Single crystals of Ba(NpO2)(PO4)(H2O) were obtained using hydrothermal synthesis techniques. The structure was determined using single-crystal X-ray diffraction data collected using MoKα radiation and an APEX II CCD detector and was refined on the basis of F2 for all unique data to R1 = 2.41%. Ba(NpO2)(PO4)(H2O) crystallized in monoclinic space group P21/n with a = 6.905(3), b = 7.108(3), c = 13.321(6) Å, β= 105.02°, and V = 631.4 Å3. The structure contains chains of edge-sharing neptunyl pentagonal bipyramids that link through phosphate tetrahedra to form infinite sheets. This sheet-type is identical to the anion topology of the uranophane group, in particular to that of oursinite, Co[(UO2)(SiO3OH)]2(H2O)6. Similarities between Ba(NpO2)(PO4)(H2O) and the uranophane group of minerals suggests a charge-balancing mechanism for incorporation of Np5+ into uranyl minerals.

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