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.

Hydrothermal conversion of mixed uranium(IV)-cerium(III) oxalates into U1-xCexO2+δ·nH2O solid solutions

Uranium-cerium oxide solid solutions, U_1-xCe_xO_2+δ·nH₂O, were prepared through hydrothermal conversion of mixed U(IV)-Ce(III) oxalate precursors, cerium being used as a surrogate for plutonium. Whatever the starting pH, the fluorite-type structure of AnO₂ was obtained after heating at 250 °C for 24 h. The initial pH of the reaction media appeared to affect significantly the oxide morphology: for pH ≤ 2, the powder was found to be composed of microspheres, whereas for more alkaline pH values, agglomerates of nanocrystallites were found. Furthermore, a study of the hydrothermal treatment duration (T = 250 °C, pH = 8, t = 1-48 h) showed that fluorite-type mixed dioxides started to form after only 1 h, and then became single phase after 3 h. SEM and TEM/EDS analyses revealed that the cationic distribution narrowed with time to finally form highly homogeneous mixed oxides. Such a preparation route was then applied to various cerium incorporation rates and it was found that the formation of U_1-xCe_xO_2+δ·nH₂O mixed oxides was possible for 0.1 ≤ x ≤ 0.75. In all the systems investigated, the speciation of uranium and cerium was questioned in both the solid and liquid phases. Thermodynamic calculations and evaluation of the O/M ratio in the final oxides led us to understand the complex redox behaviour of uranium and cerium in solution during hydrothermal processes and to propose a conversion mechanism.

Influence of Sintering Conditions on the Structure and Redox Speciation of Homogeneous (U,Ce)O2+δ Ceramics: A Synchrotron Study

Although uranium-cerium dioxides are frequently used as a surrogate material for (U,Pu)O_2-δ nuclear fuels, there is currently no reliable data regarding the oxygen stoichiometry and redox speciation of the cations in such samples. In order to fill this gap, this manuscript details a synchrotron study of highly homogeneous (U,Ce)O_2±δ sintered samples prepared by a wet-chemistry route. HERFD-XANES spectroscopy led to determining accurately the O/M ratios (with M = U + Ce). Under a reducing atmosphere (pO₂ ≈ 6 × 10^-29 atm at 650 °C), the oxides were found to be close to O/M = 2.00, while the O/M ratio varied with the sintering conditions under argon (pO₂ ≈ 3 × 10^-6 atm at 650 °C). They globally appeared to be hyperstoichiometric (i.e., O/M > 2.00) with the departure from the dioxide stoichiometry decreasing with both the cerium content in the sample and the sintering temperature. Nevertheless, such a deviation from the ideal O/M = 2.00 ratio was found to generate only moderate structural disorder from EXAFS data at the U-L₃ edge as all the samples retained the fluorite-type structure of the UO₂ and CeO₂ parent compounds. The determination of accurate lattice parameters owing to S-PXRD measurements led to complementing the data reported in the literature by various authors. These data were consistent with an empirical relation linking the unit cell parameter, the chemical composition, and the O/M stoichiometry, showing that the latter can be evaluated simply within a ± 0.02 uncertainty.

Facile Preparation of Macro-Microporous Thorium Oxide via a Colloidal Sol-Gel Route toward Safe MOX Fuel Fabrication

The identification of new colloidal sol-gel routes for the preparation of actinide oxides, which have a homogeneous and accessible porosity that can easily be impregnated by any concentrated actinide solution, opens new perspectives for the preparation of homogeneous nuclear fuel for minor actinide transmutation. This homogeneity allows us to avoid "hot spot" formation due to the local accumulation of more fissile elements. Here, we report the preparation of macro-microporous ThO₂ materials by a colloidal sol-gel route. Using a thorium salt with 6-aminocaproic acid as a complexing agent at a controlled pH, we were able to pilot the condensation of thorium hydroxo species forming colloids of tuned nanometric size and thus the sol stability. After a freeze-drying process to concentrate colloids and a thermal treatment allowing complexing agent removal and macroporosity formation by a brutal gas release during combustion, a loose packing of ThO₂ nanoparticles with an ordered distribution of interparticular porosity and a fraction of nanometric crystallites, whose size depends on the initial colloidal size, were obtained. The sols, pastes, and final materials were characterized by small- and wide-angle X-ray scattering to determine the colloidal size and the final structure of the materials, which was also confirmed by transmission electron microscopy. The most promising material was finally successfully impregnated by a simulating minor actinide solution and thermally treated to prepare a mixed actinide oxide material. This safe technology, relying on the colloidal sol-gel process and the formulation of complex fluids forming tunable precursors, opens new perspectives for the reuse of nuclear waste solutions as new fuel.

