X-Ray Diffraction and μ-Raman Investigation of the Monoclinic-Orthorhombic Phase Transition in Th1-xUx(C2O4)2•2H2O Solid Solutions

A complete Th1-xUx(C2O4)(2)center dot 2H(2)O solid solution was prepared by mild hydrothermal synthesis from a mixture of hydrochloric solutions containing cations and oxalic acid. The crystal structure has been solved from twinned single crystals for x=0, 0.5, and 1 with monoclinic symmetry, space group C2/c, leading to unit cell parameters of a approximate to 10.5 A, b approximate to 8.5 angstrom, and c approximate to 9.6 angstrom. The crystal structure consists of a two-dimensional arrangement of actinide centers connected through bis-bidentate oxalate ions forming squares, The actinide metal is coordinated by eight oxygen atoms from four oxalate entities and two water oxygen atoms forming a bicapped square antiprism. The connection between the layers is assumed by hydrogen bonds between the water molecules and the oxygen of oxalate of an adjacent layer. Under these conditions, the unit cell contains two independent oxalate ions. From high-temperature mu-Raman and X-ray diffraction studies, the compounds were found to undergo a transition to an orthorhombic form (space group Ccca). The major differences in the structural arrangement concern the symmetry of uranium, which decreases from C2 to D2, leading to a unique oxalate group. Consequently, the nu(s)(C-O) double band observed in the Raman spectra recorded at room temperature turned into a singlet. This transformation was then used to make the phase transition temperature more precise as a function of the uranium content of the sample.