Actinides in Geology, Energy, and the Environment
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摘要

The crystal structure of čejkaite, Na4(UO2)(CO3)3, from the type locality, was determined for the first time by single-crystal X-ray diffraction. In contrast to the previously reported pseudohexagonal triclinic symmetry, the current data indicate čejkaite is monoclinic, triply twinned, and belongs to the space group m>Ccm>. Refined unit-cell parameters are m>am> = 9.2919(8), m>bm> = 16.0991(11), m>cm> = 6.4436(3) Å, β = 91.404(5)°, and m>Vm> = 963.62(12) Å3. The monoclinic unit cell is also supported by the good fit to the powder diffraction data. The structure of čejkaite consists of uranyl tricarbonate clusters, forming sheets sub-parallel to (001) by sharing edges with (NaΦ) polyhedra. Sheets are interconnected through the uranyl O atoms and columns of (Na1Φ) polyhedra that share their trigonal faces. All Na atoms in the structure are in sixfold coordination. The structure refinement yielded m>Rm>obs = 0.0424 for 1687 observed reflections [m>Im>obs > 3σ(m>Im>)] and 0.0538 for all 2016 unique reflections. Refinement and bond-valence analysis of the structure confirmed the previously proposed formula Na4(UO2)(CO3)3, m>Zm> = 4.

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