A NEW RARE-EARTH-ELEMENT URANYL CARBONATE SHEET IN THE STRUCTURE OF BIJVOETITE-(Y)

The crystal structure of bijvoetite-(Y), [M83+(H2O)25(UO2)16O8(OH)8(CO3)16](H2O)14, M = (Y, REE), pseudo-orthorhombic, monoclinic, a 21.234(3), b 12.958(2), c 44.911(6) Å, b 90.00(2)°, V 12,357(6) Å3, space group B1211, Z = 4, was solved by direct methods and refined by least-squares techniques to an agreement index (R) of 8.4 % and a goodness of fit (S) of 0.81 for 6622 unique observed (|Fo |> 4sF) reflections collected for a twinned crystal using graphite-monochromatic MoKa X-radiation and a CCD area detector. There are 16 symmetrically independent U6+ positions, each of which is part of near-linear (U6+O2)2+ uranyl ion. Eight uranyl ions are coordinated by three O2 – and two (OH) – anions each, resulting in uranyl pentagonal bipyramids, and eight uranyl ions are coordinated by six O2 – anions each, forming uranyl hexagonal bipyramids. The structure contains 16 unique carbonate groups and eight unique M3+ sites that are occupied by variable amounts of Y, Dy and other REEs. The structure of bijvoetite-(Y) is based on a novel uranyl carbonate chain that is parallel to [100], involving edge-sharing dimers of uranyl pen-tagonal bipyramids, edge-sharing dimers of uranyl hexagonal bipyramids, and carbonate groups. The chains are cross-linked by irregular M3+fn (f: unspecified ligand) polyhedra, forming (Y,REE)-bearing uranyl carbonate sheets that are parallel to (010). The sheet in bijvoetite-(Y) is based upon a new sheet anion-topology. There are 39 symmetrically unique H2O groups in the structure, 25 of which are bonded to M3+ and 14 of which are located in the interlayer, where they are held in place by H bonds. The interlayer of the structure contains only H2O groups, and adjacent sheets are connected by H bonds only. The discovery of th