A neutron diffraction study of uranyl nitrate hexahydrate

Abstract

The crystal structure of uranyl nitrate hexahydrate has been determined by neutron diffraction. The structure is orthorhombic with a = 13.197 + 0.003, b = 8-035 + 0.002, c = 11.467 + 0-003 /~, space group Cmc21. Intensities of 1136 independent reflections with a 2-0< 105 ° were obt~ned with 2= 1065 A. The structure was solved from the three-dimensional neutron Patterson and Fourier syntheses, and refined by a full-matrix least squares, using a statistical weighting scheme. The final weighted R index was 3.9 Vo. The uranyl group is surrounded equatorially by a near-planar oxygen hexagon of four oxygen atoms from two non-equivalent bidentate nitrate groups and two equivalent water oxygens. In the nitrate groups, the N-O bonds involving coordinated oxygens are 0.03--0.05/~ longer than the N-O bonds involving the non-coordinated oxygen atoms; also the O-N-O angles differ significantly from 120 °. All hydrogens are involved in hydrogen bonding and these may be divided into (a) O (water) O (water) hydrogen bonds of length 268-275 A and (b) weaker O (water)O (nitrate)hydrogen bonds of length 2.93 and 2.99/1,. The water molecules associate into sheets perpendicular to the a axis.