Chemical accuracy and precision in Rietveld analysis: The crystal structure of cobalt(II) acetate tetrahydrate

The crystal structure of cobalt(II) acetate tetrahydrate, Co(C2H4O2). 4H(2)O, has been refined using single-crystal, laboratory powder, and synchrotron powder diffraction data, both individually and in various combinations. The compound crystallizes in the monoclinic space group P2(1)/c, with a = 4.80688(3), b = 11.92012(7), c = 8.45992(5) Angstrom, beta = 94.3416(4)degrees at 27 degrees C, and Z = 2. The crystal structure consists of discrete centrosymmetric trans-Co(C2H3O2)(H2O)(4) complexes, linked by a three-dimensional network of hydrogen bonds. Each complex participates in 14 hydrogen bonds, 12 intermolecular, and 2 intramolecular. Compared to the single-crystal refinement, refinement of laboratory powder data yielded an average difference in bond distances of 0.02 Angstrom, in bond angles of 3 degrees, and in root mean square atomic displacements of 0.07 Angstrom. The standard uncertainties of the bond distances were 0.01 Angstrom, compared to the 0.001-0.002 Angstrom in the single-crystal refinement. Refinement of the synchrotron powder date yielded improved accuracy and precision. It proved impossible to locate or refine hydrogen positions using a single-powder dataset, but the hydrogens could be refined using rigid groups in a joint refinement of the two powder datasets. Even from powder refinements, it is possible to obtain suitable accuracy and precision to distinguish C-O and C=O bonds, and to examine details of chemical bonding. (C) 1997 International Centre for Diffraction Data.