Separation mechanism of Am(III) from Eu(III) based on chemical bonding

Kaneko, Masashi   

Density functional theory (DFT) calculations have been employed to understand the equilibrium structures, electronic states, and stabilities of minor-actinides (MA) and lanthanides (Ln) complexes. An previous application of DFT calculation to the MA/Ln separation has indicated that the Am/Eu selectivity can be explained by the stability in complexation reaction. However, the origin of Am/Eu selectivity remains unclear. This study aims to understand the origin by correlating the stability of their complexes with the covalency in metal-ligand coordination bonds by means of scalar-relativistic DFT calculations. After an optimization of DFT method using benchmark study with Mssbauer spectroscopic parameters, we applied the DFT method to the separation of Am from Eu and analyzed the bonding states between metal and ligands in Eu and Am complexes. As a result, it was found that the wave functions of Eu ion with ligands have syn-phase overlap not depending on donor atoms, whereas those of Am ion have syn-phase overlap in the case of sulfur donors and anti-phase in the case of oxygen donors. This indicates that the difference in bonding states between metal and ligands is an origin in the Am/Eu selectivity.