Structural relationships, interconversion, and optical properties of the uranyl iodates, UO2(IO3)(2) and UO2(IO3)(2)(H2O): A comparison of reaactionsunder mild and supercritical conditions

The uranyl iodates, UO2(IO3)(2) (AU1-8) and UO2(IO3)(2)(H2O) (AU2-8) have b een prepared from the reaction of UO3 with I2O5 under hydrothermal conditio ns. Equilibrium between AU1-8 and AU2-8 is established after 3 days at 400 degreesC, resulting in the isolation of an equimolar ratio of these compoun ds. Lowering of the, reaction temperature to 180 degreesC Or shortening of the reaction duration to 1 day allows for the isolation of AU2-8 in pure fo r ln. At 327 degreesC, AU2-8 undergoes dehydration and structural rearrange ment to AU1-8, which is a reversible process. Single-crystal X-ray diffract ion, bond valence sum calculations, EDX, DSC, TGA, and vibrational and fluo rescence spectroscopy have been used to characterize these compounds. Cryst allographic data are as follows: AU1-8, monoclinic, space group P2(1)/n, a = 4.2454(8) Angstrom, b = 16.636(5) Angstrom, c = 5.384(1) Angstrom, beta = 107.57(2)degrees, and Z = 2; AU2-8, orthorhombic. space group Pbcn, a = 8. 452(2) Angstrom, b = 7.797(2) Angstrom, c = 12.271(3), and Z = 4. The struc ture of AU1-8 is pseudo-three-dimensional with one-dimensional uranyl iodat e chains forming close I . . .O interchain contacts. Each one-dimensional c hain consists of edge-sharing UO8 hexagonal bipyramids and chelating iodate anions that also bridge the UO22+ units through mu (3)-O atoms. AU2-8 has a corrugated layered structure containing pentagonal bipyramidal UO7 units that are formed from the binding of four oxygen atoms fi om four bridging i odates and the coordination of a water molecule to the UO22+ moiety. Both o f these compounds fluoresce when irradiated with UV light (365 nm, yielding emission spectra characteristic of the uranyl dication.