Spectroscopic Characterization of Tetravalent Berkelium.

We report X-ray absorption spectroscopy (XAS) studies of tetravalent berkelium (Bk). Chelation of BkIII by siderophore-inspired chelators stabilized BkIV under mild conditions, as confirmed by absorption and luminescence spectroscopies and supported by surrogate experiments with the lanthanide counterpart to Bk, cerium (Ce). The Bk complexes remained, at least partially, in the tetravalent oxidation state under the high-energy X-ray beam despite previous attempts that had resulted in immediate in situ beam reduction, owing to cryogenic measurements on solid-state complexes. Computational calculations matched the experimental data, and BkIV-O interatomic distances were determined by extended X-ray absorption fine structure measurements (EXAFS) to gain understanding into BkIV coordination behavior with organic ligands. These results pave the way for future studies of BkIV and other transplutonium compounds to investigate electronic structure and complexation behavior across the 5f series.