Flux Crystal Growth and Optical Properties of Two Uranium-Containing Silicates: A₂USiO₆ (A = Cs, Rb)

Abstract

Single crystals of two uranium silicates, Cs2USiO6 and Rb2USiO6, have been grown from molten fluoride fluxes and structurally characterized by single-crystal X-ray diffraction. Cs2USiO6 crystallizes in the body-centered orthorhombic space group, Immm, with a = 8.5812(4) Å, b = 13.0011(6) Å, and c = 13.8811(7) Å. The size of Rb is slightly too small to fit into this structural framework without effecting slight structural changes that result in a 6-fold superstructure. Sharp satellite peaks were observed in the single-crystal X-ray diffraction data, indicating the existing of a superstructure. The crystals were examined by electron diffraction, the results of which suggest that the structure can be thought of as the Immm isotype (a = 8.4916(6) Å, b = 12.6678(9) Å, and c = 13.5077(9) Å) on average, with an approximately 6-fold superstructure along the c axis. The materials were further characterized by UV-vis reflectance spectroscopy.

Synthesis and structure of the new pentanary uranium(vi) silicate, K4CaUSi4O14, a member of a structural family related to fresnoite

“…The Rb analogue, Rb 2 USiO 6 , crystallizes in the same structure, however, the Rb cations are slightly too small for the coordination environment and lead to an incommensurate structure which is believed to be caused by a modulation of the Si 4 O 12 units. 7 Such modulations and superstructures are not uncommon in silicates and, in fact, the title compound of this paper, K 4 CaUSi 4 O 14 , similarly exhibits a superstructure apparently due to modulations of the UO 6 and SiO 4 units.…”

“…6 The majority of the studied anhydrous uranium silicates contain uranium in the fully oxidized UIJVI) state. All of the reported anhydrous UIJVI) silicates, with the exception of A 2 USiO 6 (A = Rb, Cs), 7 contain isolated uranium polyhedra which connect with silicate tetrahedra to form three dimensional frameworks. The dimensionality of the silicate units varies and includes 0-D isolated SiO 4 units, 8 Si 2 O 7 units, 9 Si 4 O 12 squares, 7 Si 4 O 13 chains, 10 and Si 8 O 22 units, 11 1-D chains, 12,13 1-D columns, 14 2-D chains, 15 and 2-D slabs.…”

“…All of the reported anhydrous UIJVI) silicates, with the exception of A 2 USiO 6 (A = Rb, Cs), 7 contain isolated uranium polyhedra which connect with silicate tetrahedra to form three dimensional frameworks. The dimensionality of the silicate units varies and includes 0-D isolated SiO 4 units, 8 Si 2 O 7 units, 9 Si 4 O 12 squares, 7 Si 4 O 13 chains, 10 and Si 8 O 22 units, 11 1-D chains, 12,13 1-D columns, 14 2-D chains, 15 and 2-D slabs. 16 An exception among the known UIJVI) silicates, Cs 2 USiO 6 , 7 contains infinite 1-D chains of UO 6 polyhedra that are connected in the other two dimensions by Si 4 O 12 squares.…”

“…The majority of the known uranium silicates have been grown under hydrothermal conditions. 8,10,13,15,16,[18][19][20][21][30][31][32][33][34][35][36] Notably, A 2 USiO 6 (A = Rb, Cs) was grown from an alkali fluoride flux 7 and a series of salt-inclusion phases were grown from a mixed alkali fluoride flux. 9 Herein, we report the synthesis of a new pentanary uraniumIJVI) silicate, K 4 CaUSi 4 O 14 , grown from a eutectic CaF 2 /KF flux, discuss the structure and optical properties, and describe the extended structural family of which K 4 CaUSi 4 O 14 is a member.…”

Crystal chemistry and the role of ionic radius in rare earth tetrasilicates: Ba2RE2Si4O12F2 (RE = Er3+–Lu3+) and Ba2RE2Si4O13 (RE = La3+–Ho3+)

“…1b). The silicate building block is a cyclic [Si 4 O 12 ] unit that also has inversion symmetry ( (Colin et al, 1993), Ba 2 Cu 2 Si 4 O 12 (Janczak & Kubiak, 1992;McKeown & Bell, 1997), K 2 (NbO) 2 Si 4 O 12 (Crosnier et al, 1992), M 6 Cl 10 Si 4 O 12 (M = Sm 3+ , Gd 3+ -Dy 3+ ) (Hartenbach et al, 2006), Sr 8 Si 4 O 12 Cl 8 (Wang et al, 2001) and Cs 2 USiO 6 (Read et al, 2015) (Machida et al, 1982).…”