High-Temperature Thermodynamics of Cerium Silicates, A-Ce2Si2O7, and Ce4.67 (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 Ce(3+)bearing silicates have only been limitedly studied. In this work, we performed detailed structural and thermodynamic investigations on A-Ce2Si2O7 (tetragonal, P4(1)) and Ce 4.67 (SiO4)(3)O (hexagonal, P6(3)/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). ACe(2)Si(2)O(7) was found to be stable in N-2 and air up to 1483 K with an isotropic thermal expansion along the a- and c-axes (alpha(a) = 12.3 x 10(-6) K-1 and alpha(c).= 12.4 X 10(-6) K-1). Ce-4.67 (SiO4)(3)O had a slow partial oxidation between 533 and 873 K to a new nonstoichiometric phase Ce1.67-x3+Cex4+Ce33+(SiO4)(3)O1+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)(3)O (-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.