Polymorphism and phase transition in nanotubular uranyl phenylphosphonate:: (UO2)3(HO3PC6H5)2(O3PC6H5)2•H2O

The synthesis and crystal structure of a new nanotubular phenylphosphonate, epsilon-(UO2)(3)(HO3PC6H5)(2)(O-3- PC6H5)(2) . H2O, is reported. The compound crystallizes in the orthorhombic system, space group P2(1)2(1)2(1) with a = 16.8268(7) Angstrom, b = 7.1673(8) Angstrom, c = 28.8228(12) Angstrom, and Z = 4. The structure has been refined by the Rietveld method, and the final agreement factors were R(w)p = 13.1% and R-F = 10.1%. The structure of the epsilon-phase is very similar to that reported earlier for the delta-phase, and it consists of closed packed nanotubes. The main difference between delta- and epsilon-polymorphs is a slightly different interactions through the phenyl rings. The inside walls of these nanotubes are hydrophilic and the organic groups are projected outward, which results in hydrophobic regions between nanotubes. These nanotubes are held together only by van der Waals forces. The topotactic reversible phase transition between the two phases has been characterized by powder thermodiffractometry. P-31 MAS NMR data are also shown to display the structural relationship between both polymorphs.