Electronic states of intrinsic surface and bulk vacancies in FeS2

Abstract

Understanding the stability and reactivity of iron sulfide phases is key to developing predictive capabilities for assessing their corrosion and catalytic activity. The differences between the free surface and the bulk interior of such phases are of particular importance in this context. Here, we employ density functional theory to investigate the formation energetics and electronic structure of intrinsic Fe and S vacancies in bulk pyrite (FeS2) and on the pyrite (100) surface. The formation energies of intrinsic bulk vacancies of all charge states are found to be high, ranging from 1.7 to 3.7 eV. While the formation energies of surface vacancies are lower, varying from 1.4 to 2.1 eV for S vacancies and from 0.3 to 1.7 eV for Fe vacancies, they are too large to result in significant sub-stoichiometry in bulk pyrite at moderate temperatures. On the basis of charged defect formation energies and defect equilibria calculations, intrinsic charge carriers are expected to outnumber point defects by several orders of magnitude, and therefore, pure pyrite is not expected to demonstrate p-type or n-type conductivity. The presence of surface states is observed to cause a reduction in the band gap at the (100) surface, which was captured computationally and experimentally using tunneling spectroscopy measurements in this work. The vacancy-induced defect states behave as acceptor-like or donor-like defect states within the bulk band gap. The findings on the stoichiometry and the electronic structure of active sites on the (100) surface have important implications for the reactivity of pyrite.