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

Four Np 6+ phosphates with compositions X (NpO 2 )(PO 4 )(H 2 O) 3 , X = K + , Na + , Rb + , NH 4 + , have been synthesized by mild hydrothermal methods, and their structures have been determined. Single-crystal X-ray-diffraction data were collected using Mo K α radiation and an APEX CCD detector. The Np 6+ structures containing K + , Na + , Rb + , and NH 4 + ( KP1 , NaP1 , RbP1 , NH 4 P1 ) were refined to agreement indices R 1 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 P 4/ ncc , with a 6.9564(8), c 17.826(4) A, V 862.6(2) A 3 , Z = 4. NaP1 also is tetragonal, P 4/ ncc , with a 7.005(1), c 16.986(4) A, V 833.6(3) A 3 , Z = 4. RbP1 is tetragonal, P 4/ ncc , with a 7.031(4), c 17.87(2) A, V 884(1) A 3 , Z = 4. NH 4 P1 crystallizes in space group P 4/ ncc with a 6.9843(6), c 18.023(3) A, V 879.2(2) A 3 , Z = 4. Each of these structures contains sheets of composition [(NpO 2 )(PO 4 )] − consisting of Np 6+ 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 H 2 O group are located in the interlayer, where they balance the charge on the sheets and provide linkages between the sheets.

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