K2[(UO2)As2O7] -: the first uranium polyarsenate

Yellowish crystals of K-2[(UO2)AS(2)O(7)] (t) have been synthesized by solid-state reactions method. The structure of 1 [orthorhombic, Pmtnn, a = 12.601(2), b = 13.242(2), c = 5.621 (1) angstrom, V = 937.9(3)angstrom(3) Z = 4] has been solved by direct methods and refined to R-1 = 0.049, wR(2) = 0.1060 for 1059 observed reflections. The structure of I is based upon [(UO2)AS(2)O(7)](2-) sheets formed by corner sharing between [UO6](6-) distorted octahedra and [AS(2)O(7)](4-) polyarsenate groups. The K+ cations are either in eightfold or tenfold coordination and are located between the sheets. The topology of the uranyl arsenate sheet is related to silicate minerals of the melilite group and related synthetic silicate, aluminate and germanate compounds.