Unveiling the mechanisms of ultra-low thermal and oxygen-ion conductivity in entropy-stabilized ferroelastic rare-earth tantalates

Most current thermal and environmental barrier coating materials, even with high relative densities, exhibit high oxygen-ion conductivity, which accelerates the oxidation of the substrate and ultimately results in coating failure. Herein, high-entropy rare-earth tantalates ((6RE_1/6)TaO₄) are presented with lattice distortion that causes local disruption of the ordered stacking of oxygen-ions and cations, thereby resulting in reduced oxygen-ion and thermal conductivities. The oxygen-ion conductivity of (6RE_1/6)TaO₄ decreased by three to five orders of magnitude compared with that of 8YSZ and was further decreased by the high-entropy effect compared with that of single-RE RETaO₄ from 600 to 900 ℃, which is the result of strong bond strength, severe lattice distortion, high oxygen vacancy clusters and a high concentration of immobile vacancies. Furthermore, the intrinsic thermal conductivity of (6RE_1/6)TaO₄ is 14.3∼40.5 % less than single-RE RETaO₄ and ∼65.3 % lower than that of 8YSZ at 1200 °C. It also presents lower intrinsic thermal conductivity across the temperature range of 100 to 1200 °C. This is the result of scattering by Umklapp processes, oxygen vacancies, lattice distortions, ferroelastic domains and dislocations. Entropy-stabilized (6RE_1/6)TaO₄ also has excellent thermal stability and mechanical properties, thereby making it a promising thermal and environmental barrier coating material.