An Overview on Crystal Chemistry of Actinide Borates

In recent five years, a series of new actinide (including thorium, uranium, neptunium, plutonium, americium, and curium) borate compounds have been synthesized using molten boric acid flux reactions. These compounds possess both unusual structures and unprecedented properties. Thorium borate provides the first example of a cationic supertetrahedral framework structure and displays remarkable anion exchange properties with high-selectivity for TcO^-₄. Uranium borates contain extraordinarily rich topological relationships with the majority crystallized in noncentrosymmetric space groups. Neptunium borates are often mixed-valent with the first example of compound containing three oxidation states of actinides, which provides a new advanced waste form to stabilize most early actinides with all possible oxidation states. Plutonium borates display new coordination chemistry for trivalent actinides. Americium and curium borate both show a dramatic departure from trivalent plutonium and lanthanide borates, which could potentially lead to new An(Ⅲ)/Ln(Ⅲ) and An(Ⅲ)/An(Ⅲ) separation strategies. There are still scant studies on the bonding of transplutonium element compounds up to now. Therefore, there are several grand challenges and importance that this work addresses. A deep understanding of material chemistry of these actinide borate compounds will likely lead to the development of waste disposal and radionuclide partitioning technologies.