DFT Investigation of the Properties of Plutonium Dioxide Nanoparticles

The properties of plutonium dioxides (PuO₂) are the subject of widespread attention due in large part to it being the compound of choice used to handle and store Pu. Most computational studies of the material focus their attention on the bulk material, whilst others have studied the materials low-index surfaces. Whether the results of these studies are transferrable to samples of PuO₂ that have a high surface to volume ratio exists as an important, unanswered question. This question is posed as some facilities store PuO₂ as a powder and at a number of nuclear legacy sites, Pu has been found to migrate in subsurface environments in the form of nano-sized PuO₂ colloids.<br/>Here, we address this question by, for the first time, simulating PuO₂ nanoparticles (NPs) using density functional theory (DFT). First principles simulation of actinide NPs is computationally demanding, therefore, NPs with up to 245 atoms (equating to a particle diameter of ∼ 2.2 nm) are employed. Our NPs are constructed according to the predicted equilibrium shape (Wulff construction), which, for PuO₂, is found to be a crystal composed entirely of (111) facets. They are simulated using DFT + <i>U</i>, considering the influence of spin-orbit interaction (SOI) and non-collinear magnetic behaviour. The simulations predict that the Pu ions in the NPs are composed of Pu (IV), even when under-coordinated at the surface. A debate exists in the literature as to the nature of the Pu-O interaction in the first coordination sphere; our DFT simulation predicts that, in the first coordination sphere, there is only one Pu-O interaction, resulting in a NP structure very similar to bulk PuO₂. The density of state plots also resemble the plots of bulk PuO₂. With a DFT model of the PuO₂ NPs established, we are now in a position to make predictions about the aging characteristics of the material by modelling defects (including radiogenic impurities) and the interaction of the NPs with the surrounding environment (e.g. water).