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.

中文翻译：

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硅酸铈，A-Ce ₂ Si ₂ O ₇和Ce _4.67（SiO ₄）₃ O的高温热力学

镧系元素二硅酸盐和羟磷灰石在高温应用中具有潜在的作用，如热（TBC）和环境屏障涂层（EBC）或地质核废料库中可能的蚀变阶段。然而，仅对那些含Ce ^3+的硅酸盐进行了有限的研究。在这项工作中，我们对A-Ce ₂ Si ₂ O ₇（四边形，P 4 ₁）和Ce _4.67（SiO ₄）₃ O（六边形，P 6 ₃ / m）进行了详细的结构和热力学研究。高温结构行为和热膨胀系数由下式确定原位高温同步加速器X射线衍射（HT-XRD）结合Rietveld分析和热重分析与差示扫描量热法（TGA-DSC）一起实施。A-CE ₂的Si ₂ ö ₇被发现是在N-稳定₂和空气到~1483 K的沿各向同性热膨胀一-和Ç -axes（α_一个= 12.3×10 ^-6 ķ ^-1和α _c = 12.4×10 ^–6 K ^–1）。铈_4.67（SiO ₄）₃O在533至873 K之间缓慢氧化为新的非化学计量相Ce ³⁺ _{1.67- x} Ce ⁴⁺ _x Ce ³⁺ ₃（SiO ₄）₃ O _{1 + 0.5 x}，然后热分解为CeO ₂和SiO ₂在约1000 K的空气中。通过使用硼酸铅作为溶剂在973 K下使用高温氧化物熔体溶液量热法，确定了A-Ce ₂ Si ₂ O ₇（-3825.1±6.0 kJ / mol）和Ce _4.67（SiO ₄）的标准形成焓。₃O（-7391.3±9.5 kJ / mol）。将这些热力学参数与CeO ₂，CeSiO ₄和其他硅酸盐氧基磷灰石的热力学参数进行比较，以检查它们在与航空应用，矿物形成和核燃料循环相关的高温环境中的化学稳定性。