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 application as environmental barrier coatings (EBCs) for ceramic matrix composites (CMCs) is limited by their high thermal expansion coefficients (TECs ≥ 9.0×10⁻⁶ K⁻¹). High-entropy design provides a feasible route to tailor the thermal properties of RETaO₄; however, most reports focus on equimolar RETaO₄ high-entropy ceramics (HECs). In this study, a series of nonequimolar monoclinic-prime (m') RETaO₄ HECs are designed and synthesized to clarify the composition-structure-property relationships. The nonequimolar design is employed to regulate configurational complexity, polyhedral distortion, and lattice strain in m'-RETaO₄ within the same phase. Atomic-scale transmission electron microscopy (TEM) and geometric phase analysis (GPA) characterizations reveal the associated local distortion and lattice strain. These observations link nonequimolar 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 this is mainly attributed to 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 nonequimolar m'-RETaO₄ HEC (Sc_0.2Y_0.2Tm_0.2Ho_0.2Dy_0.1Gd_0.1)TaO₄ exhibits the lowest TEC of 6.2×10⁻⁶ K⁻¹ at 1500 °C, which is closer to that of SiC-based CMCs than the other compositions, indicating its potential as a candidate EBC material. This work proposes that polyhedral distortion and lattice strain are important structural factors for tailoring the thermal properties of nonequimolar RETaO₄ HECs, providing guidance for the design of complex oxides.