DFT + U as a reliable method for efficient ab initio calculations of nuclear materials

Synthesis and properties of anhydrous rare-earth phosphates, monazite and xenotime: a review

The synthesis methods, crystal structures, and properties of anhydrous monazite and xenotime (REPO4) crystalline materials are summarized within this review. For both monazite and xenotime, currently available Inorganic Crystal Structure Database data were used to study the effects of incorporating different RE cations on the unit cell parameters, cell volumes, densities, and bond lengths. Domains of monazite-type and xenotime-type structures and other AXO4 compounds (A = RE; X = P, As, V) are discussed with respect to cation sizes. Reported chemical and radiation durabilities are summarized. Different synthesis conditions and chemicals used for single crystals and polycrystalline powders, as well as first-principles calculations of the structures and thermophysical properties of these minerals are also provided.

First-principles study of the crystal and magnetic structures of multiferroic Cu2OCl2

Recently, the discovery of multiferroicity in pyrochlore-like compound Cu₂OCl₂has generated significant interest, and several studies have been performed in this area. This transition metal oxychloride is unique because the divalent copper atoms create anS=1/2correlated insulator and the pyrochlore lattice tends to frustrate spins. From neutron powder diffraction measurements, an incommensurate magnetic order of the ordering vectorq=(0.827,0,0)emerges below the Néel temperature of 70 K. At this temperature or slightly above, ferroelectricity (FE) or antiferroelectricity, accompanying a lattice distortion, has been observed. Experimentally, some discrepancies remain. In this paper, we report our first-principles simulation results by evaluating the possible lattice and spin spiral states. We found that theFdddstructure is not more stable thanFdd2(a), which is supported by our reexamination of the x-ray diffraction data. In addition, we find that after we include magnetism in the calculation, it predicts that theFdd2(a)lattice with a helical (proper screw) spin structure is energetically more stable than other spin configurations. Our results indicate charge-order-driven FE that subsequently induces magnetism.

Pyrochlore Compounds From Atomistic Simulations

Pyrochlore compounds (A 2 B 2O7) have a large applicability in various branches of science and technology. These materials are considered for use as effective ionic conductors for solid state batteries or as matrices for immobilization of actinide elements, amongst many other applications. In this contribution we discuss the simulation-based effort made in the Institute of Energy and Climate Research at Forschungszentrum Jülich and partner institutions regarding reliable computation of properties of pyrochlore and defect fluorite compounds. In the scope of this contribution, we focus on the investigation of dopant incorporation, defect formation and anion migration, as well as understanding of order-disorder transitions in these compounds. We present new, accurate simulated data on incorporation of U, Np, Pu, Am and Cm actinide elements into pyrochlores, activation energies for oxygen migration and radiation damage-induced structural changes in these materials. All the discussed simulation results are combined with available experimental data to provide a reliable description of properties of investigated materials. We demonstrate that a synergy of computed and experimental data leads to a superior characterization of pyrochlores, which could not be easily achieved by either of these methods when applied separately.

Interplay between London Dispersion, Hubbard U, and Metastable States for Uranium Compounds

High-throughput computational studies of lanthanide and actinide chemistry with density-functional theory are complicated by the need for Hubbard U corrections, which ensure localization of the f-electrons, but can lead to metastable states. This work presents a systematic investigation of the effects of both Hubbard U value and metastable states on the predicted structural and thermodynamic properties of four uranium compounds central to the field of nuclear fuels: UC, UN, UO₂, and UCl₃. We also assess the impact of the exchange-hole dipole moment (XDM) dispersion correction on the computed properties. Overall, the choice of Hubbard U value and inclusion of a dispersion correction cause larger variations in the computed geometric properties than result from metastable states. The weak dependence of structure optimization on metastable states should simplify future high-throughput calculations on actinides. Conversely, addition of the dispersion correction is found to offset the repulsion introduced by the Hubbard U term and provides greatly improved agreement with experiment for both cell volumes and heats of formation. The XDM dispersion correction is largely invariant to the chosen U value, making it a robust dispersion correction for actinide systems.

Americium incorporation into studtite: a theoretical and experimental study

Studtite, [UO₂(η²-O₂)(H₂O)₂]·2H₂O, and metastudtite, [UO₂(η²-O₂)(H₂O)₂], are important phase alterations of UO₂ in a spent nuclear fuel repository and have previously been shown to react with Np(v). In this work we extend the study to Am(v) on a tracer scale and show spectroscopic evidence that the Am is incorporated into the structure of studtite as Am(iii). A computational study on the possible mechanisms for the incorporation of Np and Am shows that protonation of the -yl oxygen is the favoured route and the calculated incorporation energies are large and positive. The results suggest that Am is less favoured compared to Np but energetically more favoured to incorporate both actinide ions into metastudtite rather than studtite. Finally, we have shown that once incorporated, Am readily leaches into water but spectroscopic measurements suggest subtle changes in the structure of studtite.

