U(UO2)((PO4)2, A NEW MIXED-VALENCE URANIUM ORTHOPHOSPHATE - AB-INITIO STRUCTURE DETERMINATION FROM POWDER DIFFRACTION DATA AND OPTICAL AND X-RAY PHOTOELECTRO SPECTRA

A mixed-valence uranium phosphate, U(UO2)(PO4)2, has been synthesized through dry and wet chemical processes. The crystal structure has been solved ab initio from conventional monochromatic X-ray powder diffraction. The unit cell is triclinic (space group P1BAR, Z = 2) with the cell dimensions a = 8.8212(4) angstrom, b = 9.2173(5) angstrom, c = 5.4772(3) angstrom, alpha = 102.622(4)-degrees, beta = 97.748(4)-degrees, gamma = 102.459(5)-degrees, and V = 416.55(3) angstrom3. Initial positional parameters for two independent uranium atoms were obtained by solution of the Patterson map resulting from 509 integrated intensities. Phosphorus and oxygen atoms were localized by Fourier methods. The final indicators from the Rietveld refinement was R(F) = 0.039 and R(wp) = 0.144. The overall quality of the structure was improved from the refinement of atomic coordinates from neutron diffraction data (R(F) = 0.029, R(wp) = 0.057). The structure consists of PaCl5-type chains based on a [U(1)2-U(2)2]n sequence running along [110] through edge-sharing bipyramidal pentagons around each uranium atom. The identification of a linear uranyl entity (O-U-O)2+ with two terminal oxygens in the U(2) coordination sphere allows one to attribute a +6 oxidation state for U(2) and consequently U(1) should formally be called U(IV), in accordance with the green color of the sample. The remaining apices of each polyhedron are occupied by oxygen atoms from bidentate phosphate groups, which serve to connect the chains together, generating a complex tridimensional network. Moreover, the presence of two distinct valences for uranium atoms has been confirmed by additional chemical, optical, and X-ray photoelectron spectroscopy analyses. From redox reactions the ratio U(IV)/U(VI) was found to be one; absorption spectrum in the UV-visible range qualitatively revealed the presence of both oxidation states and the 4f7/2 peak of uranium observed by XPS has shown an asymmetric form which has been interpreted with two different binding energies arising from U(IV) and U(VI).