Influence of order-disorder transition on the mechanical and thermophysical properties of A₂B₂O₇ high-entropy ceramics

The order-disorder transition (ODT) of A₂B₂O₇ compounds obtained enormous attention owing to the potential application for thermal barrier coating (TBC) design. In this work, the influence of ODT on the mechanical and thermophysical properties of dual-phase A₂B₂O₇ high-entropy ceramics was investigated by substituting Ce⁴⁺ and Hf⁴⁺ with different ionic radii on B-sites (Zr⁴⁺). The X-ray diffraction (XRD), Raman, and transmission electron microscopy (TEM) results show that r_A3+/r_B4+ = 1.47 is the critical value of ODT phase boundary with different doping B-site ion contents, and the energy dispersive spectroscopy (EDS) results further indicate the uniform distribution of elements. Interestingly, owing to the high intrinsic disorder derived from high-entropy effect, the A₂B₂O₇ high-entropy ceramics exhibit unreduced modulus (E₀ ≈ 230 GPa) and enhanced mechanical properties (HV ≈ 10 GPa, K_IC ≈ 2.3 MPa·m^0.5). A₂B₂O₇ high-entropy ceramics exhibit excellent thermal stability with relatively high thermal expansion coefficients (TECs) (Hf0.25, 11.20×10^-6 K^-1, 1000 ℃). Moreover, the matching calculation implied that the ODT further enhances the phonon scattering coefficient, leading to a relatively lower thermal conductivity of (La_0.25Eu_0.25Gd_0.25Yb_0.25)₂(Zr_0.85Ce_0.15)₂O₇ (1.48-1.51 W/(m·K), 100-500 ℃) compared with other components. This present work provides a novel composition design principle for high-entropy ceramics, as well as a material selection rule for high-temperature insulation applications.