Dose rate dependent cation & anion radiation enhanced diffusion in hematite

Abstract

Irradiation induced non-equilibrium point defect populations influence mass transport in oxides, which in turn affects their stability and performance in hostile environments. In this study, we combine theoretical predictions and experimental observations to examine the dose rate dependence (2 × 10⁻⁵ to 2 × 10⁻³ dpa s⁻¹) of mass transport in irradiated (400 keV Ar⁺ at 300 and 500 °C) hematite (α-Fe₂O₃). Both cation (⁵⁷Fe) and anion (¹⁸O) tracers are precisely embedded in an epitaxial single crystal hematite film deposited by molecular beam epitaxy. Atom probe tomography is used to experimentally measure isotope movement to determine diffusion coefficients, and a chemical rate-theory model is developed to interpret radiation enhanced transport. Radiation enhanced diffusivities are similar for both species, and are found to be at least 20 orders of magnitude faster than the Arrhenius extrapolation of high-temperature values. In the recombination dominant regime (300 °C, 2 × 10⁻⁴ dpa s⁻¹) the experimental results for the anion species deviate from the model, suggesting that the cation lattice limits the anion diffusivity in this regime. Ultimately, a strong dose rate dependence is observed driven by irradiation induced defect populations.