The defect chemistry of non-stoichiometric PuO2±x

Abstract

An increased knowledge of the chemistry of PuO₂ is imperative for the design of procedures to store, dispose, or make use of PuO₂. In this work, point defect concentrations in PuO₂ are determined by combining density functional theory (DFT) defect energies and empirical potential calculations of vibrational entropies. The obtained defect concentrations are expressed as a function of temperature and oxygen partial pressure and used to calculate non-stoichiometry in PuO_2±x. The results show that the defect chemistry of PuO₂ is dominated by oxygen vacancies and interstitials. Hypo-stoichiometric PuO_2−x is accommodated by both the uncharged oxygen vacancy and positively charged oxygen vacancy at small values of x, with increasingly dominant with increasing x. The negatively charged oxygen interstitial (O²⁻_i) is found to accommodate hyper-stoichiometry (PuO_2+x), but reluctance to form hyper-stoichiometric PuO_2+x is observed, with oxygen interstitials present only in very low concentrations irrespective of conditions. The small degree of hyper-stoichiometry found is favoured by low temperatures.