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

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

Chemical and environmental stability of monazite-cheralite solid solutions Ln1-2Ca Th PO4 (Ln = Pr, Nd; x = 0–0.15): A thermodynamic study

International audienceMonazite-cheralite ceramics are a promising waste form for actinides. To elucidate the longterm behavior of this matrix in aqueous solutions, this study measured thermodynamic data for Thrhabdophanes Ln 1-2x Ca x Th x PO 4 •nH 2 O (with Ln = Pr, Nd; x = 0-0.15) and the associated anhydrous monazite-cheralites Ln 1-2x Ca x Th x PO 4. Solubility experiments at 298 K and high temperature oxide melt solution calorimetry were combined for calculation of , and of Th-rhabdophanes and associated monazite-cheralites. Standard solubility constants were employed in a geochemical simulation using the PHREEQC software, the results of which confirmed the high chemical stability of the monazite-cheralite phases and supported their use as a specific conditioning matrix for the long-term immobilization of actinides

Thermochemistry of La1−x_{1-x}Lnx_{x}PO4_{4}-monazites (Ln = Gd, Eu)

Enthalpy of formation (ΔH°f) of single phase orthophosphate solid solutions La1-xLnxPO4 (Ln = Eu, Gd; 0<x<1) with monazite structure has been determined by high temperature oxide melt solution calorimetry. The results show a non-ideal behavior of the excess enthalphy of mixing of the solid solutions. This effect is more pronounced for the La1-xGdxPO4 solid solution series and can be attributed to the larger ion size mismatch between La and Gd. The resulting excess enthalpies of mixing given by the Margules interaction parameter of 2.5 ± 2.6 kJ·mol-1 and 11.4 ± 3.1 kJ·mol-1 for La:Eu and La:Gd cases respectively are in good agreement with data from previous modelling studies. The data are consistent with the formation of thermodynamically stable solid solution in the entire compositional range as is seen by the XRD and Raman measurements. These results are essential for validation of the thermodynamic models that are applied for characterization of the thermodynamic parameters and for the assessment of the long-term stability of monazite solid solution matrices as ceramics for immobilization of radionuclides in nuclear waste disposal

Synthesis, Crystal Structure, and Enthalpies of Formation of Churchite-type REPO 4 ·2H 2 O (RE = Gd to Lu) Materials

International audienc

Mechanical and structural properties of radiation-damaged allanite-(Ce) and the effects of thermal annealing

The onset of thermally induced, heterogeneous structural reorganization of highly radiation-damaged allanite-(Ce) begins at temperatures below 700 K. Three strongly disordered allanite samples (S74 20414: ~ 0.55 wt% ThO2, 22.1 wt% REE oxides, and maximum radiation dose 3.5 × 1018 α-decay/g; LB-1: ~1.18 wt% ThO2, 19.4 wt% REE oxides, and maximum radiation dose 2.0 × 1019 α-decay/g; R1: ~ 1.6 wt% ThO2, 19.7 wt% REE oxides, and maximum radiation dose 2.6 × 1018 α-decay/g) were step-wise annealed to 1000 K in air. Using orientation-dependent nanoindentation, synchrotron single-crystal X-ray diffraction (synchrotron XRD), X-ray powder diffraction (powder XRD), differential scanning calorimetry and thermogravimetric analysis (DSC/TG), mass spectrometry (MS), 57Fe Mössbauer spectroscopy and high-resolution transmission electron microscopy (HRTEM), a comprehensive understanding of the structural processes involved in the annealing was obtained. As a result of the overall increasing structural order, a general increase of hardness (pristine samples: 8.2–9.3 GPa, after annealing at 1000 K: 10.2–12 GPa) and elastic modulus (pristine samples: 115–127 GPa, after annealing at 1000 K: 126–137 GPa) occurred. The initially heterogeneous recrystallization process is accompanied by oxidation of iron, the related loss of hydrogen and induced stress fields in the bulk material, which cause internal and surface cracking after stepwise annealing from 800 to 1000 K. HRTEM imaging of the pristine material shows preserved nanometer-sized crystalline domains embedded in the amorphous matrix, despite the high degree of structural damage. The results show that hardness and elastic modulus are sensitive indicators for the structural reorganization proces