Kinetics of Structural and Microstructural Changes at the Solid/Solution Interface during Dissolution of Cerium(IV)–Neodymium(III) Oxides

Improving the understanding of dissolution mechanisms at the solid/solution interface of mixed Ce^IV_1–<i>x</i>Nd^III_<i>x</i>O_{2–<i>x</i>/2} dioxides is a critical step in the frame of generation IV (Gen IV) nuclear fuel elements that integrates minor actinides recycling. In order to give significant insight into the dissolution kinetics of Ce_0.4Nd_0.6O_1.7 sintered samples prepared from the thermal conversion of oxalate precursors, X-ray reflectivity (XRR) and grazing incidence X-ray diffraction (GI-XRD) were used to characterize the microstructural and structural changes of the solid/solution interface and were coupled with solution analysis. The dissolution mechanism occurred in three steps. After a short period of dissolution in which no Nd release was observed, GI-XRD revealed the existence of a 200 nm depth chemical gradient below the surface formed during the early times of the dissolution. A Nd-enriched zone was located at the surface of a Nd-depleted zone. Simultaneously, XRR data showed the development of a surface layer of 40 Å of low electron density. Both results fit well with the presence of a Nd-based thin surface layer phase. Then, the Nd-based surface layer dissolved and developed microporosity, leading to a strong increase of the surface of this layer. Thus, during the initial times, the Nd-based layer mainly contributed to dissolution, which played a crucial role in the kinetics of dissolution and explained why incongruent dissolution was first observed as a transient stage to congruent dissolution. From almost 3% of mass loss, a steady state was reached, corresponding to a congruent dissolution mechanism. The duration of the transient steps depended on the alteration conditions

From Uranothorites to Coffinite: A Solid Solution Route to the Thermodynamic Properties of USiO₄

Experiments on the solubility of intermediate members of the Th_1–<i>x</i>U_<i>x</i>SiO₄ solid solution were carried out to determine the impact of Th–U substitutions on the thermodynamic properties of the solid solution and then allow extrapolation to the coffinite end member. The ion activity products in solutions equilibrated with Th_1–<i>x</i>U_<i>x</i>SiO₄ (0 ≤ <i>x</i> ≤ 0.5) were determined by dissolution experiments conducted in 0.1 mol·L^–1 HCl under Ar atmosphere at several temperatures ranging from 298 to 346 K. For all experiments, dissolution was congruent, and a constant composition of the aqueous solution was reached after 50–200 days of dissolution. The solubility product of thorite was determined (log *<i>K</i>_S,ThSiO₄ = −5.62 ± 0.08) whereas the solubility product of coffinite was estimated (log *<i>K</i>_S,USiO₄ = −6.1 ± 0.2). The stoichiometric solubility product of Th_1–<i>x</i>U_<i>x</i>SiO₄ reached a maximum value for <i>x</i> = 0.45 ± 0.05. In terms of the standard Gibbs free energy of dissolution, solid solutions dissolve more spontaneously than the end members. The standard Gibbs free energy associated with the formation of thorite, coffinite, and intermediate members of the series were then evaluated. The standard Gibbs free energies of formation were found to increase linearly with the uranium mole fraction. Our data at low temperature clearly show that uranothorite solid solutions with <i>x</i> > 0.26, thus coffinite, are less stable than the mixture of binary oxides, which is consistent with qualitative evidence from petrographic studies of uranium ore deposits

Monoclinic Form of the Rhabdophane Compounds: REEPO₄·0.667H₂O

Hydrated rhabdophane with a general formula REEPO₄·<i>n</i>H₂O (REE: La → Dy) has been always considered to crystallize in the hexagonal system. A recent re-examination of this system by the use of synchrotron powder data of the SmPO₄·0.667 H₂O compound led to a structure crystallizing in the monoclinic <i>C</i>2 space group with <i>a</i> = 28.0903(1) Å, <i>b</i> = 6.9466(1) Å, <i>c</i> = 12.0304(1) Å, β = 115.23(1)°, and <i>V</i> = 2123.4 (1) ų with 24 formula units per unit cell. The structure consists of infinite channels oriented along the [101] direction and formed by the connection of Sm-polyhedra and P-tetrahedra by sharing O-edges. The water molecules filling the space have been localized for the first time. The monoclinic form of the hydrated rhabdophane was confirmed by studying the series with REE: La → Dy. Moreover, the dehydration of SmPO₄·0.667H₂O led to the stabilization of an anhydrous form SmPO₄ in the <i>C</i>2 space group with <i>a</i> = 12.14426 (1) Å, <i>b</i> = 7.01776(1) Å, <i>c</i> = 6.34755(1) Å, β = 90.02 (1)°, <i>V</i> = 540.97(1) ų, and <i>Z</i> = 6

