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. UI/sub 4/ is monoclinic, of space group C2/c, with a = 13.967 (6) A, b = 8.472 (4) A, c = 7.510 (3) A, and ..beta.. = 90.54 (5)/sup 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 UI/sub 2/I, 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 = ..sigma..(absolute value y/sub 0/ - y/sub c/)/..sigma..y/sub 0/ = 0.13, R(expected) = ((NO - NV)/..sigma..wy/sub 0//sup 2/)/sup 1/2/ = 0.11, and chi/sup 2/ = ..sigma..w(y/sub 0/ - y/sub c/)/sup 2//(NO - NV) = 1.7. UI/sub 4/ is not isostructural with any other known MX/sub 4/ 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.