Crystal Structures of Three Framework Alkali Metal Uranyl Phosphate Hydrates

Three homeotypic hydrated alkali metal uranyl phosphates, A2(UO2)[(UO2)(PO4)]4(H2O)2, A=Cs (CsUP), Rb (RbUP), K (KUP), were synthesized by hydrothermal methods. Intensity data were collected at room temperature using MoKα radiation and a CCD-based area detector. Their crystal structures were solved by Patterson (CsUP) and direct (RbUP, KUP) methods and refined by full-matrix least-squares techniques to agreement indices (CsUP, RbUP, KUP) wR2=0.048, 0.230, 0.072 for all data, and R1=0.023, 0.078, 0.038 calculated for 5338, 4738, 4514 unique observed reflections (∣Fo∣≥4σF), respectively. The compound CsUP is orthorhombic, space group Cmc21, Z=4, a=14.854(1), b=13.879(1), c=12.987(1) A˚, V=2677.5(3) A˚3. Both RbUP and KUP are monoclinic, space group Cm, but are presented in the unconventional pseudo-orthorhombic space group Fm11 to facilitate comparison with CsUP and to allow a model for RbUP that includes the effects of pseudo-merohedral twinning. RbUP is monoclinic, space group Fm11, Z=4, a=15.72(2), b=13.84(1), c=13.05(1) A˚, α=90.39°(2), V=2839(5) A˚3; KUP is monoclinic, space group Fm11, Z=4, a=15.257(1), b=13.831(1), c=13.007(1) A˚, α=91.760°(1), V=2743.4(3) A˚3. The structures consist of sheets of phosphate tetrahedra and uranyl pentagonal bipyramids, with composition [(UO2)(PO4)]−, that are topologically identical to the uranyl silicate sheets in uranophane-beta. These sheets are connected by a uranyl pentagonal bipyramid in the interlayer that shares corners with two phosphate tetrahedra on each of two adjacent sheets and whose fifth equatorial vertex is an H2O group, resulting in an open framework with alkali metal cations in the larger cavities of the structures. Where CsUP and RbUP have two alkali metal positions and a H2O group in these cavities, KUP has four K atoms and two H2O groups, all of which are partially occupied, in the interstitial sites. [Copyright &y& Elsevier]