Improvement of Single-Crystal Structures of Very Heavy Element Compounds by Refining Anomalous Dispersion Parameters.

Refining the anomalous dispersion parameters of the four uranium compounds NaUF 5 , NaU 2 F 9 , Cs 2 (UO 2 )TiO 4 , and Cs 2 (UO 2 )Ti 2 O 6 gave insights into the crystallographic model improvement of very heavy atoms. We found that the values for the dispersive and absorptive parts, f ' and f ″, closely followed the X-ray absorption spectra on their L 3 , L 2 , and L 1 edges. The obtained values are sensitive to the chemical environment at each crystallographically independent position. An incorrect treatment of the anomalous dispersion correction can lead to a wrong crystallographic model. The above-mentioned, already published structures were improved by this process. General guidelines were given for the crystal structure determination of very heavy compounds. When using Mo Kα radiation with uranium compounds, the proximity of its energy to the uranium L-edges causes a noticeable effect.