Non-equimolar compositional design engineered thermal expansion coefficients and conductivities of m´-RETaO₄ (RE = Sc, Y, Tm, Ho, Dy, Gd) high-entropy ceramics

Rare-earth tantalates RETaO₄ have been extensively investigated as thermal protective materials, but their applications as environmental barrier coatings (EBCs) for ceramic matrix composites (CMCs) are limited by their high thermal expansion coefficients (TECs≥9.0×10^-6 K^-1). The high-entropy design provides a feasible route to tailor the thermal properties of RETaO₄; however, most reports focus on equimolar RETaO₄ HECs. In this study, a series of non-equimolar monoclinic-prime (m´) RETaO₄ HECs are designed and synthesized to clarify the composition-structure-property relationships. The non-equimolar design is used to regulate configurational complexity, polyhedral distortion, and lattice strain in m´-RETaO4 within the same m´ phase. Atomic-scale TEM and GPA characterizations reveal the associated local distortion and lattice strain. These observations link non-equimolar composition with local structural distortion and macroscopic thermal transport behavior. The lowest thermal conductivity reaches 1.52-2.68 W·m^-1·K^-1 at 25-900 °C, and is mainly associated with RE-site disorder, lattice strain, and polyhedral distortion. Polyhedral distortion also suppresses thermal expansion by restricting atomic anharmonic vibrations. Among the designed compositions, the optimized non-equimolar m´-RETaO₄ HECs (Sc_0.2Y_0.2Tm_0.2Ho_0.2Dy_0.1Gd_0.1)TaO₄ exhibits the lowest TECs of 6.2×10^-6 K^-1 at 1500 °C, closer to SiC-based CMCs than the other compositions, indicating its potential as candidate EBCs.  This work proposes that polyhedral distortion and lattice strain are important structural factors for tailoring the thermal properties of non-equimolar RETaO₄ HECs, providing guidance for the design of complex oxides.