Ragnvald H Mathiesen | PubMed Health

PubMed2005

Three-beam resonant X-ray diffraction in germanium - Laue transmission cases.

Perturbation of the two-beam diffracted power owing to the influence of a third lattice node has been examined for various three-beam cases in a small finite germanium crystal in the vicinity of the K-absorption edge. Although the crystal was slightly imperfect, the main parts of the experimental results are very well described within the framework of the fundamental theory of X-ray diffraction in conjunction with Cromer-Liberman calculations for the resonant scattering terms. Beam divergence and dynamical block size are treated as adjustable parameters in the analysis. Observed changes in the three-beam profile asymmetry are mainly attributed to size and not to resonance effects associated with the triplet phase sum of the involved reflections. Close to the absorption edge there is however some evidence indicating that f' values should be reduced in magnitude compared to the tabulated ones.

Journal Article↗

PubMed2004

The charge density of urea from synchrotron diffraction data.

The charge density of urea is studied using very high precision single-crystal synchrotron-radiation diffraction data collected at the Swiss-Norwegian Beam Lines at ESRF. An unprecedented resolution of 1.44 A(-1) in sin theta;/lambda is obtained at 123 K. The optimization of the experiment for charge-density studies is discussed. The high precision of the data allowed the refinement of a multipole model extending to hexadecapoles and quadrupoles on the heavy and H atoms, respectively, as well as a liberal treatment of radial functions. The topological properties of the resulting electron density are analysed and compared with earlier experimental results as well as with periodic Hartree-Fock calculations. The properties of the strongly polarized C-O bond agree with trends derived from previous experimental results while the ab initio calculations differ significantly. The results indicate that the description of the C-O bond requires more flexible basis sets in the theoretical calculations. The calculated integrated atomic charges are much larger than the observed ones. It is suggested that the present experimental results provide new target values for validation of future ab initio calculations. The molecular dipole moment derived from the integrated atomic properties is the same as the one obtained from the multipole model even though the individual atomic contributions differ. Comparison with literature data for urea in solution and the gas phase yields a dipole enhancement in the solid of about 1.5 D. The thermal expansion of urea is determined using synchrotron powder diffraction data. With decreasing temperature, an increasing anisotropic strain is observed.

Journal Article↗