Contemporary Chemistry of Berkelium and Californium

Abstract

The merging of small‐scale syntheses and rapid crystallization methods have provided access to crystalline samples of berkelium (Z=97) and californium (Z=98) coordination complexes and compounds that can be interrogated with a suite of spectroscopic tools and structural elucidation approaches that have come online over the last 20 years. The combination of this experimental data with relativistic theoretical methods that capture the effects of spin‐orbit coupling and scalar relativistic effects have allowed us to understand the electronic structure of berkelium and californium compounds at a level of detail that was not previously possible. The harbinger of this new era of post‐curium chemistry was the synthesis and characterization of [Cf{B6O8(OH)5}]. This compound possesses a structure type that is distinct from earlier actinide borates, a reduction in coordination number for californium, contracted Cf−O bond lengths, a substantially reduced magnetic moment with respect to the calculated free‐ion moment and, most importantly, vibronically coupled broadband photoluminescence. Ligand‐field analysis also showed that the splitting of the ground state was larger than typically found in the f‐block elements, and when taken together places its overall electronic structure as a hybrid of d‐ and f‐block components. The discovery of the unusual properties of this compound has led to the development of large families of 4f and 5f coordination complexes, in an effort to uncover the underlying origin of the electronic structure oddities, and whether there really is a sharp onset of these changes at californium. This in turn pushed the development of far more challenging berkelium chemistry (from a radiologic standpoint) because the half‐life of the isotopes decreases from 351 years for 249Cf to 330 days for 249Bk. This short review details some of the chemistry that has been reported over the last 15 years, and its consequences for understanding the periodic table.

Probing electronic structure in berkelium and californium via an electron microscopy nanosampling approach

“…In particular, there is a growing body of evidence showing a pivot starting at Bk and Cf. Previous observations were made via solution thermodynamics experiments (complex formation constants and liquid–liquid extraction behavior), magnetic measurements, density functional theory (DFT) calculations, and structure determinations (extended X-ray absorption fine structure (EXAFS) and single crystal diffraction) with notably shorter-than-expected bond distances in the case of Cf compounds 5 , 10 , 11 , 40 , 50 .…”

“…any radioactive materials have unique properties and several of them play central roles in applications such as nuclear energy and cancer therapy [1][2][3][4][5] . However, the health hazards posed by radioactive elements have always been a major hurdle to their research.…”

“…Further, of all described actinide-containing crystal structures (~7000), only about 0.5% include elements with atomic numbers greater than that of Pu 6 . This imbalance can in part be explained by the relevance of the early actinides to nuclear power, space exploration, and nuclear weapons programs [1][2][3][4][5] , in part by the natural occurrence of thorium and uranium, on which much research has been focused. Another key factor is the extreme challenges introduced when working with the heavy actinides.…”

Thermodynamics of CeSiO4: Implications for Actinide Orthosilicates

“…Nonetheless, the tetravalent states of all the aforementioned elements are possible through either a transient state as a result of radiation-induced radiolysis [91][92][93] or a steady state being stabilized by either ligands or electrochemical methods 88,[94][95][96] . Each of these states, the transient state and steady state conditions, are achievable in the unlikely event of a failure of the geological repository and breach in radiological waste canister.…”

Covalency in Actinide Compounds

“…Covalency (and its opposite term, ionicity) often serves as an indication of bond strength in most classical non‐f‐electron systems. Although lanthanide cations are often considered as species that form predominantly ionic bonds with the ligand atoms, covalency in the actinide compounds is a much more complicated matter, which continues to fuel debates over its nature in the current scientific literature [5–7] . There is a growing body of evidence that 5f orbitals participate in covalent bonding with the surrounding ligand atoms, and this matter has subsequently evolved from the particular topic of separation chemistry to a more fundamental research direction [8] .…”