AnSe n (An = Th, U, Np, Pu, Am, or Cm) Coordination Polyhedra in Crystal Structures
- PDF / 284,535 Bytes
- 11 Pages / 612 x 792 pts (letter) Page_size
- 103 Downloads / 194 Views
n (An = Th, U, Np, Pu, Am, or Cm) Coordination Polyhedra in Crystal Structures V. N. Serezhkina,*, M. Albakajajia, D. V. Pushkina, and L. B. Serezhkinaa a Samara
National Research University named after Academician S.P. Korolev, Samara, 443011 Russia *e-mail: [email protected] Received June 10 2019; revised June 10, 2019; accepted September 10, 2019
Abstract—Voronoi–Dirichlet (VD) polyhedra have been used to make a crystal-chemical analysis of compounds containing in their crystal structure coordination polyhedra AnSen (An = Th, U, Np, Pu, Am, or Cm). It was found that atoms An(II), An(III), An(IV), and An(V) occur in selenides. These atoms coordinate from 6 to 10 selenium atoms in the form of Se2– or Se–. It was shown that the VD polyhedra make it possible to determine the valence state of An atoms in structures of crystalline substances and reveal errors in the crystal-structural information. A dependence of the multiplicity of Se–Se bonds on their length in 0D, 1D, and 2D groups composed of only selenium atoms was determined. Keywords: actinides, selenides, Voronoi–Dirichlet polyhedra, stereochemistry, actinide compression DOI: 10.1134/S1066362220040025
INTRODUCTION
X/X), exhibited by S, Se, and Te, is an important specific feature of their stereochemistry, which is due to the existence of X– anions capable of forming in crystal structures various groups affecting the structure and properties of the compounds. It has been noted more than once that the chalcogen– chalcogen contacts X/X hinder, or make at all impossible, a unique determination of the formal charges of atoms in crystal structures [8–12]. As one of examples of this kind can serve Np2Se5 {250760} [13]. Here and hereinafter, we give in curly brackets the digital or alphabetic code of a compound in the structural databases [14, 15]. According to [13], the Np2Se5 structure has Np, Se1, and Se2 atoms in a 2 : 4 : 1 ratio. Se2 atoms are Se2– ions bound to 4 Np atoms. Each Se1 atom (formally Se– ion) also binds 4 Np atoms, simultaneously being a member of linear chains –Se1–Se1–Se1– in which each atom has two neighbors at distances of 2.65 and 2.80 Å. Based on the results obtained, including the data furnished by XANES spectroscopy, the authors of [13] came to a conclusion that most of results point to an intermediate (between +3 and +4) valence state of Np, which they described by the formula [Np(3+x)+]2(Se2–) (Se4)(4+2x)– (0 < x ≤ 1), although they did not rule out that the actinide is present in the Np(IV) state in Np2Se5.
One topical problems of modern nuclear power engineering is the improvement of methods for separation of minor actinides (Np, Am, Cm) and lanthanides present in a ~1 : 40 relative amounts in highactivity wastes [1]. It has been found in recent years that the progress in solving this problem may be associated with the use of extractive agents containing “mild” donor ligands X (in particular X = S, Se, or Te), which form more covalent An–X bonds (An = actinide), compared with the similar Ln–X bonds [1–3]. Although nature of the differe
Data Loading...
