Three uranyl selenates, one mixed uranyl selenate-selenite, and two uranyl selenite compounds have been synthesized using mild hydrothermal techniques. The structure of each has been determined using single-crystal diffraction data collected with MoKαX-ray radiation and an APEX CCD-based detector. The isotypic compounds (1) Cs2Ni3(H2O)4[(UO2)8(SeO4)4O8] and (2) Cs2Co3(H2O)4[(UO2)8(SeO4)4O8] are monoclinic, space group C2/c, with cell parameters of a 8.755(3), b 14.228(5), c 17.927(8) Å, β103.739(3)° and a 8.744(2), b 14.380(4), c 17.901(5) Å, β103.654(3)°, respectively. The structure of (3) Cs2(H2O)4[(UO2)4(SeO4)2O2(OH)2] is triclinic, space group P1̅ with cell parameters a 8.6877(4), b 8.7164(4), c 8.9589(8) Å, α95.964(10), β99.155(10), and γ117.318(10)°. The mixed selenate-selenite phase (4) Cs2(H2O)5[(UO2)7(SeO4)2(SeO3)2O4]. H2O is monoclinic, space group P21/m, with cell parameters a 9.1381(3), b 15.0098(5), c 15.1732(5) Å, β91.171(10)°. The compound (5) Li2(H2O)6[(UO2)3(SeO3)2O2] is monoclinic, with cell parameters P21/c, a 7.5213(9), b 7.0071(9), c 17.3282(19) Å, β98.834(2)°. (6) Cs2(H2O)2[(UO2)4(SeO3)5] is monoclinic, P21/n, with cell parameters a 10.913(3), b 12.427(3), c 18.448(4) Å, β90.393(3)°. Compounds 1, 2, and 3 contain sheets of uranyl pentagonal bipyramids and selenate tetrahedra with the zippeite-type topology. Compound 4 is a unique mixed selenate-selenite compound that contains sheets that are topologically intermediate between the zippeite-type and phosphuranylite-type topologies. Compound 5 contains a uranyl selenite sheet based on the phosphuranylite anion-topology, as found in the minerals marthozite and guilleminite. Compound 6 contains a novel sheet consisting of uranyl pentagonal and hexagonal bipyramids, as well as SeO3 pyramids.

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