Open AccessEvolving Defect Chemistry of (Pu,Am)O2±x
Author(s) -
William D. Neilson,
Helen Steele,
Samuel T. Murphy
Publication year - 2021
Publication title -
journal of physical chemistry. c./journal of physical chemistry. c
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.401
H-Index - 289
eISSN - 1932-7455
pISSN - 1932-7447
DOI - 10.1021/acs.jpcc.1c03274
Subject(s) - valence (chemistry) , oxygen , chemistry , vacancy defect , density functional theory , oxidation state , frenkel defect , crystallographic defect , metal , crystallography , computational chemistry , organic chemistry
The β decay of 241 Pu to 241 Am results in a significant ingrowth of Am during the interim storage of PuO 2 . Consequently, the safe storage of the large stockpiles of separated Pu requires an understanding of how this ingrowth affects the chemistry of PuO 2 . This work combines density functional theory (DFT) defect energies and empirical potential calculations of vibrational entropies to create a point defect model to predict how the defect chemistry of PuO 2 evolves due to the incorporation of Am. The model predicts that Am occupies Pu sites in (Pu,Am)O 2± x in either the +III or +IV oxidation state. High temperatures, low oxygen-to-metal (O/M) ratios, or low Am concentrations favor Am in the +III oxidation state. Am (+III) exists in (Pu,Am)O 2± x as the negatively charged (Am Pu 1- ) defect, requiring charge compensation from holes in the valence band, thereby increasing the conductivity of the material compared to Am-free PuO 2 . Oxygen vacancies take over as the charge compensation mechanism at low O/M ratios. In (Pu,Am)O 2± x , hypo- and (negligible) hyperstoichiometry is found to be provided by the doubly charged oxygen vacancy (V O 2+ ) and singly charged oxygen interstitial (O i 1- ), respectively.