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THE CRYSTAL STRUCTURES OF X(NpO₂)(PO₄)(H₂O)₃ (X = K⁺, Na⁺, Rb⁺, NH₄⁺) AND THEIR RELATIONSHIP TO THE AUTUNITE GROUP

Department of Civil Engineering and Geological Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, Indiana 46556, U.S.A.

E-mail address: pburns{at}nd.edu

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 refined to agreement indices R₁ 0.042, 0.051, 0.066, and 0.023 calculated for 831, 856, 798, and 857 unique observed reflections, respectively. KP1 crystallizes in space group P4/ncc, with a 6.9564(8), c 17.826(4) Å, V 862.6(2) ų, Z = 4. NaP1 also is tetragonal, P4/ncc, with a 7.005(1), c 16.986(4) Å, V 833.6(3) ų, Z = 4. RbP1 is tetragonal, P4/ncc, with a 7.031(4), c 17.87(2) Å, V 884(1) ų, Z = 4. NH₄P1 crystallizes in space group P4/ncc with a 6.9843(6), c 18.023(3) Å, V 879.2(2) ų, 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.

Keywords: neptunium phosphate, synthetic compounds, crystal structure, neptunyl phosphate sheets, autunite group.