The defect chemistry of non-stoichiometric PuO_2±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...

Evolving Defect Chemistry of (Pu,Am)O2±x

“…The CRG potential is a many-body potential model used to describe actinide oxide systems that achieves good reproduction of thermodynamic and mechanical properties. Previous work demonstrated that the phonon DOS for PuO 2 produced using the CRG potential compares reasonably well to the experimental data of Manley et al 2 , 45 The calculation of vibrational entropies, S vib , closely follows the approach described in refs ( 40 , 46 , 47 ), where using eq 1 , defect entropies are calculated from the normal vibrational frequencies, v n , which are themselves calculated by diagonalizing the dynamical matrix of the system In this formula, h is Planck’s constant, N is the number of atoms in the crystal, T is the temperature, and k B is the Boltzmann constant. In this study, the system to calculate vibrational entropies is a 4 × 4 × 4 expansion of the PuO 2 unit cell.…”

“… 15 The decision and justification for the selection of our U and J parameters are discussed in detail in our previous work, which also reports the equilibrium properties the HSE06 and PBEsol + U ( U = 7.0 eV) functionals attain simulating PuO 2 . 2 In summary, it was chosen to reproduce the HSE06 band gap to set U as the experimental data shows a large variation, and this functional has been proven to replicate experimental band gaps. 37 In the Supporting Information , we present a comparison of the projected densities of states (DOS) obtained using the PBEsol + U and HSE06 functionals as well as evidence that while the choice of U impacts the DOS, the impact to the DFT formation energy of a defect is minimal.…”

“…Defect vibrational entropies are found by calculating the difference in vibrational entropies between the defective and perfect supercell (Δ S vib ). The Δ S vib values calculated for the Am extrinsic defects are presented in Table 2 , and the Δ S vib values for the intrinsic defects remain the same as in ref ( 2 ). As the charges assigned to the ions in an empirical simulation are fixed, the same value of Δ S vib is given to all charge states of a given defect.…”

“…Several correction methods exist; it was previously found that the scheme developed by Kumagai and Oba 49 was very successful at accounting for finite size effects exhibited in PuO 2 , and therefore it is applied in this work. 2 The scheme of Kumagai and Oba is an extension of that developed by Freysoldt et al 50 and uses the atomic site electronic potentials of supercells with ( V defect,q ) and without ( V bulk ) defects to calculate the correction. The energy correction, E corr , for a defect with charge q is summarized following ref ( 49 ) as where Δ V PC,q/b is the potential difference between the defect induced potential V q/b and the point charge (PC) potential V PC,q .…”

“…1 Previous theoretical investigations of the defect chemistry of PuO 2 suggest that pure PuO 2 is very reluctant to form hyperstoichiometric PuO 2+ x . 2 However, under storage conditions, aged PuO 2 contains significant ingrowth of Am produced by 241 Pu decaying into 241 Am. Am builds up relatively quickly due to the short half-life of 241 Pu (14.4 years), with Am concentrations peaking after approximately 70 years, at which point, it too begins to decay faster than it is produced.…”