Synthesis and crystal structure of a new Pb uranyl oxide hydrate with a framework structure that contains channels

Using lead oxide, uranyl nitrate and ultrapure water, seven hydrothermal synthesis experiments were performed with Pb:U molar ratios ranging from 1:1 to 1:10. Reaction products were characterized by powder and single-crystal X-ray diffraction. Two novel Pb uranyl oxide hydrates were found, with structural formulae Pb-2(H2O)[(UO2)(10)UO12(OH)(6)(H2O)(2)] and Pb-0.5[(UO2)(4)O-2(OH)(5)](H2O)(4) (Pb-poor fourmarierite). The synthesis products are related to the Pb:U molar ratio of the solution, and indicate a trend from curite, Pb3+x(H2O)(2)[(UO2)(4)O4+x(OH)(3-x)](2), to Pb-2(H2O)[(UO2)(10)UO12(OH)(6)(H2O)(2)], then to Pb-0.5[UO2)(4)O-2(OH)(5)](H2O)(4) with decreasing Pb in the solution. The structure of Pb-2(H2O)[(UO2)(10)UO12(OH)(6)(H2O)(2)], Z = 4, monoclinic, space group C2/c, a 13.281(5), b 10.223(4), c 26.10(1) Angstrom, beta 103.202(6)degrees, V 3450(2) Angstrom (3), was solved by direct methods and refined on the basis of F-2 using 4065 unique reflections. The agreement index (R1) was 6.38%, calculated for 1306 unique observed reflections (/F-o/ greater than or equal to 4(sigmaF)). The structure contains complex chains of edge- and vertex-sharing uranyl pentagonal and square bipyramids, as well as distorted U-6+O-6 octahedra, along both the [110] and [110] directions. The chains in the two orientations are linked at z approximate to 0.25 and z approximate to 0.75, forming a framework that contains elongate open channels. The Pb2+ cations occur in the channels between adjacent chains.