Crystal chemistry of uranyl molybdates; V, Topologically distinct uranyl dimolybdate sheets in the structures of Na (sub 2) [UO (sub 2) )(MoO (sub 4) ) (sub 2) ](H (sub 2) O)

Two alkali uranyl dimolybdates, Na (sub 2) [(UO (sub 2) )(MoO (sub 4) ) (sub 2) ] and K (sub 2) [(UO (sub 2) )(MoO (sub 4) ) (sub 2) ](H (sub 2) O), 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 (sub 2) for all unique data collected with monochromatic MoKalpha X-radiation and a CCD (charge-coupled device) detector. The structure of Na (sub 2) [(UO (sub 2) )(MoO (sub 4) ) (sub 2) ] was refined to an agreement factor (R1) of 3.1%, calculated from 2089 unique reflections (F (sub o) > or =4sigma (sub F) ); it is orthorhombic, space group P2 (sub 1) 2 (sub 1) 2 (sub 1) , a 7.2298(5), b 11.3240(8), c 12.0134(8) Aa, V 983.5(1) Aa (super 3) , Z = 4. The structure of K (sub 2) [(UO (sub 2) )(MoO (sub 4) ) (sub 2) ](H (sub 2) O) was refined to an R1 of 3.7%, calculated from 2181 unique reflections (F (sub o) > or =4sigma (sub F) ); it is monoclinic, space group P2 (sub 1) /c, a 7.893(2), b 10.907(2), c 13.558(3) Aa, beta 98.70(3) degrees , V 1153.8(4) Aa (super 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.