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 Pb2(H2O)[(UO2)10UO12(OH)6(H2O)2] and Pb0.5[(UO2)4O2(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)4O4+x(OH)3–x]2, to Pb2(H2O)[(UO2)10UO12(OH)6(H2O)2], then to Pb0.5[(UO2)4O2(OH)5](H2O)4 with decreasing Pb in the solution. The structure of Pb2(H2O)[(UO2)10UO12(OH)6(H2O)2], Z = 4, monoclinic, space group C2/c, a 13.281(5), b 10.223(4), c 26.10(1) Å, 103.202(6)°, V 3450(2) Å3 , was solved by direct methods and refined on the basis of F2 using 4065 unique reflections. The agreement index (R1) was 6.38%, calculated for 1306 unique observed reflections (|Fo| ≥ 4 F). The structure contains complex chains of edgeand vertex-sharing uranyl pentagonal and square bipyramids, as well as distorted UO6 octahedra, along both the [110] and [1̄10] directions. The chains in the two orientations are linked at z ≈ 0.25 and z ≈ 0.75, forming a framework that contains elongate open channels. The Pb2+ cations occur in the channels between adjacent chains.