Monoclinic Form of the Rhabdophane Compounds: REEPO₄·0.667H₂O

Hydrated rhabdophane with a general formula REEPO₄·<i>n</i>H₂O (REE: La → Dy) has been always considered to crystallize in the hexagonal system. A recent re-examination of this system by the use of synchrotron powder data of the SmPO₄·0.667 H₂O compound led to a structure crystallizing in the monoclinic <i>C</i>2 space group with <i>a</i> = 28.0903(1) Å, <i>b</i> = 6.9466(1) Å, <i>c</i> = 12.0304(1) Å, β = 115.23(1)°, and <i>V</i> = 2123.4 (1) ų with 24 formula units per unit cell. The structure consists of infinite channels oriented along the [101] direction and formed by the connection of Sm-polyhedra and P-tetrahedra by sharing O-edges. The water molecules filling the space have been localized for the first time. The monoclinic form of the hydrated rhabdophane was confirmed by studying the series with REE: La → Dy. Moreover, the dehydration of SmPO₄·0.667H₂O led to the stabilization of an anhydrous form SmPO₄ in the <i>C</i>2 space group with <i>a</i> = 12.14426 (1) Å, <i>b</i> = 7.01776(1) Å, <i>c</i> = 6.34755(1) Å, β = 90.02 (1)°, <i>V</i> = 540.97(1) ų, and <i>Z</i> = 6

The Flexible Ba₇UM₂S_12.5O_0.5 (M = V, Fe) Compounds: Syntheses, Structures and Spectroscopic, Resistivity, and Electronic Properties

Two new compounds, Ba₇UV₂­S_12.5O_0.5 and Ba₇UFe₂­S_12.5O_0.5, 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 Ba₇UV₂­S_12.5O_0.5 crystallizes at 100 K in the Cs₇Cd₃­Br₁₇ structure type in space group <i>D</i>_4<i>h</i>¹⁸–<i>I</i>4<i>/mcm</i> of the tetragonal system. The compound Ba₇UFe₂­S_12.5O_0.5 crystallizes at 100 K in space group <i>D</i>_4<i>h</i>⁵–<i>P</i>4<i>/mbm</i> 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 S^2–, S₂^2–, and 0.5 O^2–, whereas the V⁴⁺ compound charge balances with 7 Ba²⁺, U⁴⁺, 2 V⁴⁺, 12.5 S^2–, and 0.5 O^2–. 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 Ba₇UV₂­S_12.5O_0.5 and Ba₇UFe₂­S_12.5O_0.5 offers a remarkable flexibility in terms of the oxidation state of the incorporated transition metal

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

Coffinite, USiO₄, is the second most abundant U⁴⁺ 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, <i>T</i>, 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

Triclinic–Cubic Phase Transition and Negative Expansion in the Actinide IV (Th, U, Np, Pu) Diphosphates

The <i>An</i>P₂O₇ diphosphates (<i>An</i> = Th, U, Np, Pu) have been synthesized by various routes depending on the stability of the <i>An</i>^IV cation and its suitability for the unusual octahedral environment. Synchrotron and X-ray diffraction, thermal analysis, Raman spectroscopy, and ³¹P nuclear magnetic resonance reveal them as a new family of diphosphates which probably includes the recently studied CeP₂O₇. Although they adopt at high temperature the same cubic archetypal cell as the other known MP₂O₇ diphosphates, they differ by a very faint triclinic distortion at room temperature that results from an ordering of the P₂O₇ units, as shown using high-resolution synchrotron diffraction for UP₂O₇. The uncommon triclinic–cubic phase transition is first order, and its temperature is very sensitive to the ionic radius of <i>An</i>^IV. The conflicting effects which control the thermal variations of the P–O–P angle are responsible for a strong expansion of the cell followed by a contraction at higher temperature. This inversion of expansion occurs at a temperature significantly higher than the phase transition, at variance with the parent compounds with smaller M^IV cations in which the two phenomena coincide. As shown by various approaches, the P–O_b–P linkage remains bent in the cubic form