Structural evolution and stability of plutonium oxide clusters

Plutonium oxide clusters have attracted great interest as potential complex for the separation or storage of radioactive plutonium elements. However, the structural stability, chemical bonding mechanism and maximum oxygen adsorption capacity for plutonium oxygen clusters are not well understood due to the difference between the radial distribution function and orbital energy of the plutonium atom. Here, we systematically study the structural evolution and electronic properties of plutonium oxygen clusters with cluster sizes n from 2 to 15 by using the CALYPSO cluster structural prediction method in combination with density functional theory (DFT) calculations. The low-lying isomers searched by the CALYPSO method are re-optimised at the theoretical level of B3LYP/ECP60MWB(Pu)/aug-cc-pVTZ(O). Relative stability results indicate that the PuO₈ cluster with C_S symmetry is the most stable cluster due to the large HOMO–LUMO gap (of 4.84 eV). The high stability of PuO₈ cluster is predominantly attributed to the strong interactions between Pu-5f orbitals and O-2p orbitals. The Pu atom can chemically adsorb up to eight O atoms, and the corresponding adsorption energy is −3.84 eV. The present findings shed light on the complex chemical bonding and structural evolution mechanisms of plutonium oxide clusters, which may facilitate the rational design and the synthesis of other actinide-oxygen clusters. Plutonium chemically adsorbs eight oxygen atoms, and its high stability is attributed to the interactions between Pu-5f and O-2p orbitals.