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RadiochSeveral La1–xGdxPO4 solid solutions were prepared in the monazite- or rhabdophane-type structures for various x values using three methods of preparation (direct evaporation, synthesis in closed PTFE containers on a sand bath or in autoclaves). Samples of rhabdophane-type La1–xGdxPO4·nH2O (n0.5) were prepared at 150°C only for x0.4. For x0.3, the solids were precipitated as the monazite-type structure. These results were confirmed by the study of pure rare earth phosphates synthesized under the same conditions. By these means, well-crystallized and monophase samples of MPO4·nH2O (n0.5–1) in the monazite (La, Ce), rhabdophane (Nd, Sm, Eu, Gd, Tb, Dy) or churchite (Ho, Er, Tm, Yb, Lu) forms were prepared.On the basis of the variation of the specific area versus the holding temperature and of the dilatometric studies, the optimal temperature of sintering for these solids was found to be between 1250 and 1400°C. The effective relative densities of the pellets of GdPO4 prepared using a two-step procedure (pressing between 200 and 700 MPa, then heat treatment at 1300°C) reached 96% of the value calculated from the XRD data. The chemical durability of sintered samples of GdPO4 was evaluated in several acidic media between room temperature and 90°C. The very low normalized dissolution rates RL(between 10–6 and 10–3 g m–2 day–1) measured even in very acidic media confirmed the very good retention properties of this kind of phosphate-based matrix for the immobilization of radionuclides and especially of trivalent actinides

Several CeO(2)-based mixed oxides with general composition Ce(1-x)Ln(x)O(2-x/2) (for 0 ≤ x ≤ 1 and Ln = La, Nd, Sm, Eu, Gd, Dy, Er, or Yb) were prepared using an initial oxalic precipitation leading to a homogeneous distribution of cations in the oxides. After characterization of the Ce/Nd oxalate precursors and then thermal conversion to oxides at T = 1000 °C, investigation of the crystalline structure of these oxides was carried out by XRD and μ-Raman spectroscopy. Typical fluorite Fm ̅3m structure was obtained for relatively low Ln(III) contents, while a cubic Ia ̅3̅ superstructure was evidenced above x ≈ 0.4. Moreover, since Nd(2)O(3) does not crystallize with the Ia ̅3̅-type structure, two-phase systems composed with additional hexagonal Nd(2)O(3) were obtained for x(Nd) ≥ 0.73 in the Ce(1-x)Nd(x)O(2-x/2) series. The effect of heat treatment temperature on these limits was explored through μ-Raman spectroscopy, which allowed determining the presence of small amounts of the different crystal structures observed. In addition, the variation of the Ce(1-x)Ln(x)O(2-x/2) unit cell parameter was found to follow a quadratic relation as a result of the combination between increasing cationic radius, modifications of cation coordination, and decreasing O-O repulsion caused by oxygen vacancies.

On the basis of optimized grinding/heating cycles developed for several phosphate-based ceramics, the preparation of brabantite and then monazite/brabantite solid solutions loaded with tetravalent thorium, uranium, and cerium (as a plutonium surrogate) was examined versus the heating temperature. The chemical reactions and transformations occurring when heating the initial mixtures of AnO2/CeO2, CaHPO(4).2H2O (or CaO), and NH4H2PO4 were identified through X-ray diffraction (XRD) and thermogravimetric/differential thermal analysis experiments. The incorporation of thorium, which presents only one stabilized oxidation state, occurs at 1100 degrees C. At this temperature, all the thorium-brabantite samples appear to be pure and single phase as suggested by XRD, electron probe microanalyses, and micro-Raman spectroscopy. By the same method, tetravalent uranium can be also stabilized in uranium-brabantite, i.e., Ca0.5U0.5PO4, after heating at 1200 degrees C. Both brabantites, Ca0.5Th0.5PO4 and Ca0.5U0.5PO4, begin to decompose when increasing the temperature to 1400 and 1300 degrees C, respectively, leading to a mixture of CaO and AnO2 by the volatilization of P4O10. In contrast to the cases of thorium and uranium, cerium(IV) is not stabilized during the heating treatment at high temperature. Indeed, the formation of Ca0.5Ce0.5PO4 appears impossible, due to the partial reduction of cerium(IV) into cerium(III) above 840 degrees C. Consequently, the systems always appear polyphase, with compositions of CeIII1-2xCeIVxCaxPO4 and Ca2P2O7. The same conclusion can be also given when discussing the incorporation of cerium(IV) into La1-2xCeIIIx-yCeIVyCay(PO4)1-x+y. This incomplete incorporation of cerium(IV) confirms the results obtained when trying to stabilize tetravalent plutonium in Ca0.5PuIV0.5PO4 samples.

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To underline the potential links between the crystallization state and the microstructure of powdered cerium-neodymium oxides and their chemical durability, several Ce(IV)(1-x)Nd(III)(x)O(2-x/2) mixed dioxides were prepared in various operating conditions from oxalate precursors and then leached. The powdered samples were first examined through several physicochemical properties (crystallization state and associated crystallite size, reactive surface area, porosity...). The dependence of the normalized dissolution rates on various parameters (including temperature, nitric acid concentration, crystallization state) was examined for pure CeO(2) and Ce(1-x)Nd(x)O(2-x/2) solid solutions (with x = 0.09 and 0.16). For CeO(2), either the partial order related to the proton activity (n = 0.63) or the activation energy (E(A) = 37 kJ·mol(-1)) suggested that the dissolution was mainly driven by surface reactions occurring at the solid-liquid interface. The chemical durability of the cerium-neodymium oxides was also strongly affected by chemical composition. The initial normalized dissolution rates were also found to slightly depend on the crystallization state of the powders, suggesting the role played by the crystal defects in the dissolution mechanisms. On the contrary, the crystallite size had no important effect on the chemical durability. Finally, the normalized dissolution rates measured near the establishment of saturation conditions were less affected, which may be due to the formation of a gelatinous protective layer at the solid/liquid interface.