Structure determination of uranium halogenide phosphates UXPO4 center dot 2H(2)O (X = Cl, Br) from powder diffraction data

The structure of the two isostructural uranium halogenide phosphates UClPO4 . 2H(2)O and UBrPO4 . 2H(2)O was determined from conventional monochromatic X-ray powder diffraction data, The crystal symmetry is tetragonal (space group I4/m, Z = 8) with lattice constants a = 14.631(2) and c = 6.662(1) Angstrom for the chloride phase and a = 14.7480(7) and c = 6.6810(4) Angstrom for the bromide compound, The structure solution was found from Patterson and Fourier syntheses and refinements by the Rietveld method (38 parameters, 760 reflections) converged to R-F = 0.03 for both examples, The configuration around uranium(IV) is derived from the typical pentagonal bipyramid formed by two water oxygen atoms, four phosphate oxygen atoms, and one halide atom, Due to steric considerations, the halide atom deviates from the pentagonal base, leading to a repulsion of the two apical oxygen atoms, The interatomic distances and angles are usual and agree with the values found from neutron diffraction in related compounds, The discrete UX1O6 (X = Cl, Br) polyhedra are linked together by phosphate tetrahedra which are viewed as the unique bonding units in the structure, This arrangement results in a 3D network based on two kinds of tunnels along [001]. The widest tunnel contains the X atoms, while half of the water molecules lie in the smallest tunnel. (C) 1997 Academic Press.