Surface defects and self-diffusion on pyrite {100}: An ultra-high vacuum scanning tunneling microscopy and theoretical modeling study

Various defects on {100} cleavage surfaces of pyrite (FeS2) are observed di rectly using ultra high vacuum scanning tunneling microscopy. Step edges ar e aligned along [10] and [11] surface directions. Atomic scale images indic ate that the atomic structure, with respect to the Fe lattice, and local de nsity of occupied states is unchanged at a step edge, including kink and co rner sites. The inferred presence of monosulfides at step edges, based on X -ray photoelectron spectra on similar surfaces elsewhere, does not lead to occupied states higher in energy than d(z2) dangling bond states at Fe site s. A sequence of consecutive images at the atomic scale captured evidence of d ynamic structural changes at defects on this surface at room temperature. S tep edges appear generally stable over the course of the STM observations, whereas vacancies, their surrounding sites, and corner step edge sites are not. Theoretical maps of the attachment energy for an Fe adatom over a {100 } surface cell indicate the presence of low energy diffusion channels along the topology of the closest S atoms in the uppermost atomic S monolayer. C alculation of the activation energy barriers for the self-diffusion of an F e adatom over a {100} terrace predict low 0.10 eV diffusion barriers along channels and 0.24 eV across channels. Subsequently, calculated Fe adatom mo bilities over the time scale of the STM observations are very high, ranging from 10(5)-10(6) Angstrom over the course of one minute, calculated for ro om temperature and depending on the diffusion direction. The structural cha nges documented in the STM images are attributed to the natural process of surface self-diffusion.