Refinement of the crystal structure of rutherfordine

Rutherfordine, UO2CO3, is orthorhombic, a 4.840(1), b 9.273(2), c 4.298(1) Angstrom, V 192.90(7) Angstrom(3), space group Imm2, Z = 2. The structure was refined to an R index of 2.2% on the basis of 306 unique data [\F-o\/sigma(\F-o\) > 5] measured with MoK alpha X-radiation on a single-crystal diffractometer. The structure consists of neutral sheets of edge- and corner-sharing (UO8) hexagonal bipyramids and (CO3) triangles, as originally proposed by Christ et al. (1955); our refinement, however, shows that (CO3) groups in alternate layers have the same orientation, not opposite orientations as originally reported. The refined value of the U-O(uranyl) distance is strongly affected by the details of the absorption correction, ranging from 1.71 to 1.80 a as a function of the plate-glancing angle used in an empirical psi-scan absorption correction and as a function of the type of weighting scheme used in the refinement. The Gaussian-quadrature method of integration also shows similar problems, but they are less extreme. The preferred value for the U-O(uranyl) distance in rutherfordine is similar to 1.745 Angstrom; as rutherfordine contains no H atoms, the O(uranyl) atom is [1]coordinated, and should have the shortest U-O(uranyl) distance stereochemically possible. The current work suggests that U-O(uranyl) values less than 1.745 Angstrom reported in other studies are adversely affected by less-than-optimum absorption corrections.