Mild Hydrothermal Crystal Growth, Structure, and Magnetic Properties of Ternary U(IV) Containing Fluorides: LiUF₅, KU₂F₉, K₇U₆F₃₁, RbUF₅, RbU₂F₉, and RbU₃F₁₃

Abstract

Single crystals of several ternary alkali uranium fluorides, LiUF5, KU2F9, K7U6F31, RbUF5, RbU2F9, and RbU3F13, have been obtained in a mild hydrothermal process using UO2(CH3CO2)2(H2O)2 as the uranium source. Their crystal structures were determined by single crystal X-ray diffraction. The uranium in the starting reagent was successfully reduced from U(6+) to U(4+) in a dilute hydrofluoric acid environment, aided by the presence of a copper salt. All materials exhibit highly complex crystal structures that range from two-dimensional to three-dimensional. The U(4+) cations are found in high (UF8 and UF9) coordination environments. The magnetic susceptibility measurements yielded effective magnetic moments of 3.01-3.83 μB for the U(4+) cations. The temperature dependent magnetic susceptibility measurements confirmed that the U(4+) cation exhibits a nonmagnetic singlet ground state at low temperatures. No long-range magnetic order was observed for any of the above compositions down to 2 K. Optical and thermal behaviors of the fluorides were also investigated.

A Molecular Octafluoridoneptunate(IV) Anion in (NH4)4[NpF8] and Theoretical Investigations of the [MF8]4– System (M = Th – Bk)

“…Th, U) complex anions forming infinite layers or networks, respectively, are observed. [1,[3][4][5] The Np-F distances in [NpF 8 ] 4are observed in the range of 2.253(1) to 2.308(1) Å, whereas in [UF 8 ] 4the U-F distances at 150 K lie in between 2.257(1) and 2.335(1) Å. [6] An EXAFS study of (NH 4 ) 4 [NpF 8 ] at room temperature gave an average Np-F distance of 2.26(1) Å which nicely agrees with our results.…”

“…[1,2] In these ternary fluorides the anions usually form layer or network structures such as for example 2 ∞ [UF 3/3 F 4/2 F 2/1 ]in RbUF 5 or 3 ∞ [UF 8/2 F 1/1 ]in LiUF 5 . [3] In recent years single-crystal structure determinations of ternary fluoridometallates(IV) have mainly been carried out for Th and U compounds, whereas such studies and thus detailed structural parameters for compounds of the transuranium elements are lacking. [3][4][5][6][7] For the latter, usually only data obtained from powder diffraction studies have been reported, with the exception of a few single-crystal structure determinations.…”

“…[3] In recent years single-crystal structure determinations of ternary fluoridometallates(IV) have mainly been carried out for Th and U compounds, whereas such studies and thus detailed structural parameters for compounds of the transuranium elements are lacking. [3][4][5][6][7] For the latter, usually only data obtained from powder diffraction studies have been reported, with the exception of a few single-crystal structure determinations. [1,2,[8][9][10][11][12][13] The coordination number of the actinide atom in ternary fluoridometallates(IV) is usually nine, with a tricapped-trigonal prismatic arrangement of the fluorido ligands.…”

“…[1,2,[8][9][10][11][12][13] The coordination number of the actinide atom in ternary fluoridometallates(IV) is usually nine, with a tricapped-trigonal prismatic arrangement of the fluorido ligands. [3,4] Compounds with a coordination number of eight of the actinide atom are scarce for fluoridometallates(IV), examples being Li 4 [ [6,11,[14][15][16] In these compounds the complex anions are spatially separated and not connected to each other. For coordination number eight commonly four different ligand arrangements are observed -the ideal polyhedra being either a cube (O h ), a square antiprism (D 4d ), a snub disphenoid (D 2d , Johnson solid J 84 ) or a bicapped trigonal prism (C 2v ).…”

“…A cubic arrangement of ligands in molecular homoleptic complex anions has so far only been observed for the actinide compounds Na 3 [PaF 8 ] and Na 2 [UF 8 ]. [20,21] The square-antiprismatic arrangement has been observed in many more compounds, such as in Na 3 [22][23][24][25][26][27][28][29] The snub disphenoid arrangement is observed in cyanido complexes such as K 4 . [30][31][32] The bicapped trigonal-prismatic arrangement is observed in Li 4 [MF 8 ] (M = Zr, Ce, Tb, U).…”