Effect of Annealing on Structural and Thermodynamic Properties of ThSiO4-ErPO4 Xenotime Solid Solution

The effect of annealing on structural and thermochemical properties of a thorite-xenotime solid solution Th_1-xEr_x(SiO₄)_1-x(PO₄)_x was assessed. The samples synthesized at low temperatures and stored at room temperature for 2 years retained their tetragonal structures. This structure was also maintained after heating to 1100 °C. During annealing, the structure lost water and exsolved some thorianite phases. The thermodynamic parameters did not change much after annealing, suggesting that xenotime was not a low-temperature metastable phase but rather a stable structure able to withstand elevated temperatures regardless of the thorium content. The solid solution exhibited subregular behavior with the Margules function W(x) = (73.1 ± 20.1) - (125.7 ± 49.8)·x.

A multiscale in situ high temperature high resolution transmission electron microscopy study of ThO2 sintering

Two-grain model systems formed by ThO₂ nanospheres have been used to experimentally study for the first time the initial stage of sintering from room temperature to 1050 °C using high temperature high resolution transmission electron microscopy. In each grain, oriented attachment drove the reorganization and growth of the crystallites up to 300 °C to form a pseudo single crystal. Crystallite size kept growing up to 950 °C. At this temperature, a fast transformation probably corresponding to the elimination of stacking faults or dislocation walls led to the formation of single-crystals. The contact formed at room temperature between the two grains was stabilized during heat treatment by a slight reorientation of the crystallographic planes (T≈ 400 °C), leading the neck to be formed by numerous boundaries between the crystallites. At higher temperatures, the neck evolved and stabilized in the form of a plane of crystallographic orientation mismatch between the grains, which corresponds to the usual definition of the grain boundary. The growth of the neck by the addition of atomic columns was further observed in real time and quantified. At T = 950 °C, the evolution of the microscopic sintering parameter λ was obtained from HT-HRTEM images and indicated that the neck formation mostly proceeded through volume diffusion.

The Role of Water and Hydroxyl Groups in the Structures of Stetindite and Coffinite, MSiO4 (M = Ce, U)

Orthosilicates adopt the zircon structure types (I4₁/amd), consisting of isolated SiO₄ tetrahedra joined by A-site metal cations, such as Ce and U. They are of significant interest in the fields of geochemistry, mineralogy, nuclear waste form development, and material science. Stetindite (CeSiO₄) and coffinite (USiO₄) can be formed under hydrothermal conditions despite both being thermodynamically metastable. Water has been hypothesized to play a significant role in stabilizing and forming these orthosilicate phases, though little experimental evidence exists. To understand the effects of hydration or hydroxylation on these orthosilicates, in situ high-temperature synchrotron and laboratory-based X-ray diffraction was conducted from 25 to ∼850 °C. Stetindite maintains its I4₁/amd symmetry with increasing temperature but exhibits a discontinuous expansion along the a-axis during heating, presumably due to the removal of water confined in the [001] channels, which shrink against thermal expansion along the a-axis. Additional in situ high-temperature Raman and Fourier transform infrared spectroscopy also confirmed the presence of the confined water. Coffinite was also found to expand nonlinearly up to 600 °C and then thermally decompose into a mixture of UO₂ and SiO₂. A combination of dehydration and dehydroxylation is proposed for explaining the thermal behavior of coffinite synthesized hydrothermally. Additionally, we investigated high-temperature structures of two coffinite-thorite solid solutions, uranothorite (U_xTh_1-xSiO₄), which displayed complex variations in composition during heating that was attributed to the negative enthalpy of mixing. Lastly, for the first time, the coefficients of thermal expansion of CeSiO₄, USiO₄, U_0.46Th_0.54SiO₄, and U_0.9Th_0.1SiO₄ were determined to be α_V = 14.49 × 10^-6, 14.29 × 10^-6, 17.21 × 10^-6, and 17.23 × 10^-6 °C^-1, respectively.

