Harmonic transition state theory applied to vacancy diffusion pre-exponential factors in a concentrated solid-solution alloy

High-entropy alloys are solid solutions composed of five or more elements in near-equimolar proportions that demonstrate a number of unusual properties that have yet to be fully explained. Among these, the origin and existence of diffusion qualified as sluggish have been debated since the first measurements of diffusion in disordered systems became available. To better understand the potential role of entropy in this phenomenon, we analyze vacancy diffusion in a ternary concentrated solid-solution alloy, FeNiCr. This is done through atomistic simulations using the kinetic activation-relaxation technique, an off-lattice kinetic Monte Carlo algorithm combined with an embedded atom method potential. Through an analysis of millions of activated events, we compare the kinetics of a vacancy when the activation prefactor is computed specifically within the harmonic approximation to a system where activated prefactors are set at a constant value, regardless of the environment. This allows us to identify the role of disorder on energetic barriers and prefactor distributions, particularly in the case of defect kinetics. More precisely, through the emerging statistical evidence of a compensation between the barrier and prefactor, we show that disorder strongly perturbs the system's vibrational entropy, contributing to explain sluggish diffusion.