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In molecular solids derived from Prussian blue, intermetallic charge transfer is fostered through a cyano bridge two metal ions. In this study, isostructural trivalent lanthanide and tetravalent actinide Prussian blue analogs' valence orbitals are probed by soft X-ray absorption measurements.

Fundamental knowledge on intrinsic plutonium colloids is important for the prediction of plutonium behaviour in the geosphere and in engineered systems. The first synthetic route to obtain salt-free intrinsic plutonium colloids by ultrasonic treatment of PuO2 suspensions in pure water is reported. Kinetics showed that both chemical and mechanical effects of ultrasound contribute to the mechanism of Pu colloid formation. In the first stage, fragmentation of initial PuO2 particles provides larger surface contact between cavitation bubbles and solids. Furthermore, hydrogen formed during sonochemical water splitting enables reduction of Pu(IV) to more soluble Pu(III), which then re-oxidizes yielding Pu(IV) colloid. A comparative study of nanostructured PuO2 and Pu colloids produced by sonochemical and hydrolytic methods, has been conducted using HRTEM, Pu LIII-edge XAS, and O K-edge NEXAFS/STXM. Characterization of Pu colloids revealed a correlation between the number of Pu-O and Pu-Pu contacts and the atomic surface-to-volume ratio of the PuO2 nanoparticles. NEXAFS indicated that oxygen state in hydrolytic Pu colloid is influenced by hydrolysed Pu(IV) species to a greater extent than in sonochemical PuO2 nanoparticles. In general, hydrolytic and sonochemical Pu colloids can be described as core-shell nanoparticles composed of quasi-stoichiometric PuO2 cores and hydrolyzed Pu(IV) moieties at the surface shell.

The preparation of synthetic (Zr,U)SiO4 solid solution is challenging, as the conventional high-temperature solid-state method limits the solubility of uranium (4 ± 1 mol%) in the orthosilicate phase due to its thermodynamic instability. However, these compounds are of great interest as a result of (Zr,U)SiO4 solid solutions, with uranium contents exceeding this concentration, being observed as corium phases formed during nuclear accidents. It has been identified that hydrothermal synthesis pathways can be used for the formation of the metastable phase, such as USiO4. The investigation carried out in this study has indeed led to the confirmation of metastable (Zr,U)SiO4 compounds with high uranium contents being formed. It was found that (Zr,U)SiO4 forms a close-to-ideal solid solution with uranium loading of up to 60 mol% by means of hydrothermal treatment for 7 days at 250 °C, at pH = 3 and starting from an equimolar reactant concentration equal to 0.2 mol L−1. A purification procedure was developed to obtain pure silicate compounds. After purification, these compounds were found to be stable up to 1000 °C under an inert atmosphere (argon). The characterisation methods used to explore the synthesis and thermal stability included powder X-ray diffraction (PXRD), Fourier transform infrared (FTIR) and Raman spectroscopies, scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). [ABSTRACT FROM AUTHOR]

The electronic properties of actinide cations are of fundamental interest to describe intramolecular interactions and chemical bonding in the context of nuclear waste reprocessing or direct storage. The 5f and 6d orbitals are the first partially or totally vacant states in these elements, and the nature of the actinide ligand bonds is related to their ability to overlap with ligand orbitals. Because of its chemical and orbital selectivities, X-ray absorption spectroscopy (XAS) is an effective probe of actinide species frontier orbitals and for understanding actinide cation reactivity toward chelating ligands. The soft X-ray probes of the light elements provide better resolution than actinide L3-edges to obtain electronic information from the ligand. Thus coupling simulations to experimental soft X-ray spectral measurements and complementary quantum chemical calculations yields quantitative information on chemical bonding. In this study, soft X-ray XAS at the K-edges of C and N, and the L2,3-edges of Fe was used to investigate the electronic structures of the well-known ferrocyanide complexes K4Fe(II)(CN)6, thorium hexacyanoferrate Th(IV)Fe(II)(CN)6, and neodymium hexacyanoferrate KNd(III)Fe(II)(CN)6. The soft X-ray spectra were simulated based on quantum chemical calculations. Our results highlight the orbital overlapping effects and atomic effective charges in the Fe(II)(CN)6 building block. In addition to providing a detailed description of the electronic structure of the ferrocyanide complex (K4Fe(II)(CN)6), the results strongly contribute to confirming the actinide 5f and 6d orbital oddity in comparison to lanthanide 4f and 5d.