Hydrothermal Conversion of Thorium Oxalate into ThO2·nH2O Oxide

Hydrothermal conversion of thorium oxalate, Th(C₂O₄)₂·nH₂O, into thorium dioxide was explored through a multiparametric study, leading to some guidelines for the preparation of crystallized samples with the minimum amount of impurities. As the formation of the oxide appeared to be operated through the hydrolysis of Th⁴⁺ after decomposition of oxalate fractions, pH values typically above 1 must be considered to recover a solid phase. Also, because of the high stability of the thorium oxalate precursor, hydrothermal treatments of more than 5 h at a temperature above 220 °C were required. All the ThO₂·nH₂O samples prepared presented amounts of residual carbon and water in the range 0.2-0.3 wt % and n ≈ 0.5, respectively. A combined FTIR, PXRD, and EXAFS study showed that these impurities mainly consisted of carbonates trapped between elementary nanosized crystallites, rather than substituted directly in the lattice, which generated a tensile effect over the crystal lattice. The presence of carbonates at the surface of the elementary crystallites could also explain their tendency to self-assembly, leading to the formation of spherical aggregates. Hydrothermal conversion of oxalates could then find its place in different processes of the nuclear fuel cycle, where it will provide an interesting opportunity to set up dustless routes leading from ions in solution to dioxide powders in a limited number of steps.

Uranium removal from mining water using Cu substituted hydroxyapatite

In this study, synthetic copper substituted hydroxyapatite (Cu-Hap), Cu_xCa_10-x(PO₄)₆(OH)₂ were prepared by co-precipitation method and were used as reactive materials in batch experiments to immobilize uranyl. The limit of incorporation of Cu into a single-phased Cu-Hap reached x_Cu ≤1.59. The synthetic Cu-Hap samples obtained with various Cu contents were contacted with synthetic uranyl doped solutions and with real mining waters showing various pH and chemical compositions. A fast and strong decrease of the uranium concentration was observed, followed by the establishment of an equilibrium after 1-4 days of contact with the solutions. Examination of the solid phase after uranium uptake was performed using a combination of techniques. Depending on the composition of the solution and the copper content of the Cu-Hap, various mechanisms of uranium removal were observed. Based on the experimental results and geochemical simulations, it appeared that the main interest for using Cu-Hap is to enlarge the domain of water compositions for which the precipitation of meta-torbernite, (H₃O)_0.4Cu_0.8(UO₂)₂(PO₄)₂·7.6 H₂O is the predominant mechanism associated to the uranium removal, especially for pH > 6.7 where carbonate uranium species are predominant.

Structural and Thermodynamic Investigation of the Perovskite Ba2NaMoO5.5

Neutron diffraction, X-ray absorption spectroscopy (XAS), and Raman spectroscopy measurements of the quaternary perovskite phase Ba₂NaMoO_5.5 have been performed in this work. The cubic crystal structure in space group Fm3̅m has been refined using the Rietveld method. X-ray absorption near-edge structure spectroscopy (XANES) measurements at the Mo K-edge have confirmed the hexavalent state of molybdenum. The local structure of the molybdenum octahedra has been studied in detail using extended X-ray absorption fine structure (EXAFS) spectroscopy. The Mo–O and Mo–Ba distances have been compared to the neutron diffraction data with good agreement. The coefficient of thermal expansion measured in the temperature range of 303–923 K, using high temperature X-ray diffraction (HT-XRD) (α_V = 55.8 × 10^–6 K), has been determined to be ∼2 times higher than that of the barium molybdates BaMoO₃ and BaMoO₄. Moreover, no phase transition nor melting have been observed, neither by HT-XRD nor Raman spectroscopy nor differential scanning calorimetry, up to 1473 K. Furthermore, the standard enthalpy of formation (Δ_fH_m°) for Ba₂NaMoO_5.5(cr) has been determined to be −(2524.75 ± 4.15) kJ mol⁻¹ at 298.15 K, using solution calorimetry. Finally, the margin for safe operation of sodium-cooled fast reactors (SFRs) has been assessed by calculating the threshold oxygen potential needed, in liquid sodium, to form the quaternary compound, following an interaction between irradiated mixed oxide (U,Pu)O₂ fuel and sodium coolant.

The structure of the cubic perovskite Ba₂NaMoO_5.5 has been refined from neutron diffraction data and EXAFS measurements. The thermal expansion of Ba₂NaMoO_5.5 has been measured using high-temperature XRD. The stability of the compound has been investigated by DSC, XRD, and Raman data, up to 1473 K. The enthalpy of formation has been derived from solution calorimetry measurements as −(2524.75 ± 4.15) kJ mol⁻¹. The oxygen potential threshold to form Ba₂NaMoO_5.5 in liquid sodium has been evaluated.

