Crystal chemistry of uranyl molybdates.: V.: Topologically distinct uranyl dimolybdate sheets in the structures of Na2[(UO2)(MoO4)2] and K2[(UO2)3O(MO4)2](H2O)

Two alkali uranyl dimolybdates, Na-2[(UO2)(MoO4)(2)] and K-2[(UO2)(MoO4)(2)](H2O), have been synthesized by high-temperature solid-state reactions and hydrothermal methods, respectively. The structures of both compounds were solved by direct methods and refined on the basis of F-2 for all unique data collected with monochromatic MoKalpha X-radiation and a CCD (charge-coupled device) detector. The structure of Na-2[(UO2)(MoO4)(2)] was refined to an agreement factor (RI) of 3.1%, calculated from 2089 unique reflections (\F-o\ greater than or equal to 4sigma(F)); it is orthorhombic, space group P2(1)2(1)2(1), a 7.2298(5), b 11.3240(8), c 12.0134(8) Angstrom, V 983.5(I) Angstrom(3), Z = 4. The structure of K-2[(UO2)(MoO4)(2)](H2O) was refined to an RI of 3.7%, calculated from 2181 unique reflections (F-o greater than or equal to 4sigma(F)); it is monoclinic, space group P2(1)/c, a 7.893(2), b 10.907(2), c 11558(3) Angstrom, beta 98.70(3)degrees, V 1153.8(4) Angstrom(3), Z = 4. Both structures are based upon sheets of uranyl pentagonal bipyramids and molybdate tetrahedra. The structural sheets in the two compounds are isochemical but topologically distinct.