Investigation of the Iron–Sulfide Phase Transformation in Nanoscale

Iron sulfides, polymorphous substances, play a key role in geology, marine systems, material science, and the life sciences. The crystal structures and phase transitions of iron–sulfur compounds are complex and elusive. In this study, we fabricated most known iron sulfides, mackinawite was the first reaction product, which declined gradually until it disappeared; the following Cubic FeS and troilite presented a characteristic of layer growth. Using high-resolution transmission electron microscopy, we directly observed for the first time the transition of cubic FeS into greigite and that of mackinawite into greigite. Greigite, which is extremely unstable in an oxygen-free H2S environment, was found to play an intermediate role in the transition into a more stable phase.