Manipulating RE‐O Bonds Engineered Thermal/Oxygen Transport Characteristics of Monoclinic‐Prime RETaO 4 High‐Entropy Ceramics

ABSTRACT Multifunctional thermal protective ceramic thermal/environmental barrier coatings (T/EBCs) are of great significance for applications of various high‐temperature components, and we propose an effective strategy for designing high‐entropy ceramics (HECs) as T/EBCs for SiC ceramic matrix composites (CMCs), where low thermal expansion coefficients (TECs ≤ 7 × 10 −6 K −1 ) and thermal conductivity (1.0 W m −1 K −1 ) as well as oxygen ionic conductivity (≤ 10 −5 S cm −1 ) are needed. The monoclinic‐prime (m′) (ScYTmHoDyGd) 1/6 TaO 4 6HECs are designed based on manipulating RE–O bonds, and the misfits in RE 3+ ionic radius and atomic weight result in high lattice strains and dislocations, which service as the dominators for engineering their thermophysical properties. The low TECs of RETaO 4 are governed by the highly distorted m′‐phase crystal, when the free of intrinsic oxygen vacancies and sluggish diffusion effect of HECs have synergistically resulted in the low oxygen ionic conductivity (20.29 × 10 −6 S cm −1 ), which can slow oxygen transport and delay the oxidation of bond coat and SiC CMCs. Atom‐scale characterizations reveal that fluctuating RE–O bonds and dislocations induce high lattice strains, which enhance phonon scattering, reduce phonon propagation speed, and depress thermal conductivity. Manipulating RE–O bonds can effectively regulate thermal/oxygen transport of HECs, and this mechanism may be extended to various ceramics.