Oxygen ion diffusion in RE3TaO7: Why long-range migration of O2− is prohibited in the defective-fluorite structure?

Oxides with low O ²⁻ lattice diffusion rate are critical for the development of topcoat materials for next generation thermal barrier coatings (TBCs) working at > 1600 °C to prevent the fast growth of thermally grown oxide (TGO) at the bondcoat/topcoat interface. Here we delve into a comprehensive analysis of the oxygen ion transport characteristics of the recently developed low-, medium- and high-entropy rare earth tantalates (RE ₃TaO ₇) for TBCs by a combination of impedance spectroscopy, ¹⁸O isotope diffusion tests and atomic-scale simulations. Results show that the RE ₃TaO ₇ series display remarkably low oxygen ion diffusion rate (> 3 orders of magnitude lower than the state-of-the-art topcoat material yttria stabilized zirconia, YSZ) despite the presence of abundant oxygen vacancies in the structure. Molecular dynamic (MD) simulations combined with first principles calculations reveal that oxygen ions are strongly trapped by the potential well at the Ta-Ta edge in the tantalates. In addition, the high electrostatic potential (EP) surrounding oxygen vacancies and the high diffusion barriers between rare earth elements also impede oxygen ion diffusion. With low oxygen ion diffusion rate, along with exceptional thermal and mechanical properties reported previously, RE ₃TaO ₇ are promising topcoat materials for next-generation TBCs working at higher temperatures. Moreover, this study also provides valuable insights for understanding the O ²⁻ lattice diffusion behaviour in RE ₃TaO ₇ with a defective fluorite structure, which may benefit the exploration of oxide-ion conductors.