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
Ac. Bean et al., 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, CHEM MATER, 13(4), 2001, pp. 1266-1272
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.