First-principles prediction of diffusion coefficients in off-stoichiometric tantalum carbide

The tantalum carbide TaC is a promising ceramic material for ultra-high-temperature applications. Diffusion coefficients in this carbide are critically important for the synthesis optimization and under service conditions. Unfortunately, they remain largely unknown because their experimental measurements are highly challenging. This work aims to understand the atomistic mechanisms of Ta and C diffusion in TaC and evaluate the diffusion coefficients by combining first-principles calculations with a statistical–mechanical model of point defects and a diffusion kinetics theory. We focus on practically important carbon-deficient compositions and high temperatures ranging between 2000 K and 4000 K. The calculations show that carbon diffusion is extremely fast and mediated by single-vacancy jumps on the carbon sublattice. Ta diffusion is much slower and occurs predominantly by atomic exchanges with divacancies with a smaller contribution of trivacancies. Deviations from the perfect stoichiometry accelerate carbon diffusion but have little effect on the rate of Ta diffusion.