Theoretical probe into universal tetrad effect for complexation discrepancy between lanthanide elements and organophosphoric extractant diethyl phosphoric acid

The deep understanding of 4f-correlated electron motion behavior is experimentally limited due to their similar physicochemical properties of rare earth elements (REEs). While the solvent extraction behavior originated from the continuous occupation of 4f electrons along the lanthanide family provide a particular platform to probe the 4f electrons motion behavior and its correlations to their versatile functions. Herein, the complexation between REEs and the prototypical extractant of diethyl phosphoric acid (DEP) has been substantially studied based on quantum mechanical calculation. The results firstly show that the average bond lengths between Ln and six coordinated oxygen atoms are consistent with lanthanide contraction phenomenon, and gadolinium break effect is also obviously observed. Secondly, the "tetrad effect" is figured out based on the subtle discrepancy of 4f electrons occupation, namely La–Nd, Sm–Eu, Gd–Dy and Ho–Lu. Specifically, the main composition is 4f5d6p for La–Nd, while the discrepancy is due to the increasing electrons in the 5d shell, while it is 4f5d for Pm–Eu. For Gd–Dy, their 4f orbitals accept little electrons and the donating electrons flow into 6s5d subshells due to the half-filled stability and the enhancement of shielding effect. The electrons donate into 5d6p subshells due to multiply electrons occupation of 4f orbitals for Ho–Lu. These detailed analyses obviously clarify the "tetrad effect" along the lanthanide family due to the different variation of 4f orbital occupation. It is expected that these microscopic understanding provide important guidance for the separation of REEs and the design of new extractants.