The Crystal Chemistry of Neptunium Compounds: Structural Relationships to U6+ Mineralogy

Our understanding of solid state neptunium chemistry is still in its infancy with only a handful of structures determined. This lack of knowledge poses a problem in regards to accurately describing and interpreting the physical and chemical properties of neptunium compounds. Pentavalent neptunium is of particular interest due to its importance in the long-term storage of commercial spent nuclear fuel in a geological repository and its potential release into the environment. The research presented herein focuses on the crystal chemistry of pentavalent neptunium that contains ligands of environmental importance. Hydrothermal synthesis techniques and single-crystal X-ray diffraction were used to study the structural details of 19 neptunium compounds. Several environmental significant anions were investigated, including sulfate, phosphate, oxygen, hydroxide, chloride, and silicate. The magnetic susceptibilities of five Np5+ compounds were measured and magnetic ordering was observed in compounds containing special linkages commonly referred to 'cation-cation' interactions. The nature of the magnetic ordering for Np5+ was found to be related to the structural details of the compound. Similarities in the crystal chemistry of pentavalent neptunium and hexavalent uranium has lead to the idea that the crystal chemistry of Np5+ will mirror that of U6+. Most notably both compounds contain a nearly linear dioxo cation that is further coordinated by four, five, or six equatorial ligands to create square, pentagonal, and hexagonal bipyramids. A structural hierarchy of U6+ crystal chemistry has been previously established as a means for comparison between structures with vastly different chemistries. A similar hierarchy has been constructed here for Np5+crystal chemistry, providing a tool to evaluate the structural topologies of Np5+and U6+ compounds. The difference in valency of the actinyl ion combined with a prevalence for cation-cation interactions in Np5+compounds causes a significant divergence in the crystal chemistries of Np5+and U6+ compounds.