Trigonal Na4[UO2(CO3)3]

Tetrasodium tricarbonatodioxouranate(VI), Na-4 [UO2(CO3)(3)], crystallizes in the trigonal space group P(3) over bar c1. Though the symmetry differs from other similar compounds (e.g. the NH4+, K+ and Tl+ salts) which are monoclinic, there is a common structure motif consisting of UO2 (CO3)3 groups with a trigonal outline when viewed along the shortest O-U-O bond pair. In Na-4[UO2 (CO3)(3)], there are three non-equivalent Na atoms; Na1 (site symmetry (3) over bar) and Na2 (site symmetry 3) are in centres of face-sharing octahedra, which form a chain running parallel to the c axis at each unit-cell corner, whereas the Na3 atom is surrounded by a deformed square pyramid of O atoms, forming edge-sharing triplets. The title compound has also a natural dimorph, namely the recently approved triclinic mineral cejkaite.