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

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 MoK  α 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.

Arrhenius plot of the electrical conductivity of tunneled compounds Cs3U2PO10 and CsU2Nb2O11.5. Figure optionsDownload as PowerPoint slideHighlights
► The reaction of (UO2)3(PO4)2(H2O)4 in excess of molten CsI leads to single-crystals of new tunneled compound Cs3(UO2)2(PO4)O2.
► Ionic conductivity measurements and crystal structure analysis indicate a strong connection of the Cs+ cations to the tunnels.
► A low symmetry in Cs3(UO2)2(PO4)O2 is the cause of IR activation and splitting of the bands in the IR spectrum.