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 NaUF5, NaU2F9, Cs2(UO2)TiO4, and Cs2(UO2)Ti2O6 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 L3, L2, and L1 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.