High-Temperature, High-Pressure Hydrothermal Synthesis, Characterization, and Structural Relationships of Layered Uranyl Arsenates

Abstract

Five new uranyl arsenates, Na14[(UO2)5(AsO4)8]·2H2O (1), K6[(UO2)5O5(AsO4)2] (2a), K4[(UO2)3O2(AsO4)2] (2b), Rb4[(UO2)3O2(AsO4)2] (3), and Cs6[(UO2)5O2(AsO4)4] (4), were synthesized by high-temperature, high-pressure hydrothermal reactions at about 560 °C and 1440 bar and were characterized by single-crystal X-ray diffraction, thermogravimetric analysis, and photoluminescence spectroscopy. Crystal data for compound 1: triclinic, P1, a = 7.0005(3) Å, b = 12.1324(4) Å, c = 13.7428(5) Å, α = 64.175(2)°, β = 89.092(2)°, γ = 85.548(2)°, V = 1047.26(7) Å(3), Z = 1, R1 = 0.0185; compound 2a: monoclinic, P2₁/c, a = 6.8615(3) Å, b = 24.702(1) Å, c = 7.1269(3) Å, β = 98.749(2)°, V = 1193.89(9) Å(3), Z = 2, R1 = 0.0225; compound 2b: monoclinic, P2₁/c, a = 6.7852(3) Å, b = 17.3640(8) Å, c = 7.1151(3) Å, β = 98.801(3)°, V = 828.42(6) Å(3), Z = 2, R1 = 0.0269; compound 3: monoclinic, P2₁/m, a = 6.9783(3) Å, b = 17.4513(8) Å, c = 7.0867(3) Å, β = 90.808(3)°, V = 862.94(7) Å(3), Z = 2, R1 = 0.0269; compound 4: triclinic, P1, a = 7.7628(3) Å, b = 9.3324(4) Å, c = 11.9336(4) Å, α = 75.611(2)°, β = 73.136(2)°, γ = 86.329(2)°, V = 801.37(5) Å(3), Z = 1, R1 = 0.0336. The five compounds have layer structures consisting of uranyl square, pentagonal, and hexagonal bipyramids as well as AsO4 tetrahedra. Compound 1 contains chains of discrete uranyl square and pentagonal bipyramids, 2a contains three-polyhedron-wide ribbons of edge- and corner-sharing uranyl square and pentagonal bipyramids, 2b and 3 contain dimers of edge-shairing pentagonal bipyramids that share edges with hexagonal bipyramids to form chains, and 4 contains one-polyhedron-wide zigzag chains of edge-sharing uranyl polyhedra. The double sheet structure of 1 is new, but the chain topology has been observed in an organically templated uranyl sulfate. Compound 2b is a new geometrical isomer of the phosphuranylite group. The sheet anion topologies of 2a and 4 can be obtained by splitting the β-U3O8-type sheet into complex chains and connecting the chains by arsenates.

A Family of Layered Phosphates Crystallizing in a Rare Geometrical Isomer of the Phosphuranylite Topology: Synthesis, Characterization, and Computational Modeling of A4[(UO2)3O2(PO4)2] (A = Alkali Metal) Exhibiting Intralayer Ion Exchange

“…This sheet topology can be described as a combination of the uranophane and β -U 3 O 8 topologies and had been previously observed in K 6 [(UO 2 ) 5 O 5 (AsO 4 ) 2 ], which is isostructural with 7 and 8. 10 As seen in structures 1−6, adjacent phosphate tetrahedra point in the same direction and stack in the same direction of tetrahedra in the adjacent layers, creating a larger space between the phosphate tetrahedra, compared to the uranyl polyhedra. For this reason, the alkali site directly below the phosphate tetrahedra is partially occupied by Cs/K and Rb/ K accommodating the larger alkali cation, while K cations lie between uranyl polyhedra.…”

“…Interestingly, none of the four known geometrical isomers observed in uranyl minerals match the layers in the six compounds; however, this geometrical isomer of the phosphuranylite anion-topology has been observed in a uranyl arsenate, K 4 [(UO 2 ) 3 O 2 (AsO 4 ) 2 ]. 10 Structures 1, 2, and 4−6 all have mixed-alkali sites, where the larger alkali metal is located between the phosphate tetrahedra, while the smaller alkali metal is located between the uranyl polyhedra. This is likely a space filling issue, since, between the layers, there is more space between the phosphate tetrahedra than between the uranyl polyhedra.…”

“…3 Within the phosphuranylite class, there are four geometric isomers that occur in minerals, 9 and a fifth that has been reported in a uranyl arsenate, K 4 [(UO 2 ) 3 O 2 (AsO 4 ) 2 ]. 10 These geometric isomers primarily differ by the orientations of the phosphate (or other nonsheet ligand) tetrahedra. Historically, such layered phosphates have been described by looking at the chains of phosphate tetrahedra and the pair of tetrahedra that edge share with the hexagonal uranyl bipyramid.…”

“…The pentagonal- and hexagonal-uranyl bipyramids edge share to form chains that are connected together by edge- and corner-sharing phosphate tetrahedra . Within the phosphuranylite class, there are four geometric isomers that occur in minerals, and a fifth that has been reported in a uranyl arsenate, K 4 [(UO 2 ) 3 O 2 (AsO 4 ) 2 ] . These geometric isomers primarily differ by the orientations of the phosphate (or other nonsheet ligand) tetrahedra.…”

Influence of extreme conditions on the formation and structures of caesium uranium(vi) arsenates

“…Such studies were performed mainly at relatively low pressures and temperatures. [27][28][29][30][31][32] In this report we present the results of the U-As-Cs oxo-system investigations, aiming to explore the synthetic and coordination chemistry of uranium under extreme (HT/HP) conditions. Herein, we obtained four new uranium(VI) arsenates, Cs 4), respectively.…”