Probing the Subtle Differences between Promethium and Curium

Abstract Curium (²⁴⁴Cm) from the plutonium (²³⁸Pu) waste stream at Oak Ridge National Laboratory is coextracted with promethium (¹⁴⁷Pm) because of similar chemical and physical properties. To gain insight for improving this separation, their fundamental properties are experimentally and computationally probed in a 2,2’:6’,2”-terpyridine crystal system. Analysis of the isostructural compounds via single crystal X-ray diffraction and quantum theory of atoms in molecules (QTAIM) revealed that although promethium (¹⁴⁷Pm) and ²⁴⁴Cm share the same predicted VI-coordinate ionic radius, they have minute structural differences. Although the differences between Pm and Cm are subtle in this crystal system, insights for potentially optimizing the separation between ¹⁴⁷Pm and ²⁴⁴Cm are gained. Furthermore, this study develops the chemistries of two rare elements in the solid state and experimentally portrays promethium’s often omitted position within the lanthanide series.