The crystal structure of Na-4(UO2)(CO3)(3) and its relationship to schrockingerite

Crystals of the compound Na-4(UO2)(CO3)(3) have been synthesized and the st ructure has been solved. It is trigonal with a = 9.3417(6), c = 12.824(1) A ngstrom, 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 f inal R1 was 2.0%, calculated for 534 unique observed (F-o greater than or e qual 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 hexa gonal 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 relat ed to a uranyl carbonate phase with the same composition which has been rep orted as an alteration phase on the surface of Chernobyl 'lava', and as a m ineral in the Jachymov ore district, Czech Republic.