Influence of Ga on H Reactivity with Ga-Stabilized δ-Pu Alloys

Abstract

The influence of Ga on the adsorption and solubility of H on Ga-stabilized δ-Pu alloys is investigated using density functional theory and experimentally via a Sieverts' apparatus. Allelectron DFT calculations, with spin−orbit coupling and full relaxations of the surface, are implemented to calculate H adsorption on δ-Pu−Ga alloy (111) surfaces. Chemisorption energies indicate that H prefers a 3-fold hollow site that binds strongly to a Pu-rich environment regardless of Ga concentration of the surface. Sites that include a Ga−H bond yield an increase in chemisorption energies as opposed to chemisorption energies when the H is only bonded to Pu atoms. Also, solution energies for interstitial sites are less exothermic than surface adsorption energies. Experimentally, H-vacancy binding is more exothermic than interstitial binding, and since vacancy binding is the analogue of surface binding, the DFT results are in agreement. Both DFT and experimental results show that with increasing Ga content to 7 at. % Ga there is an increase in H solution energies, thus reducing H solubility. The maximal reduction of H solubility is between 4 and 5 at. % Ga stabilized δ-Pu. The bonding nature of H on the Pu−Ga surface, via analysis of the partial density of states, shows weak hybridizations between the Pu 6d and H 1s states, with a slight decrease of the Pu 5f states near the Fermi level. Overall, Ga will affect the surface adsorption and solubility of H in δ-Pu−Ga alloys.

Quantum Accurate Prediction of Plutonium–Plutonium Dihydride Phase Equilibrium Using a Lattice Gas Model

“…In general, our simulations overpredict the hydrogen saturation of δ-Pu by a factor of 3–6 compared to experimental values. It is unclear whether the lack of gallium in our simulations contributes to or minimizes this discrepancy, as a recent report found that Pu–Ga alloys absorb hydrogen less exothermically than unalloyed Pu in contrast to earlier reports, which found that Pu–Ga have higher hydrogen solubility limits than Pu. , Alternatively, this discrepancy could be a consequence of our decision to limit H–H interactions in our model to nearest neighbor O sites. The binding of a H in one O site effectively blocks the 12 neighboring O sites from being occupied because of the high energy required to insert a hydrogen in these locations.…”

Ab Initio Study of the Effect of Mono-Vacancies on the Metastability of Ga-Stabilized δ-Pu

“…While there are some structural distortions due to the mono-vacancy, such as a monoclinic or tetragonal distortion, the electronic effects indicate a complex balance between the Pu 5f and 6d states. Due to the strong coupling of the Pu 5f and 6d states [50][51][52][53], in order for the Ga to stabilize the fcc matrix, hybridization of the Ga 4p and Pu 6d states must occur. With the introduction of a mono-vacancy, 5f-6d states decouple, thus intensifying the 5f states near the Fermi energy and broadening the 6d states, enabling hybridization with the Ga 4p states.…”

A first-principles study of hydrogen surface coverage on δ-Pu (100), (111), and (110) surfaces

“…Previous DFT studies of adsorption of a single H atom or H 2 molecule on clean (100) and (111) surfaces of δ-Pu have all been conducted at low coverage conditions. [3][4][5][6][11][12][13] High surface coverage will be expected when P H 2 P eq H 2 . The equilibrium pressure P eq H 2 at room temperature, has not been experimentally verified but was recently predicted from simulations to be O(10 −19 ) bar.…”