Darstellung, gitterkonstanten und chemische eigenschaften einiger ternärer oxide des plutoniums, americiums und curiums vom typ MeIIIXVO4

Investigation of Radiation Damage in the Monazite-Type Solid Solution La1-xCexPO4

Crystalline materials such as monazite have been considered for the storage of radionuclides due to their favorable radiation stability. Understanding their structural chemical response to radiation damage as solid solutions is a key component of determining their suitability for radionuclide immobilization. Herein, high-resolution structural studies were performed on ceramics of the monazite solid solution La1-xCexPO4 (x = 0.25, 0.5, 0.75, 1) in order to understand the role of structural chemistry on irradiation stability. Ceramic samples were irradiated with 14 MeV Au ions with 1014 ions/cm2 and 1015 ions/cm2 to simulate the recoil of daughter nuclei from the alpha decay of actinide radionuclides. The extent of radiation damage was analyzed in detail using scanning electron microscopy (SEM), Raman spectroscopy, grazing incidence X-ray diffraction (GI-XRD), and high-energy-resolution fluorescence detection extended X-ray absorption fine structure (HERFD-EXAFS) spectroscopy. SEM and Raman spectroscopy revealed extensive structural damage as well as the importance of grain boundary regions, which appear to impede the propagation of defects. Both radiation-induced amorphization and recrystallization were studied by GI-XRD, highlighting the ability of monazite to remain crystalline at high fluences throughout the solid solution. Both, diffraction and HERFD-EXAFS experiments show that while atomic disorder is increased in irradiated samples compared to pristine ceramics, the short-range order was found to be largely preserved, facilitating recrystallization. However, the extent of recrystallization was found to be dependent on the solid solution composition. Particularly, the samples with uneven ratios of solute cations, La0.75Ce0.25PO4 and La0.25Ce0.75PO4 were observed to exhibit the least apparent radiation damage resistance. The findings of this work are discussed in the context of the monazite solid solution chemistry and their appropriateness for radionuclide immobilization.

Incorporation of U(IV) in monazite-cheralite ceramics under oxidizing and inert atmospheres

This work is the first attempt to prepare Nd1-xCaxUxPO4 monazite-cheralite with 0 < x ≤ 0.1 by a wet chemistry method. This method relies on the precipitation under hydrothermal conditions (T = 110 °C for four days) of the Nd1-xCaxUxPO4·nH2O rhabdophane precursor, followed by its thermal conversion for 6 h at 1100 °C in air or Ar atmosphere. The optimized synthesis protocol led to the incorporation of U and Ca in the rhabdophane structure. After heating at 1100 °C for 6 h in air, single-phase monazite-cheralite samples were obtained. However, α-UP2O7 was identified as a secondary minor phase in the samples heated under Ar atmosphere. The U speciation in the samples converted in an oxidising atmosphere was carefully characterized using synchrotron radiation by combining HERFD-XANES and XRD. These results showed the presence of a minor secondary phase containing hexavalent uranium and phosphate with a stoichiometry of U : P = 0.78. This highly labile uranyl phosphate phase incorporated 21 mol% of the uranium initially precipitated with the rhabdophane precursor. This phase was completely removed by a washing protocol. Thus, single-phase monazite-cheralite was obtained through the wet chemistry route described in this work with a maximum U loading of x = 0.08.

Periodic trends in trivalent actinide halides, phosphates, and arsenates

Due to the limited abundance of the actinide elements, computational methods, for now, remain an exclusive avenue to investigate the periodic trends across the actinide series. As every actinide element can exhibit a +3-oxidation state, we have explored model systems of gas-phase actinide trihalides, phosphates, and arsenates across the series to capture the periodic trends. By doing so, we were able to capture the periodic trends down the halogen series as well, and for the first time we are reporting a study on actinide astatides. Using scalar and spin-orbit relativistic Density Functional Theory (DFT) calculations, we have explored the variations in bond lengths, bond angles, and the charges on actinides (An). Despite the use of different sets of ligands, the trends remain similar. The properties of trivalent Pa, U, Np, and Pu are nearly identical; similar ionic radii could be the reason. The actinide elements show a tendency to exhibit a pre-Pu and a post-Cm behaviour, with Am acting as a switch. This could be due to the change in the behaviour from d-f-type to f-filling/d-type at around Pu-Cm in the actinides as already proposed in the previous literature. Bond lengths in the AnX3 increase down the halide series, and the atomic charges decrease on the actinide elements.

Synthesis, Characterization, and Stability of Two Americium Vanadates, AmVO3 and AmVO4

In search for chemically stable americium compounds with high power densities for radioisotope sources for space applications, AmVO₃ and AmVO₄ were prepared by a solid-state reaction. We present here their crystal structure at room temperature solved by powder X-ray diffraction combined with Rietveld refinement. Their thermal and self-irradiation stabilities have been studied. The oxidation states of americium were confirmed by the Am M₅ edge high-resolution X-ray absorption near-edge structure (HR-XANES) technique. Such ceramics are investigated as potential power sources for space applications like radioisotope thermoelectric generators, and they have to endure extreme conditions including vacuum, high or low temperatures, and internal irradiation. Thus, their stability under self-irradiation and heat treatment in inert and oxidizing atmospheres was tested and discussed relative to other compounds with a high content of americium.

Synthesis, Characterization, and Stability of Americium Phosphate, AmPO4

AmPO₄ was prepared by a solid-state reaction method, and its crystal structure at room temperature was solved by powder X-ray diffraction combined with Rietveld refinement. The purity of the monazite-like phase was confirmed by spectroscopic (high-resolution solid-state ³¹P NMR and Raman) and microscopic (SEM-EDX and TEM) techniques. The thermal and self-irradiation stability have been studied. The compound is stable under argon and air atmosphere at least up to 1773 K. It remains crystalline under self-irradiation for circa two months, with a crystallographic volume swelling of ∼1.5%, and then is amorphizing over a year. However, microcrystals are present in the amorphous material even after a two year period of time. All these characteristics are discussed in relation to the potential application of AmPO₄ as a stable form of Am in radioisotope power sources for space exploration and of behavior of the monazites under irradiation.

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.

Phase transitions among the ABX4compounds*,1

This paper brings together available data on phase transitions involving compounds with ABX4stoichiometry.