Ba(NpO2)(PO4)(H2O), its relationship to the uranophane group, and implications for Np incorporation in uranyl minerals

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 ų. 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.