The Crystal Structure of Na6[(UO2)(SO4)4](H2O)4: X-Ray and Raman Spectroscopy Study

Abstract

A new uranyl sulfate of composition Na6[(UO2)(SO4)4](H2O)4 has been synthesized at room temperature and its structure has been determined, along with a Raman spectroscopy characterization. The new compound is monoclinic, space group P 21/ c , with a 9.8005(3), b 9.7183(3), c 20.8263(6) A, β 98.794(2)°, V 1960.27(10) A3, and Z = 4. The structure was solved and refined from single-crystal X-ray diffraction data on the basis of F 2 for 3996 unique reflections collected with monochromatic Mo K α radiation to R (all) = 2.92%. The structure of this synthetic compound contains a uranyl sulfate cluster with composition [(UO2)(SO4)4]6–. This cluster consists of a uranyl pentagonal bipyramid and four sulfate tetrahedra, three of which are linked to the uranyl pentagonal bipyramid by the sharing of vertices, while the other is linked bidentately, sharing an equatorial edge of the uranyl pentagonal bipyramid. The uranyl sulfate clusters are linked through bonds to Na+ cations and by an extensive H-bonding network. The bidentate linkage in uranyl compounds is an unusual attribute, which has not previously been observed for uranyl minerals. Raman spectroscopy of this novel compound and selected uranyl minerals with similar structures was undertaken in order to compare them in detail. The results are interpreted with regard to the structural features of the studied substances, i.e. , the mode of linkage and corresponding distortions of the tetrahedral units.