Crystallographic Evidence for Formation of M₆O₈/M₆O₉ f-Element Clusters in Hydrolysis Reactions

Understanding the formation of f-block element colloids via hydrolysis reactions, which form actinide oxo clusters, has implications in the nuclear fuel cycle, especially in high-level radioactive nuclear waste storage management. Recently, the chemistry of these species has gained attention in the development of new nuclear waste remediation technologies due to the fact that oxo and oxo–hydroxo clusters are prominent structural models for the actinides. In this contribution, we have conducted an examination of crystallographic data which includes 184 4f- and 5f-structures containing the M6O8 or M6O9 moiety. Since much of the work in this field has actually been done to explore the formation and properties of metal organic frameworks (MOFs), we hope that this collection can be used by actinide scientists to further investigate structural and mechanistic trends and gain insight into the formation of these compounds. Here we discuss the general trends in coordination, ligands used, and bonding within the M6O8 or M6O9 core in the hope that further computational work can benefit. We recognize, however, that the available crystallographic data needs a lot of improvement. In addition, use of crystallographic software (e.g., SQUEEZE) to essentially hide the rather smeared out electron density in the refinement can improve the crystallographic refinement parameters obtained, but it leads to a lot of possible interpretations of the overall structure, including charge balance. This does not bode well for predicting the exact phases which may solidify from complex nuclear waste with a variety of anions and decomposition products present, and much more study using advanced analytical techniques beyond crystallography is warranted.