Thermodynamics of CeSiO₄: Implications for Actinide Orthosilicates

Abstract

The mineral zircon (ZrSiO 4 : I4 1 /amd) can accommodate natural actinides, such as thorium and uranium. The zircon structure has also been obtained 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 (CeSiO 4), a surrogate of PuSiO 4. For the first time, the standard enthalpy of formation of CeSiO 4 was obtained by high temperature oxide melt solution calorimetry to be-1971.9 ± 3.6 kJ/mol. Stetindite is energetically metastable with respect to CeO 2 and SiO 2 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 CeSiO 4 from this work, along with those previously reported for USiO 4 and ThSiO 4 , we developed an empirical energetic relation for actinide orthosilicates. The predicted enthalpies of formation of AnSiO 4 are then made with a discussion of future strategies to efficiently immobilize Pu or minor actinides in the zircon structure.

The Role of Water and Hydroxyl Groups in the Structures of Stetindite and Coffinite, MSiO4 (M = Ce, U)

“…The observed three-stage high temperature behavior of stetindite revealed by XRD (section 3.1) is in excellent agreement with the in situ high temperature Raman spectroscopy, the FTIR spectroscopy performed on the calcinated samples, and the previous TGA-DSC analysis by Strzelecki et al 23 Indeed, these authors suggested that a portion of water was energetically more strongly associated with stetindite than that of surface adsorbed water as it was not fully removed until around 700 °C, 23 well corresponding to the first and second regimes (25 to 634 °C) shown by the XRD result (Figure 2a). The high temperature needed for removing this water also indicates the additional energetic contributions arise from the waterlattice interaction due to confinement that helps overcome the enthalpic penalty encountered by the anhydrous phases.…”

“…Likewise, uranothorite (UxTh1-xSiO4) and stetindite (CeSiO4), which are isostructural to coffinite (I41/amd, Figure 1), exhibit similar hydration dependencies that again were revealed by calorimetry. 23,24 Investigations into the structural and thermodynamic properties of coffinite, uranothorite, and stetindite have also significant implications outside the fields of geochemistry and mineralogy, because these mineral phases are considered as potential ceramic waste forms for the immobilization of actinides. [25][26][27][28][29][30] Indeed, zircon (ZrSiO4) has demonstrated advantages of extreme chemical and physical durability across geologic time, [31][32][33][34][35][36] implying considerable durability as an actinide host.…”

“…[48][49][50][51][52] Thus, a fundamental understanding of the thermal behavior of such solid solution is needed. Previous thermogravimetric analysis (TGA) coupled with differential scanning calorimetry (DSC) on stetindite performed by Strzelecki et al (2020) 23 showed a sluggish dehydration from room temperature to 900 °C. 23 However, the structural information accompanying the dehydration process and beyond that dictates the measured mass and energetic events is largely lacking.…”

“…Previous thermogravimetric analysis (TGA) coupled with differential scanning calorimetry (DSC) on stetindite performed by Strzelecki et al (2020) 23 showed a sluggish dehydration from room temperature to 900 °C. 23 However, the structural information accompanying the dehydration process and beyond that dictates the measured mass and energetic events is largely lacking. Similarly, the calorimetric studies reported by Guo et al 22 and Reynolds et al 53 showed that there may be two water-bonded sites associated with coffinite.…”

Thermodynamic non-ideality and disorder heterogeneity in actinide silicate solid solutions

“…Uranothorite, coffinite, and thorite all crystallize in the zircon structure (I4 1 /amd), MSiO 4 , isostructural with zircon (Fig. 1), where the large MO 8 polyhedra can be occupied by many tetravalent cations, such as transition metals (Zr, Hf), lanthanides (Ce), and actinides (Th, U, Np, Pu) [1][2][3]5,[8][9][10][11][12][34][35][36][37][38][39][40][41][42][43][44][45] . This is due to the high oxygen coordination of the MO 8 site that favors the presence of large cations, following Pauling's rules 46 .…”