Crystal-chemical features of baric polymorphism of actinides

Abstract

Crystal-chemical analysis of actinide baric polymorphs was performed on the basis of Voronoi–Dirichlet (VD) tessellation. The second moment of inertia of the VD polyhedron (G₃) and the displacement of the atom nucleus from the center of gravity of its VD polyhedron (D_A) were suggested as descriptors acting as criteria for revealing 5f interactions between the actinide atoms. Pressure elevation is accompanied, as a rule, by an increase in G₃ for Th, Pa, Am, Cm, Bk, and Cf, whereas for U and Pu it leads to a decrease in G₃. Topologically equivalent flat (Th-hP) or corrugated (Pa-hP, α-U, Вk-III) 4⁴ networks are formed in these high-pressure phases owing to the shortest (covalent) bonds between the actinide atoms, whereas a diamond-like framework is formed in the structures of An-III or An-IV (An = Am, Cm, Cf) and Cm-V, and Pu₄ molecules are formed in hexagonal Pu-hP.