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Revision of the crystal structure of ulrichite, CaCu2+ (UO2)(PO4)2·4H2O

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The crystal structure of ulrichite, CaCu2+(UO2)(PO4)2·4H2O (space group P21/c, a = 12.784(3), b = 6.996(1), c = 13.007(3) Å,  = 91.92(1)°, V = 1162.7(4) Å3, Z = 4) was redetermined using X-ray diffraction data measured from a twinned crystal with Mo-Kα radiation and a CCD area detector (2510 unique reflections with Fo > 4(Fo), R1 = 8.8%). Ulrichite crystallizes in space group P21/c rather than C2/m reported previously. The newly determined atomic positions give reasonable coordination polyhedra. One unique Ca atom is irregularly coordinated by eight O atoms ( = 2.46 Å). One unique U atom shows a {2+5} coordination with characteristic bond angles and lengths (1.806(11) Å, 1.842(12) Åand five bonds between 2.252(15) and 2.441(11) Å). Furthermore, the structure contains groups in which strongly elongated CuO6 'octahedra' (also describable as CuO4 squares) are cornerlinked to two PO4 tetrahedra via two opposite, equatorial O atoms. Edge- and corner-sharing UO7, CaO8 and PO4 polyhedra form heteropolyhedral sheets parallel to (001) that are linked to adjacent sheets via the CuO6 'octahedra' and hydrogen bonds.
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Keywords: CRYSTAL STRUCTURE; PHOSPHATES; TWINNING; ULRICHITE; URANIUM

Document Type: Research Article

Publication date: 2001-12-01

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