Rare-Earth Orthophosphates From Atomistic Simulations

Lanthanide phosphates (LnPO₄) are considered as a potential nuclear waste form for immobilization of Pu and minor actinides (Np, Am, and Cm). In that respect, in the recent years we have applied advanced atomistic simulation methods to investigate various properties of these materials on the atomic scale. In particular, we computed several structural, thermochemical, thermodynamic and radiation damage related parameters. From a theoretical point of view, these materials turn out to be excellent systems for testing quantum mechanics-based computational methods for strongly correlated electronic systems. On the other hand, by conducting joint atomistic modeling and experimental research, we have been able to obtain enhanced understanding of the properties of lanthanide phosphates. Here we discuss joint initiatives directed at understanding the thermodynamically driven long-term performance of these materials, including long-term stability of solid solutions with actinides and studies of structural incorporation of f elements into these materials. In particular, we discuss the maximum load of Pu into the lanthanide-phosphate monazites. We also address the importance of our results for applications of lanthanide-phosphates beyond nuclear waste applications, in particular the monazite-xenotime systems in geothermometry. For this we have derived a state-of-the-art model of monazite-xenotime solubilities. Last but not least, we discuss the advantage of usage of atomistic simulations and the modern computational facilities for understanding of behavior of nuclear waste-related materials.

Optical and Chemical Characterization of Uranium Dioxide (UO2) and Uraninite Mineral: Calculation of the Fundamental Optical Constants

Uranium dioxide (UO₂) is a material with historical and emerging applications in numerous areas such as photonics, nuclear energy, and aerospace electronics. While often grown synthetically as single-crystal UO₂, the mineralogical form of UO₂ called uraninite is of interest as a precursor to various chemical processes involving uranium-bearing chemicals. Here, we investigate the optical and chemical properties of a series of three UO₂ specimens: synthetic single-crystal UO₂, uraninite ore of relatively high purity, and massive uraninite mineral containing numerous impurities. An optical technique called single-angle reflectance spectroscopy was used to derive the optical constants n and k of these uranium specimens by measuring the specular reflectance spectra of a polished surface across the mid- and far-infrared spectral domains (ca. 7000-50 cm^-1). X-ray diffractometry, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were further used to analyze the surface composition of the mineralogical forms of UO₂. Most notably, the massive uraninite mineral was observed to contain significant deposits of calcite and quartz in addition to UO₂ (as well as other metal oxides and radioactive decay products). Knowledge of the infrared optical constants for this series of uranium chemicals facilitates nondestructive, noncontact detection of UO₂ under a variety of conditions.

A Spectroscopic and Computational Study of Cm3+ Incorporation in Lanthanide Phosphate Rhabdophane (LnPO4·0.67H2O) and Monazite (LnPO4)

This study investigates the incorporation of the minor actinide curium (Cm³⁺) in a series of synthetic La_{1- x}Gd _xPO₄ ( x = 0, 0.24, 0.54, 0.83, 1) monazite and rhabdophane solid-solutions. To obtain information on the incorporation process on the molecular scale and to understand the distribution of the dopant in the synthetic phosphate phases, combined time-resolved laser fluorescence spectroscopy and X-ray absorption fine structure spectroscopy investigations were conducted and complemented with ab initio atomistic simulations. We found that Cm³⁺ is incorporated in the monazite endmembers (LaPO₄ and GdPO₄) on one specific, highly ordered lattice site. The intermediate solid-solutions, however, display increasing disorder around the Cm³⁺ dopant as a result of random variations in nearest neighbor distances. In hydrated rhabdophane, and especially its La-rich solid-solutions, Cm³⁺ is preferentially incorporated on nonhydrated lattice sites. This site occupancy is not in agreement with the hydrated rhabdophane structure, where two-thirds of the lattice sites are associated with water of hydration (LnPO₄·0.67H₂O), implying that structural substitution reactions cannot be predicted based on the structure of the host matrix only.

Trends in the valence band electronic structures of mixed uranium oxides

The valence band electronic structures of mixed uranium oxides (UO₂, U₄O₉, U₃O₇, U₃O₈, and β-UO₃) have been studied using the resonant inelastic X-ray scattering (RIXS) technique at the U M₅ edge and computational methods.

New insights into phosphate based materials for the immobilisation of actinides

This paper focuses on major phosphate-based ceramic materials relevant for the immobilisation of Pu, minor actinides, fission and activation products. Key points addressed include the recent progress regarding synthesis methods, the formation of solid solutions by structural incorporation of actinides or their non-radioactive surrogates and waste form fabrication by advanced sintering techniques. Particular attention is paid to the properties that govern the long-term stability of the waste forms under conditions relevant to geological disposal. The paper highlights the benefits gained from synergies of state-of-the-art experimental approaches and advanced atomistic modeling tools for addressing properties and stability of f-element-bearing phosphate materials. In conclusion, this article provides a perspective on the recent advancements in the understanding of phosphate based ceramics and their properties with respect to their application as nuclear waste forms.