Abstract
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.
Highlights
Rare earth orthophosphates (REPO4 · n H2O, where RE is a rare earth element, i.e., lanthanide plus yttrium and scandium) are widespread minerals
Analysis of the powder X-ray diffraction (PXRD) patterns (Figure 1A) confirms that all the synthesized samples were single phases crystallizing with the rhabdophane structure type (Mesbah et al, 2014, 2017)
The unit cell volume of rhabdophanes evolves linearly with increase of the ionic radius from La to Gd, which is in agreement with earlier observations (Mesbah et al, 2014, 2017)
Summary
Rare earth orthophosphates (REPO4 · n H2O, where RE is a rare earth element, i.e., lanthanide plus yttrium and scandium) are widespread minerals Their hydrated forms are commonly known as rhabdophane (Mooney, 1948, 1950; Mesbah et al, 2014) and churchite (Kohlmann et al, 1994), while their anhydrous forms are monazite (Clavier et al, 2011) and xenotime (Ni et al, 1995). These phosphate minerals are a primary source of rare earths and thorium. This radiation resistance makes monazite ceramics promising candidates for the specific immobilization of tetravalent and trivalent actinides coming from the reprocessing of spent nuclear fuel or from the management of plutonium (Ewing, 1999)
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