High-Temperature Thermodynamics of Cerium Silicates, A-Ce2Si2O7, and Ce4.67(SiO4)3O

Abstract

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