Molten salt flux synthesis and crystal structure of a new open-framework uranyl phosphate Cs3(UO2)2(PO4)O2: Spectroscopic characterization and cationic mobility studies

Abstract

The reaction of triuranyl diphosphate tetrahydrate precursor (UO2)3(PO4)2(H2O)4 with a CsI flux at 750°C yields a yellow single crystals of new compound Cs3(UO2)2(PO4)O2. The crystal structure (monoclinic, space group C2/c, a=13.6261 (13)Å, b=8.1081(8)Å, c=12.3983(12)Å, β=114.61(12)°, V=1245.41(20)Å3 with Z=4) has been solved using direct methods and Fourier difference techniques. A full-matrix least-squares refinement on the basis of F2 yielded R1=0.028 and wR2=0.071 for 79 parameters and 1352 independent reflections with I≥2σ(I) collected on a BRUKER AXS diffractometer with MoKα radiation and a charge-coupled device detector. The crystal structure is built by two independent uranium atoms in square bipyramidal coordination, connected by two opposite corners to form infinite chains [UO5]∞1 and by one phosphorus atom in a tetrahedral environment PO4. The two last entities [UO5]∞1 and PO4 are linked by sharing corners to form a three-dimensional structure presenting different types of channels occupied by Cs+ alkaline cations. Their mobility within the tunnels were studied between 280 and 800°C and compared with other tunneled uranyl minerals. The infrared spectrum shows a good agreement with the values inferred from the single crystal structure analysis of uranyl phosphate compound.