Spectroscopic Characterization of Tetravalent Berkelium

We report X-ray absorption spectroscopy (XAS) studies of tetravalent berkelium (Bk). Chelation of Bk^III by siderophore-inspired chelators stabilized Bk^IV 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 Bk^IV-O interatomic distances were determined by extended X-ray absorption fine structure measurements (EXAFS) to gain understanding into Bk^IV coordination behavior with organic ligands. These results pave the way for future studies of Bk^IV and other transplutonium compounds to investigate electronic structure and complexation behavior across the 5f series.