Atomistic Modeling of Sulfur Vacancy Diffusion Near Iron Pyrite Surfaces

Through density functional calculations, we investigated the diffusion of isolated sulfur vacancies (V S ) from the bulk of iron pyrite (cubic FeS 2 ) to the (100) and (111) surfaces. The influence of vacancy depth on the vacancy formation energy and the activation energy for vacancy diffusion are discussed. Significantly, we find that V S defects tend to migrate toward stoichiometric and sulfur-rich surfaces through sequential “intra-dimer” and “inter-dimer” hops. We find a pre-exponential constant ( D 0 ) of 9 × 10 –7 m 2 s –1 and an activation energy ( E ) of 1.95 eV for sulfur vacancy diffusion in bulk pyrite, corresponding to a vacancy diffusion coefficient D V = D 0 exp(− E / kT ) = 9 × 10 –40 m 2 s –1 at 25 °C and 5 × 10 –18 m 2 s –1 at 600 °C. The activation energy is smaller near the surface (e.g., E = 1.5 eV near the stoichiometric (100) surface), resulting in faster vacancy diffusion near the surface than in the bulk. Using the formation enthalpy of V S at the (100) surface, E = 2.37 eV, we find a sulfur diffusivity in bulk pyrite D S = 7 × 10 –47 m 2 s –1 at 25 °C and 2 × 10 –20 m 2 s –1 at 600 °C. The calculated D S values are in reasonable agreement with experiment only at intermediate temperatures (∼275–625 °C). Our results show that bulk and near-surface sulfur vacancies can be healed in sulfur-rich conditions at reasonably high temperatures. The mechanism of vacancy diffusion presented here should be useful in managing V S defects during the fabrication of high-quality pyrite samples for solar energy conversion applications.