Dynamics of point defect formation, clustering and pit initiation on the pyrite surface

The collective behavior of point defects formed on the free surfaces of ionic crystals under redox conditions can lead to initiation of local breakdown by pitting. Here, we controllably generated sulfur vacancies on single crystal FeS2(100) through in vacuo annealing, and investigated the resulting evolution of surface chemistry using synchrotron x-ray photoelectron spectroscopy (XPS). By measuring the S 2p photoemission signal intensity arising from sulfur defects as a function of temperature, the enthalpy of formation of sulfur vacancies was found to be 0.1±0.03eV, significantly lower than the reduction enthalpy of bulk FeS2. Above 200°C, the created sulfur vacancies together with preexisting iron vacancies condensed into nm-scale defect clusters, or pits, at the surface, as evidenced by scanning tunneling microscopy (STM). We provide a mechanistic description for the initiation of pits that requires the concerted behavior of both the sulfur and iron vacancies, and validate this model with kinetic Monte Carlo (kMC) simulations. The model probes realistic length and time scales, providing good agreement with the experimental results from XPS and STM measurements. Our results mechanistically and quantitatively describe the atomic scale processes occurring at pyrite surfaces under chemically reducing environments, important in many natural and technological settings, ranging from its role as a passivating film in corrosion to its potential use as a photovoltaic absorber in solar energy conversion.