Determination of the isotopic composition of single sub-micrometer-sized uranium particles by laser ablation coupled with multi-collector inductively coupled plasma mass spectrometry

RATIONALE

A multi-collector inductively coupled plasma (MC-ICP) mass spectrometer coupled to a UV ns-laser ablation (LA) system was used to measure uranium isotopic ratios (²³⁴ U/²³⁸ U, ²³⁵ U/²³⁸ U and ²³⁶ U/²³⁸ U) in single uranium particles of various sizes and isotopic compositions, including home-made sub-micrometric natural uranium particles of narrow size distribution (415 ± 60 nm).

METHODS

The LA-ICP mass spectrometer was operated in wet plasma conditions thanks to simultaneous injection of the laser aerosol and water vapor through a desolvating nebulizer. The isotopic ratios were corrected for mass bias and gain factors between detectors. The ²³⁶ U/²³⁸ U ratios were also corrected for the presence of ²³⁵ U hydrides and tailing of the ²³⁸ U⁺ peak.

RESULTS

²³⁶ U/²³⁸ U ratios were successfully measured in micrometer-sized particles from the NBS U050 certified standard material with a ²³⁶ U/²³⁸ U ratio of ~5 × 10^-4 . The analysis of 77 natural uranium sub-μm-sized particles yielded a very good trueness with respect to the expected ²³⁴ U/²³⁸ U and ²³⁵ U/²³⁸ U ratios, while the measurement errors for single particles ranged from -2.7% to +2.1% for ²³⁵ U/²³⁸ U and from -17% to +33% for the ²³⁴ U/²³⁸ U ratios. Their relative combined standard uncertainties ranged from 3.3% to 32.8% and from 0.4% to 4.0% for ²³⁴ U/²³⁸ U and ²³⁵ U/²³⁸ U ratios, respectively. In addition, extremely low detection limits, in the attogram range, were achieved.

CONCLUSIONS

This study demonstrates that coupling of a ns-laser ablation system with a MC-ICP mass spectrometer allows measurements of the isotopic composition in natural uranium particles of a few hundreds of nm with very good trueness, average combined standard uncertainties of ~1% for ²³⁵ U/²³⁸ U ratios and 12% for ²³⁴ U/²³⁸ U ratios, and detections limits of a few ag for minor isotopes.

Thermodynamics and Stability of Rhabdophanes, Hydrated Rare Earth Phosphates REPO4 · n H2O

Rare earth phosphates comprise a large family of compounds proposed as possible nuclear waste disposal forms. We report structural and thermodynamic properties of a series of rare earth rhabdophanes and monazites. The water content of the rhabdophanes, including both adsorbed and structural water, decreases linearly with increase in ionic radius of the rare earth. The energetics of the transformation of rhabdophane to monazite plus water and the enthalpy of formation of rhabdophane from the constituent oxides was determined by high temperature drop solution calorimetry. The former varies linearly with the ionic radius of the lanthanide, except for cerium. By combining the enthalpy of formation determined by high temperature drop solution calorimetry and the free energy of formation determined previously by solubility experiments, a complete set of thermodynamic data was derived for the rhabdophanes. They are thermodynamically metastable with respect to the corresponding monazites plus water at all temperatures under ambient pressure conditions. This conclusion strengthens the case for monazites being an excellent nuclear waste form.

Monazite, rhabdophane, xenotime & churchite: Vibrational spectroscopy of gadolinium phosphate polymorphs

Rare-earth phosphates with the general formula REEPO₄·nH₂O belong to four distinct structural types: monazite, rhabdophane, churchite, and xenotime. We report herein the first direct comparison between vibrational spectra of these compounds for the same metal cation i.e. gadolinium. The four GdPO₄·nH₂O samples were prepared through wet chemistry methods and first characterized by X-ray diffraction. Three distinct spectral domains, associated to the deformation and stretching modes of phosphate tetrahedra (PO₄) and to water molecules vibrations were then analyzed from FTIR and Raman data, and discussed regarding the structural characteristics of each sample. The most obvious differences between the spectra were associated to δ(H₂O) and δ_s(PO₄) modes and led to propose a simple method to rapidly and unambiguously discriminate the four polymorphs.

