title: Multipole electron-density modelling of synchrotron powder diffraction data: the case of diamond.
abstract: Accurate structure factors are extracted from synchrotron powder diffraction data measured on crystalline diamond based on a novel multipole model division of overlapping reflection intensities. The approach limits the spherical-atom bias in structure factors extracted from overlapping powder data using conventional spherical-atom Rietveld refinement. The structure factors are subsequently used for multipole electron-density modelling, and both the structure factors and the derived density are compared with results from ab initio theoretical calculations. Overall, excellent agreement is obtained between experiment and theory, and the study therefore demonstrates that synchrotron powder diffraction can indeed provide accurate structure-factor values based on data measured in minutes with limited sample preparation. Thus, potential systematic errors such as extinction and twinning commonly encountered in single-crystal studies of small-unit-cell inorganic structures can be overcome with synchrotron powder diffraction. It is shown that the standard Hansen-Coppens multipole model is not flexible enough to fit the static theoretical structure factors, whereas fitting of thermally smeared structure factors has much lower residuals. If thermally smeared structure factors (experimental or theoretical) are fitted with a slightly wrong radial model (s(2)p(2) instead of sp(3)) the radial scaling parameters (kappa' parameters) are found to be inadequate and the ;error' is absorbed into the atomic displacement parameter. This directly exposes a correlation between electron density and thermal parameters even for a light atom such as carbon, and it also underlines that in organic systems proper deconvolution of thermal motion is important for obtaining correct static electron densities.
authors.affiliation: Center for Materials Crystallography, Department of Chemistry and iNANO, Aarhus University, DK-8000 Aarhus C, Denmark.
authors.forename: H
authors.initials: H
authors.lastname: Svendsen
authors.forename: J
authors.initials: J
authors.lastname: Overgaard
authors.forename: R
authors.initials: R
authors.lastname: Busselez
authors.forename: B
authors.initials: B
authors.lastname: Arnaud
authors.forename: P
authors.initials: P
authors.lastname: Rabiller
authors.forename: A
authors.initials: A
authors.lastname: Kurita
authors.forename: E
authors.initials: E
authors.lastname: Nishibori
authors.forename: M
authors.initials: M
authors.lastname: Sakata
authors.forename: M
authors.initials: M
authors.lastname: Takata
authors.forename: B B
authors.initials: BB
authors.lastname: Iversen