Ferroelastic Rare-Earth Tantalates: Physical Mechanisms and Multilayer Architectures for Extreme Solar Thermal Protection

The Solar Close Observations and Proximity Experiments (SCOPE) mission imposes unprecedented thermal challenges on advanced protection systems, necessitating sustained resistance to temperatures exceeding 2000°C and radiative heat fluxes above 2.52 MW[Formula: see text]. However, current experimental validations for candidate ceramic systems are primarily limited to the 1200.1600 °C range. This critical review systematically evaluates rare-earth tantalates (RETs) as a transformative class of materials for next-generation thermal/environmental barrier coatings (T/EBCs). We comprehensively analyze the diverse crystallographic stoichiometries of RETs—including ferroelastic [Formula: see text]-RETaO 4 , weberite/defect-fluorite RE 3 TaO 7 , and perovskite-like RETa 3 O[Formula: see text] and their correlation with thermophysical and fracture-related properties. RETs exhibit intrinsically low thermal conductivity, with reported values in the range of [Formula: see text] W[Formula: see text] ⋅K[Formula: see text] at 800.1200°C depending on composition, density, and processing route, which approaches the theoretical amorphous lower bound predicted by Cahill’s minimum thermal conductivity model. Additionally, RETs show engineered coefficients of thermal expansion ([Formula: see text]) and ferroelastic domain-switching toughening, yielding fracture toughness values of approximately 2−3 MPa⋅m[Formula: see text]. In stark contrast to conventional yttria-stabilized zirconia, RETs demonstrate clear advantages in high-temperature phase stability and resistance to CMAS-induced degradation. This work elucidates the dominant thermo-mechanical mechanisms based on fundamental crystal physics: from anharmonic phonon scattering induced by mass-difference and high-entropy lattice distortion, to energy dissipation via ferroelastic domain switching as predicted by co-elastic theory. Furthermore, we highlight the role of chemical stability, specifically the formation of apatite phases during CMAS interactions and the inherently low oxygen ionic conductivity ([Formula: see text] 10[Formula: see text] S⋅cm[Formula: see text]). Finally, the review addresses scalability through high-entropy engineering—leveraging the “cocktail effect” for stability—and proposes advanced multilayer architectures (e.g., C/C-Ir-RET-Al[Formula: see text]) tailored for SCOPE-specific extremes. This synthesis establishes RETs as pivotal candidates for ultra-high-temperature aerospace applications and identifies future directions for compositional tailoring and cross-scale simulations.