THE CRYSTAL STRUCTURES OF X ( NpO

Four Np6+ phosphates with compositions X(NpO2)(PO4)(H2O)3, X = K+, Na+, Rb+, NH4, have been synthesized by mild hydrothermal methods, and their structures have been determined. Single-crystal X-ray-diffraction data were collected using MoK radiation and an APEX CCD detector. The Np6+ structures containing K+, Na+, Rb+, and NH4 (KP1, NaP1, RbP1, NH4P1) were refi ned to agreement indices R1 0.042, 0.051, 0.066, and 0.023 calculated for 831, 856, 798, and 857 unique observed refl ections, respectively. KP1 crystallizes in space group P4/ncc, with a 6.9564(8), c 17.826(4) Å, V 862.6(2) Å3, Z = 4. NaP1 also is tetragonal, P4/ncc, with a 7.005(1), c 16.986(4) Å, V 833.6(3) Å3, Z = 4. RbP1 is tetragonal, P4/ncc, with a 7.031(4), c 17.87(2) Å, V 884(1) Å3, Z = 4. NH4P1 crystallizes in space group P4/ncc with a 6.9843(6), c 18.023(3) Å, V 879.2(2) Å3, Z = 4. Each of these structures contains sheets of composition [(NpO2)(PO4)] consisting of Np6+ in square bipyramidal coordination and phosphate tetrahedra. These sheets are topologically identical to the well-known uranyl phosphate sheets of the autunite group. The monovalent cations and H2O group are located in the interlayer, where they balance the charge on the sheets and provide linkages between the sheets.

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