Thermodynamics of CeSiO₄: Implications for Actinide Orthosilicates

Zircon (ZrSiO₄, I4₁/amd) can accommodate actinides, such as thorium, uranium, and plutonium. The zircon structure has been determined for several of the end-member compositions of other actinides, such as plutonium and neptunium. However, the thermodynamic properties of these actinide zircon structure types are largely unknown due to the difficulties in synthesizing these materials and handling transuranium actinides. Thus, we have completed a thermodynamic study of cerium orthosilicate, stetindite (CeSiO₄), a surrogate of PuSiO₄. For the first time, the standard enthalpy of formation of CeSiO₄ was obtained by high temperature oxide melt solution calorimetry to be -1971.9 ± 3.6 kJ/mol. Stetindite is energetically metastable with respect to CeO₂ and SiO₂ by 27.5 ± 3.1 kJ/mol. The metastability explains the rarity of the natural occurrence of stetindite and the difficulty of its synthesis. Applying the obtained enthalpy of formation of CeSiO₄ from this work, along with those previously reported for USiO₄ and ThSiO₄, we developed an empirical energetic relation for actinide orthosilicates. The predicted enthalpies of formation of AnSiO₄ are then determined with a discussion of future strategies for efficiently immobilizing Pu or minor actinides in the zircon structure.