The crystal structure of Na4(UO2)(CO3)3 and its relationship to schrockingerite

Crystals of the compound Na-4(UO2)(CO3)(3) have been synthesized and the structure has been solved. It is trigonal with a = 9.3417(6), c = 12.824(1) Angstrom, V= 969.2(1) Angstrom (3), space group p (3) over bar cl and Z = 4. The structure was refined on the basis of F-2 (wR2 = 4.2%) for all unique data collected using Mo-K alpha X-radiation and a CCD-based detector. The final R1 was 2.0%, calculated for 534 unique observed (F-o greater than or equal to 4 sigma (F)) reflections, and the goodness-of-fit (S) was 0.91. The structure contains a uranyl tricarbonate cluster composed of a uranyl hexagonal bipyramid that shares three equatorial edges with CO3 triangles. The uranyl tricarbonate clusters are connected through NaO6 and NaO5 polyhedra, forming a heteropolyhedral framework structure. This compound may be related to a uranyl carbonate phase with the same composition which has been reported as an alteration phase on the surface of Chernobyl 'lava', and as a mineral in the Jachymov ore district, Czech Republic.