Inorganic Chemistry, Vol.59, No.18, 13174-13183, 2020
Thermodynamics of CeSiO4: Implications for Actinide Orthosilicates
Zircon (ZrSiO4, I4(1)/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 AnSiO(4) are then determined with a discussion of future strategies for efficiently immobilizing Pu or minor actinides in the zircon structure.