High-Temperature Thermodynamics of Cerium Silicates, A‑Ce2Si2O7, and Ce4.67(SiO4)3O
journal contributionposted on 30.10.2020, 15:07 by Andrew C. Strzelecki, Kyle Kriegsman, Paul Estevenon, Vitaliy Goncharov, Jianming Bai, Stephanie Szenknect, Adel Mesbah, Di Wu, John S. McCloy, Nicolas Dacheux, Xiaofeng Guo
Lanthanide disilicates and oxyapatites have potential roles in high-temperature applications as thermal (TBC) and environmental barrier coatings (EBC) or possible alteration phases in geological nuclear waste repositories. However, those Ce3+-bearing silicates have only been limitedly studied. In this work, we performed detailed structural and thermodynamic investigations on A-Ce2Si2O7 (tetragonal, P41) and Ce4.67(SiO4)3O (hexagonal, P63/m). The high-temperature structural behaviors and coefficients of thermal expansion were determined by in situ high-temperature synchrotron X-ray diffraction (HT-XRD) implemented with Rietveld analysis and thermogravimetric analysis coupled with differential scanning calorimetry (TGA-DSC). A-Ce2Si2O7 was found to be stable in N2 and air up to ∼1483 K with an isotropic thermal expansion along the a- and c-axes (αa = 12.3 × 10–6 K–1 and αc= 12.4 × 10–6 K–1). Ce4.67(SiO4)3O had a slow partial oxidation between 533 and 873 K to a new nonstoichiometric phase Ce3+1.67‑xCe4+xCe3+3(SiO4)3O1+0.5x, followed by a thermal decomposition to CeO2 and SiO2 at ∼1000 K in air. By using high temperature oxide melt solution calorimetry at 973 K with lead borate as the solvent, the standard enthalpy of formation was determined for A-Ce2Si2O7 (−3825.1 ± 6.0 kJ/mol) and Ce4.67(SiO4)3O (−7391.3 ± 9.5 kJ/mol). These thermodynamic parameters were compared with those of CeO2, CeSiO4, and other silicate oxyapatites for examining their chemical stability in high-temperature environments relevant for aeronautical applications, mineral formation, and nuclear fuel cycle.