A computational study on the superionic behaviour of ThO2

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.

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.

Dependency of f states in fluorite-type XO2 (X = Ce, Th, U) on the stability and electronic state of doped transition metals

Transition metals (TMs) exhibit different quantum-mechanical oxidation state (OS_qm) population when doped into fluorite-type CeO₂, ThO₂ and UO₂.

A computational study of high pressure polymorphic transformations in monazite-type LaPO4

Polymorphic transformations in LaPO₄ are investigated as a function of pressure using density functional theory (DFT) based calculations under the generalized gradient approximation. The monazite-type (P2₁/n) → barite-type (Pbnm) structural transformation is identified at 16.2 GPa and experimentally, no transformation is observed near this pressure. A discontinuity in the pressure-volume relation (of 4.16% volume discontinuity compared to the monazite structure at the same pressure) and unit-cell dimensions is observed around 28 GPa, which matches well with the previous experimental results. The pressure of discontinuity matches the DFT calculated monazite-type (P2₁/n) → post barite-type (P2₁2₁2₁) structural transformation pressure. The equation of state, single crystal elastic constants and phonon dispersion curves of the different polymorphs as a function of pressure are determined. Both the barite-type (Pbnm) and post barite-type (P2₁2₁2₁) structures are mechanically and dynamically stable at 27 GPa indicating that the monazite-type (P2₁/n) → barite-type (Pbnm) phase transformation may be hindered by a kinetic barrier. The phase transformation in monazite-type LaPO₄ is driven by a softening of the C₂₅ single crystal elastic constant. Moreover, a small displacement and tilting of PO₄ tetrahedra as a function of pressure leads to a change in the La chemical environment and creates space for the construction of LaO₁₂ polyhedra from LaO₉ due to a phase transformation.

Phase stability, electronic structures and elastic properties of (U,Np)O2 and (Th,Np)O2 mixed oxides

Mixing enthalpies (ΔH_mix) of U_1−xNp_xO₂ and Th_1−xNp_xO₂ solid solutions are derived from atomic scale simulations based on density functional theory (DFT) employing the generalised gradient approximation corrected with an effective Hubbard parameter (U_eff).