Revealing the low thermal conductivity of high-entropy rare-earth tantalates via multiscale defects analysis

Thermal barrier coatings (TBCs) materials can improve energy conversion efficiency and reduce fossil fuel use. Herein, the novel rare earth tantalates RETaO₄, as promising candidates for TBCs, were reassembled into multi-component solid solutions with a monoclinic structure to further depress the thermal conductivity via an entropy strategy. The formation mechanisms of oxygen vacancy defects, dislocations and ferroelastic domains associated with thermal conductivity are demonstrated by aberration-corrected scanning transmission electron microscopy. Compared to single-RE RETaO₄ and 8YSZ, the intrinsic thermal conductivity of (5RE_1/5)TaO₄ was decreased by 35% ~ 47% and 57% ~ 69% at 1200°C, respectively, which is likely attributed to the multi-scale phonon scattering from Umklapp phonon–phonon, point defects, domain structures and dislocations.  and low-temperature thermal conductivity are negatively correlated, as are E/κ and high-temperature thermal conductivity. Meanwhile, the high defects' concentration and lattice distortion in high-entropy ceramics enhances the scattering of transverse-wave phonons and reduces the transverse-wave sound velocity, leading to a decrease in the thermal conductivity and Young's modulus. In addition, 5HEC-1 has ultra-low thermal conductivity, moderate thermal expansion coefficients and high hardness among the three five-component high-entropy samples. Thus, 5HEC-1 with superior thermal barrier and mechanical properties can be used as a promising thermal insulating material.