Actinide Contraction in Oxygen-Containing An(VI) Compounds

Abstract

The actinide contraction effect in the structures of 55 crystals containing \(\rm{AnO}_2^{2+}\) dioxocations (An = U, Np, Pu, Am) and belonging to 18 series of isostructural An(VI) compounds was analyzed using Voronoi-Dirichlet (VD) polyhedra. Analysis shows that the actinide contraction, as a rule, is accompanied by a regular decrease in the radii of the spherical domains (R_sd) of the volume equal to that of the VD polyhedra of the An(VI) atoms in the crystal structures. In the series U-Np-Pu, such contraction is accompanied by a regular increase in the second moment of inertia (G₃) of the VD polyhedra, suggesting a decrease in the extent of their sphericity (due to stronger flattening of the coordination polyhedron in the form of bipyramid), whereas in going from Pu to Am the extent of sphericity of the VD polyhedron increases instead of decreasing. The results obtained confirm the viewpoint that the contraction and localization of the 5f shell start specifically from Am. The lack of correlation between the actinide contraction and relative strength of An-O bonds in the series U-Np-Pu-Am is noted.