Super Abundant Vacancies in fcc-Pu

Super abundant vacancies (SAVs) are phases where a material has an excess number of vacancies throughout the lattice with respect to the thermalvacancy concentration, driven by the presence of hydrogen in the lattice. High, ca. 10 at. %, concentration of metal vacancies are known to form in transition metal hydrides. Such hydrogen induced super abundant vacancy phases enhanced metal atom diffusion and creep. While SAVs are rarely observed in metallic phases, anomalous hydrogen solubility data has been interpreted to suggest SAVs form in the fcc phase of plutonium in the presence of 1-2 at. % dissolved H. To date, however, it is unclear if the SAV mechanism operates in fcc-Pu and whether SAVs form at low H chemical potentials before hydride formation occurs. Using density functional theory (DFT), we study vacancy formation and H adsorption in the noncollinear ground state of fcc-Pu. As part of this study, we establish a range of vacancy formation energies dependent on the magnetic and structural state surrounding the vacancy. We then use the DFT energetics to parameterize thermodynamic models as a means of establishing whether H atoms drive vacancy formation to obtain stable and metastable SAV phases in fcc-Pu. With these calculations and publically available datasets, we establish constraints on the physical conditions needed to thermodynamically stabilize SAVs in fcc-Pu. We find that, when H in the lattice is equilibrated with gaseous H2, the formation of SAVs in fcc-Pu is not thermodynamically possible and any excess vacancy concentration beyond thermal vacancies would need to occur by a different mechanism.