Improvement\nof Single-Crystal Structures of Very Heavy\nElement Compounds by Refining Anomalous Dispersion Parameters

Refining the anomalous dispersion parameters of the four\nuranium\ncompounds NaUF<sub>5</sub>, NaU<sub>2</sub>F<sub>9</sub>, Cs<sub>2</sub>(UO<sub>2</sub>)TiO<sub>4</sub>, and Cs<sub>2</sub>(UO<sub>2</sub>)Ti<sub>2</sub>O<sub>6</sub> gave insights into the crystallographic\nmodel improvement of very heavy atoms. We found that the values for\nthe dispersive and absorptive parts, <i>f</i>′ and <i>f</i>″, closely followed the X-ray absorption spectra\non their L<sub>3</sub>, L<sub>2</sub>, and L<sub>1</sub> edges. The\nobtained values are sensitive to the chemical environment at each\ncrystallographically independent position. An incorrect treatment\nof the anomalous dispersion correction can lead to a wrong crystallographic\nmodel. The above-mentioned, already published structures were improved\nby this process. General guidelines were given for the crystal structure\ndetermination of very heavy compounds. When using Mo Kα radiation\nwith uranium compounds, the proximity of its energy to the uranium\nL-edges causes a noticeable effect.