High-Temperature, High-Pressure Hydrothermal Synthesis and Characterization of an Open-Framework Uranyl Silicate with Nine-Ring Channels: Cs₂UO₂Si₁₀O₂₂

Abstract

A new uranium(VI) silicate, Cs(2)UO(2)Si(10)O(22), has been synthesized by a high-temperature, high-pressure hydrothermal method and characterized by single-crystal X-ray diffraction, luminescence, and solid state NMR spectroscopy. It crystallizes in the monoclinic space group P2(1)/c (No. 14) with a = 12.2506(4) Å, b = 8.0518(3) Å, c = 23.3796(8) Å, β = 90.011(2)°, and Z = 4. Its structure consists of silicate double layers in the ab plane which are connected by UO(6) tetragonal bipyramids via four equatorial oxygen atoms to form a 3D framework with nine-ring channels parallel to the b axis where the Cs(+) cations are located. The photoluminescence emission spectrum at room temperature consists of one broad structured band which is typical of uranyl. The (29)Si MAS NMR spectrum is consistent with the crystal structure as determined from X-ray diffraction, and the resonances in the spectrum are assigned. A comparison of related uranyl silicate structures is made.

Single crystal growth and structural characterization of ternary transition-metal uranium oxides: MnUO4, FeUO4, and NiU2O6

“…Single crystals of uranium containing oxides have been grown by a variety of methods, including solid-state reactions [8,, hydrothermal [6][7][8][9][10][11][12], chemical vapor transport [1][2][3][4][5], and flux methods [15,[34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50]. Most of the crystals reported to date were obtained from solid-state reactions, which require very high temperatures and which work best for congruently melting solids.…”

“…The single crystal growth of complex uranium-containing oxides has garnered recent attention, due to a strong desire to understand better existing structure-property relationships. Complex uranium-containing oxides have been grown as single crystals using a variety of methods, including chemical vapor transport [1][2][3][4][5], hydrothermal synthesis [6][7][8][9][10][11][12], traditional solid-state processing [8,, and flux crystal growth [15,[34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50]. Much of the reported crystal growth utilized the solid-state approach, however, there are some limitations with this method.…”

Hydrothermal Synthesis and Structural Investigation of a Crystalline Uranyl Borosilicate

“…Among the many compound classes that have been studied for waste form applications, uranyl silicates and borates have received considerable attention. The variable arrangement of SiO 4 tetrahedra in uranyl silicate compounds has resulted in a multitude of different structure types, many of which are three-dimensional [9][10][11][12][13][14][15][16][17]. Likewise, the ability of BO 3 and BO 4 units to polymerize to form polyborate units has given rise to a rich structural chemistry in uranyl borates [3,[18][19][20][21][22][23][24].…”

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

“…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 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.…”

“…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.…”