Synthesis of size-controlled UO2 microspheres from the hydrothermal conversion of U(iv) aspartate

A wet chemistry route towards UO₂ spherical particles was designed through the hydrothermal conversion of uranium(iv) aspartate. A multi-parametric study led us to point out the conditions leading to monodisperse and size-controlled particles in the 400–2500 nm range. This simple protocol paves the way to applications in various scientific areas.

Incorporation of Thorium in the Zircon Structure Type through the Th1-xErx(SiO4)1-x(PO4)x Thorite-Xenotime Solid Solution

Pure powdered compounds with a general formula Th_1-xEr_x(SiO₄)_1-x(PO₄)_x belonging to the zircon-xenotime family were successfully synthesized under hydrothermal conditions (250 °C, 7 days) as recently reported for the preparation of coffinite. Therefore, a thorough, combined PXRD, EDX, EXAFS, Raman, and FTIR analysis showed the formation of a solid solution in agreement with Vegard's law. Moreover, the examination of the local structure shows that the Th-O distances remain close to those found in ThSiO₄, whereas the Er-O distances show a significant decrease from 2.38(14) to 2.34(7) Å when increasing the erbium content from x = 0.2 to x = 1. The variation of the local structure also affects the PO₄^3- groups that are surely distorted in the structure.

Novel approaches for the in situ study of the sintering of nuclear oxide fuel materials and their surrogates

Sintering is one of the key-points of the processing of ceramic materials. It is then of primary interest for the nuclear fuel cycle, in which it constitutes an important step in the fabrication of either UO2 or (U,Pu)O2 pellets used in current PWR reactors. The sintering of actinides oxides not only drives the final density and microstructure of the fuels, but also several characteristics that can impact significantly their behavior in the reactor. Dedicated tools are then needed to monitor the microstructure of such materials and forecast their evolution. In this frame, this paper presents the new potentialities offered by the use of environmental scanning electron microscope at high temperature (HT-ESEM) for the study of nuclear ceramics sintering. First, the results obtained from bulk pellets are detailed, either regarding original fundamental data at the grain level (such as grain boundaries and pores motion), or design of dedicated microstructures through the assessment of grain growth kinetics. Acquisition of sintering maps thanks to the combination of HT-ESEM observations and classical dilatometric measurements are also addressed. In a second part, observations undertaken at the 2-grain scale to monitor the first stage of sintering, dedicated to neck elaboration, are presented, and compared to the results currently provided by numerical models.

Vibrational spectroscopy of synthetic analogues of ankoleite, chernikovite and intermediate solid solution

Ankoleite (K(UO2)PO4·nH2O), chernikovite (H3O(UO2)PO4·nH2O) and intermediate solid solutions are frequently encountered in the uranium ores that result from the alteration of uranium primary minerals. This paper reports a thorough FTIR and Raman study related to synthetic analogues for these minerals. First, the vibration bands associated to the UO2(2 +) uranyl ion were used to calculate the U = O bond length which appeared in good agreement with the data coming from PXRD. Then, the examination of the phosphate vibration modes in both sets of spectra confirmed the general formulation of the samples and ruled out the presence of hydrogenphosphate groups. Finally, the presence of H2O as well as protonated H3O(+) and/or H5O2(+) species was also pointed out, and could be used to clearly differentiate the various phases prepared. Vibrational spectroscopy then appeared as an efficient method for the investigation of such analogues of natural samples. It should be particularly relevant when identifying these phases in mineral ores or assemblies.

Coffinite, USiO4, Is Abundant in Nature: So Why Is It So Difficult To Synthesize?

Coffinite, USiO4, is the second most abundant U(4+) mineral on Earth, and its formation by the alteration of the UO2 in spent nuclear fuel in a geologic repository may control the release of radionuclides to the environment. Despite its abundance in nature, the synthesis and characterization of coffinite have eluded researchers for decades. On the basis of the recent synthesis of USiO4, we can now define the experimental conditions under which coffinite is most efficiently formed. Optimal formation conditions are defined for four parameters: pH, T, heating time, and U/Si molar ratio. The adjustment of pH between 10 and 12 leads probably to the formation of a uranium(IV) hydroxo-silicate complex that acts as a precursor of uranium(IV) silicate colloids and then of coffinite. Moreover, in this pH range, the largest yield of coffinite formation (as compared with those of the two competing byproduct phases, nanometer-scale UO2 and amorphous SiO2) is obtained for 250 °C, 7 days, and 100% excess silica.

