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.

5f Covalency Synergistically Boosting Oxygen Evolution of UCoO4 Catalyst

Electronic structure modulation among multiple metal sites is key to the design of efficient catalysts. Most studies have focused on regulating 3d transition-metal active ions through other d-block metals, while few have utilized f-block metals. Herein, we report a new class of catalyst, namely, UCoO₄ with alternative CoO₆ and 5f-related UO₆ octahedra, as a unique example of a 5f-covalent compound that exhibits enhanced electrocatalytic oxygen evolution reaction (OER) activity because of the presence of the U 5f–O 2p–Co 3d network. UCoO₄ exhibits a low overpotential of 250 mV at 10 mA cm^–2, surpassing other unitary cobalt-based catalysts ever reported. X-ray absorption spectroscopy revealed that the Co²⁺ ion in pristine UCoO₄ was converted to high-valence Co^3+/4+, while U⁶⁺ remained unchanged during the OER, indicating that only Co was the active site. Density functional theory calculations demonstrated that the OER activity of Co^3+/4+ was synergistically enhanced by the covalent bonding of U⁶⁺-5f in the U 5f–O 2p–Co 3d network. This study opens new avenues for the realization of electronic structure manipulation via unique 5f involvement.

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.

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

The formation energies (ΔH_f) of fluorite PuO₂, α-Pu₂O₃ and sub-oxides PuO_2-x (0.0 < x < 0.5) are determined from atomic scale simulations based on density functional theory (DFT) employing the generalised gradient approximation (GGA) corrected with an effective Hubbard parameter (U_eff). The variation of structural and electronic properties of PuO₂ and α-Pu₂O₃ is determined while ramping up U_eff from 0 eV to 5 eV (U_eff-ramping method) to treat the presence of metastable magnetic states and to determine the most suitable U_eff value matching the experiments. The GGA+U calculated lattice parameter variation as a function of stoichiometry (a(x)) for PuO_2-x shows a positive volume of relaxation and an almost linear variation presented by the relation a(x) = a₀- 0.522738x, where a₀ is the equilibrium lattice parameter of PuO₂. The GGA+U calculated ΔH_f values of PuO_2-x lie above the tie line connecting the ΔH_f of PuO₂ and Pu₂O₃, and with decreasing O/Pu ratio, the stability of the sub-oxides increases. The crystal and electronic structure analysis of the oxygen vacancy in PuO₂ shows outward anisotropic relaxation of four Pu atoms around the vacancy site. The electronic charges within the Wigner-Seitz sphere around these Pu atoms show an overall gain of only (0.12-0.22)e per Pu atom, signifying an incomplete localization of charges. Finally, the GGA+U calculated single crystal elastic constant values decrease continuously with decreasing O/Pu ratio from 2.0 to 1.5. The rate of decrease of the average C₁₁ is almost 11-15 times higher compared to the rate of decrease of C₁₂ and C₄₄.

Magnetic structure of UO2and NpO2by first-principle methods

The magnetic structure of the actinide dioxides (AnO₂) remains a field of intense research. A noncollinear relativistic computational study of the AnO₂(An = U, Np) magnetic structure has been completed.