ca, Sr) that VDP characteristics make it possible to reveal essential differences in the crystal structures even for isostructural compounds. The FeIVO6 octahedra, unlike isoelectronic MnIIIO6 or CrIIO6 octahedra, display no Jahn-Teller effect. In was shown in relation to CaFeO3 that VDP parameters can be used for the quantitative description of the charge disproportionation in the crystal structure.