Crystal structure of uranium(IV) tetraiodide by x-ray and neutron diffraction

The crystal structure of uranium tetraiodide has been solved by a combination of x-ray and neutron diffraction techniques. UI4 is monoclinic, of space group C2/c, with a = 13.967 (6) A, b = 8.472 (4) A, c = 7.510 (3) A, and β = 90.54 (5)0. The uranium atom is at position 4(e) with y = 0.152 (2), and the iodine atoms are in general positions 8(f). There are two types of iodine atoms with I(1) at (0.123 (1), 0.118 (2), -0.086 (2)) and I(2) at (-0.134 (1), 0.382 (3), and 0.100 (2)). The structure, based on hexagonal close packing of the iodine atoms, consists of zigzag chains UI2I, with the terminal iodine atoms in cis positions. The U-I(1) (bridging) distances are 3.08 (2) and 3.11 (2) A, and the U-I(2) (terminal) distances are both 2.92 (2) A. The parameters were determined by the neutron powder profile refinement technique, and R = Σ(absolute value y0 - y/sub c/)/Σy0 = 0.13, R(expected) = ((NO - NV)/Σwy02)/sup 1/2/ = 0.11, and chi2 = Σw(y0 - y/sub c/)2/(NO - NV) = 1.7. UI4 is not isostructural with any other known MX4 halide and is the only actinide tetrahalide to have a chain structure, although chains are common in the transition-metal tetrahalide structures. 4 figures, 1 table