High-Temperature Thermodynamics of Cerium Silicates, A‑Ce₂Si₂O₇, and Ce_4.67(SiO₄)₃O

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 Ce_4.67(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 (α_a = 12.3 × 10^–6 K^–1 and α_c= 12.4 × 10^–6 K^–1). Ce_4.67(SiO₄)₃O had a slow partial oxidation between 533 and 873 K to a new nonstoichiometric phase Ce³⁺_1.67‑xCe⁴⁺_xCe³⁺₃(SiO₄)₃O_1+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 Ce_4.67(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.