First-principles study of phase stability, electronic and mechanical properties of plutonium sub-oxides

Evidence of vacancy ordered structures in PuO2-x and AmO2-x from first-principles calculations

A combination of first-principles calculations and cluster expansion method is used to study ordering of oxygen vacancies in PuO_2-x and AmO_2-x. Vacancy ordered stable/metastable structures of composition Pu₈O₁₅ (PuO_1.875), Pu₆O₁₁ (PuO_1.833), Pu₈O₁₄ (PuO_1.75) and Am₁₀O₁₉ (AmO_1.90), Am₈O₁₅ (AmO_1.875), Am₁₀O₁₈ (AmO_1.80), Am₈O₁₃ (AmO_1.625) are identified in PuO_2-x and AmO_2-x, respectively, from cluster expansion calculations. A comparison of formation enthalpies of vacancy ordered and vacancy disordered structures shows that Am₈O₁₅ (AmO_1.875) and Am₈O₁₃ (AmO_1.625) are more stable by 52 and 55 meV per atom, respectively, compared to their disordered counterparts. Similarly, vacancy ordered Pu₈O₁₅ (PuO_1.875) and Pu₈O₁₄ (PuO_1.75) structures are more stable compared to the disordered structures by 10 and 8 meV per atom, respectively. In contrast, the disordered PuO_1.625 structure is more stable compared to the cluster expansion generated structures. The vacancy ordered structures are mechanically stable and their bulk modulus, Young's modulus, shear modulus and Poisson's ratio are reported.

First-Principles Study on Mechanical and Optical Behavior of Plutonium Oxide under Typical Structural Phases and Vacancy Defects

The chemical corrosion aging of plutonium is a very important topic. It is easy to be corroded and produces oxidation products of various valence states because of its 5f electron orbit between local and non-local. On the one hand, the phase diagram of plutonium and oxygen is complex, so there is still not enough research on typical structural phases. On the other hand, most of the studies on plutonium oxide focus on PuO₂ and Pu₂O₃ with stoichiometric ratio, while the understanding of non-stoichiometric ratio, especially for Pu₂O_3-x, is not deep enough. Based on this, using the DFT + U theoretical scheme of density functional theory, we have systematically studied the structural stability, lattice parameters, electronic structure, mechanical and optical properties of six typical high temperature phases of β-Pu₂O₃, α-Pu₂O_3,γ-Pu₂O_3, PuO, α-PuO_2,γ-PuO₂. Further, the mechanical properties and optical behavior of Pu₂O_3-x under different oxygen vacancy concentrations are analyzed and discussed in detail. The result shows that the elasticity modulus of single crystal in mechanical properties is directly related to the oxygen/plutonium ratio and crystal system. As the number of oxygen vacancies increases, the mechanical constants continue to increase. In terms of optical properties, PuO has the best optical properties, and the light absorption rate decreases with the increase of oxygen vacancy concentration.

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

The β decay of ²⁴¹Pu to ²⁴¹Am results in a significant ingrowth of Am during the interim storage of PuO₂. Consequently, the safe storage of the large stockpiles of separated Pu requires an understanding of how this ingrowth affects the chemistry of PuO₂. 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 PuO₂ 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 PuO₂. 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.

The defect chemistry of non-stoichiometric PuO2±x

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 PuO2±x. The results show that the defect chemistry of PuO2 is dominated by oxygen vacancies and interstitials. Hypo-stoichiometric PuO2-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 (PuO2+x), but reluctance to form hyper-stoichiometric PuO2+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.

First-Principles Study of the Hydrogen Resistance Mechanism of PuO2

The in-depth investigation of hydrogen behaviors in Pu-oxide overlayers (mainly PuO₂ and α-Pu₂O₃) is critical for modeling the complex induction period of Pu hydriding. Within density functional theory (DFT) + U + D3 schemes, our systematic first-principles calculations and ab initio thermodynamic evaluations reveal that the hydrogen incorporation, dissolution behaviors, and diffusion mechanism in PuO₂ are quite different from those in α-Pu₂O₃, among which the highly endothermic incorporation and dissolution of hydrogen are the primary hydrogen resistance mechanism of PuO₂. Since its difficult recombination, atomic H is the preferred existence state in PuO₂, but H will recombine spontaneously in α-Pu₂O₃. In PuO₂, H diffusion is always clinging to O anions, whereas in α-Pu₂O₃, H₂ prefers to migrate along O vacancies with higher barriers. H dissolution in intact PuO₂ is very difficult, which can only be driven by extremely high pressure P_H2 and temperature. Based on a series of theoretical studies, we conclude that the main interactions between hydrogen and Pu-oxide overlayers are not involved with chemical reactions, and intact PuO₂ can effectively inhibit hydrogen permeation.

Structural, thermodynamic, electronic and elastic properties of Th1-xUxO2 and Th1-xPuxO2 mixed oxides

The structural, thermodynamic, electronic, and elastic properties of Th1-xUxO2 and Th1-xPuxO2 mixed oxides (MOX) have been calculated with Hubbard corrected density functional theory (DFT+U) to account for the strong 5f electron correlations. The ideal solid solution is approximated by special quasi-random structures and the U-ramping method is used to account for the presence of metastable states in the self-consistent field solution of the DFT+U approach. The mixing enthalpy (ΔHmix) is positive throughout the composition range of the Th1-xUxO2 MOX, consistent with a simple miscibility gap (at low temperature) phase diagram. The behavior of the Th1-xPuxO2 MOX is more complex, where ΔHmix is positive in the ThO2-rich region and negative in the PuO2-rich region. Electronic structure analysis shows that substitution of Th by U/Pu in ThO2 leads to a reduction of the average Th-O bond lengths, causing distortion in the crystal structure. The distortion in the crystal structure results in an increase in the conduction bandwidth and a reduction of the band-gap in the MOX. Good agreement of our DFT+U calculated elastic properties of ThO2, UO2 and PuO2 compounds with experiments leads to convincing prediction of these properties for Th1-xUxO2 and Th1-xPuxO2 MOX.