Although hydrogen peroxide (H2O2) has been highly used in nuclear chemistry for more than 75 years, the preparation and literature description of tetravalent actinide peroxides remain surprisingly scarce. A new insight is given in this topic through the synthesis and thorough structural characterization of a new peroxo compound of Pu(IV). [ABSTRACT FROM AUTHOR]

Complex formation between uranyl ion, UO22+, and a hydrophilic anionic form of SO3-Ph-BTP4- ligand, L4-, in water was studied by liquid-liquid extraction experiments performed over a range of the ligand and HNO3 concentrations in the aqueous phase, at a constant concentration of nitrate anions at 25°C . The competition for UO22+ ions between the lipophilic TODGA extractant and the hydrophilic L4- ligand leads to the decrease in the uranyl distribution ratios, D, with an increasing L4- concentration. The model of the solvent extraction process used accounts - apart from uranyl complexation by TODGA and SO3-Ph-BTP4- - also for uranyl complexation by nitrates and for the decrease in the concentration of the free L4- ligand in the aqueous phase, due to its protonation, bonding in the uranyl complex and the distribution between the two liquid phases. The unusually strong dependence of the D values on the acidity, found in the experiment, could hardly be explained as due to L4- protonation merely. Three hypotheses were experimentally tested, striving to interpret the data in terms of additional extraction to the organic phase of ion associates of protonated TODGA cation with either partly protonated anionic L4- ligands or anionic UO22+ complexes with NO3 - or L4-. None of them has been confirmed. The analysis of the results, based on the formal correction of free ligand concentrations, points to the formation of 1 : 1 and 1 : 2 uranyl - SO3-Ph-BTP complexes in the aqueous phase. The conditional formation constant of the 1:1 complex has been determined, logßL,1 = 2.95 ± 0.15.

Although ZrSiO4 is the most well-known compound in the zircon-structured family (space group I41/amd), the experimental conditions for preparing pure and well-crystallized phases that are doped with a tetravalent element via hydrothermal synthesis have never been clearly discussed in the literature. With the aim to answer this question, the experimental conditions of the preparation of ZrSiO4 and (Zr,Ce)SiO4 were investigated in order to synthesize well-crystallized and pure phases. A multiparametric study has been carried out using soft hydrothermal conditions with variables including reactant concentration, initial pH of the reactive medium, and duration of the hydrothermal treatment. Pure ZrSiO4 was obtained through hydrothermal treatment for 7 days at 250 °C, within a large acidity range (1.0 ≤ pH ≤ 9.0) and starting from CSi ≈ CZr ≥ 0.2 mol L−1. As hydrothermally prepared zircon structured phases can be both hydrated and hydroxylated, its annealed form was also studied after heating to 1000 °C. Based on these results, the synthesis of (Zr,Ce)SiO4 solid solutions was also investigated. The optimal hydrothermal conditions to acquire pure and crystallized phases were as follows: 7 days at 250 °C with initial pH = 1 and concentration of the reactants equal to 0.2 mol L−1. This led to Zr1−xCexSiO4 solid solutions with the incorporated Ce content up to 40 mol%. Samples were characterized using multiple methods, including laboratory and synchrotron PXRD, IR and Raman spectroscopies, SEM, and TGA. Moreover, it was found that these phases were thermally stable in air up to at least 1000 °C. [ABSTRACT FROM AUTHOR]

Actinide colloids and nanoparticles (NPs) currently constitute a topic of strong interest due to their potential role in advanced nuclear energetics and the environmental migration of radioactivity. A better understanding of the physico-chemical properties of nanoscale actinide oxides requires robust synthesis approaches. In this work, UO2+x NPs were successfully prepared by sonochemistry from U(IV) solutions previously stabilised in a hydrochloric medium (20 kHz, 65 °C, Ar/(10%)CO). Colloidal suspensions were found to be composed of crystalline and spherical NPs showing a UO2-like structure and measuring 18.0 ± 0.1 nm (SAXS, HR-TEM and PXRD techniques). In comparison with the controlled hydrolysis approach used as a reference, sonochemistry appears to be a simple and original synthesis route providing larger, better defined and more crystalline UO2+x NPs with a narrower size distribution. These well-defined NPs offer new opportunities for the preparation of reference actinide materials devoted to fundamental, technological and environmental studies. [ABSTRACT FROM AUTHOR]