Experimental and Computational Characterization of the (NH4)4AnF8 (An = U, Np, Pu) Family

Actinide fluoride compounds have been integral to the development of historical and advanced nuclear technologies, with their unique chemistries well suited for gaseous enrichment, molten salt fuels, and chemical purification techniques. Thorough characterization of tetraammonium octafluoroneptunate and octafluoroplutonate single crystals by X-ray diffraction, absorption spectroscopy, and magnetic susceptibility analysis is presented herein with comparison to the previously characterized isostructural tetraammonium octafluorouranate. The isolated [AnF8]4- structures of the tetraammonium salts are well positioned as benchmark compounds for analysis of early 5f-electron bonding. Detailed theoretical treatment of the reported compounds was conducted by CASSCF, with calculations aimed at discerning the nature of the Np-F bond by the interacting quantum atoms method (IQA). These calculations indicated a slight increase in covalent character with decreasing ionic radius, driven by reduced overall bond energy.