First principles investigation of the structural and bonding properties of hydrated actinide (IV) oxalates, An(C2O4)2·6H2O (An = U, Pu)

Abstract

The structural and bonding properties of actinide (IV) oxalates, U(C2O4)2·6H2O and Pu(C2O4)2·6H2O, are investigated using the generalized gradient approximation (GGA) to spin-polarized density functional theory (DFT) with van der Waals corrections. The GGA optimized structures, ground state magnetic moments, site-projected density of states, and Bader charges are reported. We calculate the energy differences between ferromagnetic (FM) and antiferromagnetic (AFM) spin configurations on the Pu or U sites to determine the preferred magnetic structure of these materials. The relaxed AFM-spin structure of Pu(C2O4)2·6H2O was found to be considerably lower in energy than the corresponding relaxed FM-spin structure; whereas, there was negligible energy difference in the relaxed AFM and FM-spin structures of U(C2O4)2·6H2O. Weak hybridization between the actinide (Pu or U) 5f and O (2p) states in the site-projected density of states suggests that these systems are ionic. Furthermore, Bader charge analysis reveals charges on the actinide and oxalate oxygen sites in both U(C2O4)2·6H2O and Pu(C2O4)2·6H2O that are similar to literature data on other actinide species which are ionic, in particular the actinide dioxides. Calculating the density of states using a Hubbard correction parameter of U = 4.0 eV based on the GGA + U method shows band gaps of ∼1 eV for Pu(C2O4)2·6H2O and ∼3 eV for U(C2O4)2·6H2O. Both systems are predicted to be charge-transfer insulators.