From thorite to coffinite: a spectroscopic study of Th(1-x)U(x)SiO4 solid solutions

Coffinite (USiO4), along with Th(1-x)U(x)SiO4 uranothorite solid solutions, are frequently present in reduced economically exploitable uranium ores. They could also control the concentration of uranium in the environment in the case of accidental release from underground radwaste repository. This paper reports for the first time a thorough FTIR and Raman study relative to the Th(1-x)U(x)SiO4 system, including synthetic analogues of thorite and coffinite end-members. Both sets of spectra confirmed the formulation of the samples and allowed to rule out the presence of structural water molecules and/or hydroxyl groups in the coffinite. Also, no characteristic signal of UO2(2+) uranyl ion was recorded, ensuring that uranium was fully incorporated under its tetravalent oxidation state. The variation of the positions corresponding to SiO4 internal vibration modes was then followed versus the chemical composition of the samples. If the FTIR spectra did not revealed any significant shift in the bands position, several Raman modes followed a linear trend as a function of the uranium incorporation rate. On this basis, Raman spectroscopy could be considered as a promising tool for the semi-quantitative determination of chemical composition of uranothorite samples, particularly for those coming from mineral ores. Finally, the data collected for the coffinite end-member, as the first to be obtained on pure synthetic samples, allowed a review of the results previously reported in the literature for this compound.

Preparation and characterisation of uranium oxides with spherical shapes and hierarchical structures

One of the first reports on shape-controlled uranium oxides with hierarchical structures and their mechanism of formation.

The flexible Ba7UM2S12.5O0.5 (M = V, Fe) compounds: syntheses, structures and spectroscopic, resistivity, and electronic properties

Two new compounds, Ba7UV2S12.5O0.5 and Ba7UFe2S12.5O0.5, have been synthesized in fused-silica tubes by the direct combinations of V or Fe with U, BaS, and S at 1223 K. The compound Ba7UV2S12.5O0.5 crystallizes at 100 K in the Cs7Cd3Br17 structure type in space group D4h(18)–I4/mcm of the tetragonal system. The compound Ba7UFe2S12.5O0.5 crystallizes at 100 K in space group D4h(5)–P4/mbm of the tetragonal system. The structures are very similar with V/S or Fe/S networks in which Ba atoms reside as well as channels large enough to accommodate additional Ba atoms and infinite linear US5O chains. Each U atom is octahedrally coordinated to four equatorial S atoms, one axial S atom, and one axial O atom. The Fe/S network contains a S–S single bond, whereas the V/S network does not. The result is that the Fe3+ compound charge balances with 7 Ba2+, U4+, 2 Fe3+, 10.5 S2–, S2(2–), and 0.5 O2–, whereas the V4+ compound charge balances with 7 Ba2+, U4+, 2 V4+, 12.5 S2–, and 0.5 O2–. Other differences between these two compounds have been characterized by Raman spectroscopy and resistivity measurements. DFT calculations have provided insight into the nature of their bonding. The overall structural motif of Ba7UV2S12.5O0.5 and Ba7UFe2S12.5O0.5 offers a remarkable flexibility in terms of the oxidation state of the incorporated transition metal.

Dissolution of cerium(IV)-lanthanide(III) oxides: comparative effect of chemical composition, temperature, and acidity

The dissolution of Ce(1-x)Ln(x)O(2-x/2) solid solutions was undertaken in various acid media in order to evaluate the effects of several physicochemical parameters such as chemical composition, temperature, and acidity on the reaction kinetics. The normalized dissolution rates (R(L,0)) were found to be strongly modified by the trivalent lanthanide incorporation rate, due to the presence of oxygen vacancies decreasing the samples cohesion. Conversely, the nature of the trivalent cation considered only weakly impacted the R(L,0) values. The dependence of the normalized dissolution rates on the temperature then appeared to be of the same order of magnitude than that of chemical composition. Moreover, it allowed determining the corresponding activation energy (E(A) ≈ 60-85 kJ·mol(-1)) which accounts for a dissolution driven by surface-controlled reactions. A similar conclusion was made regarding the acidity of the solution: the partial order related to (H(3)O(+)) reaching about 0.7. Finally, the prevailing effect of the incorporation of aliovalent cations in the fluorite-type CeO(2) matrix on the dissolution kinetics precluded the observation of slight effects such as those linked to the complexing agents or to the crystal structure of the samples.