Characterization and Simulation of Natural Pyrite Surfaces: A Combined Experimental and Theoretical Study

The electronic structure of a natural pyrite sample from Navajún, Spain was studied using both core-level x-ray and valence band ultraviolet photoelectron spectroscopies (XPS and UPS, respectively). High-resolution XPS spectra of the S-2p and Fe-2p photoelectron regions for a cut surface of natural pyrite are used to characterize the chemical state of the surface after (i) exposure to atmosphere, (ii) dry polishing, and (iii) Ar + ion sputtering under ultra high vacuum conditions. The polished samples are predominantly stoichiometric pyrite (FeS 2 ), while sputtering preferentially removes sulfur, leading to the formation of abundant sulfur-reduced phases. Additionally, density-functional theory (DFT) calculations that include Hubbard- U corrections on the Fe-3d electrons are applied to compute iron sulfide electronic structures. The partial density of states within the uppermost approximately 8 Å are weighted with the photoionization cross sections to produce approximate UPS spectra. The DFT+ U calculations were performed on pristine and S vacancy containing FeS 2 (001) surfaces, as well as the sulfur-reduced FeS(001), Fe 7 S 8 (001), and Fe 3 S 4 (001) phases. These are compared to the experimentally collected UPS spectra, and it is shown that sputtered surfaces are best described by the reduced phases.