Sonochemical synthesis of mackinawite and the role of Cu addition on phase transformations in the Fe–S system

We studied the influence of Cu addition on the first precipitate, near-amorphous mackinawite- type FeS and subsequent phase transformations in the Fe-S system. Despite many studies on the formation and phase transition of Fe sulphides, the structure and physical properties of this first precipitate are still not well explained. To investigate the reaction products, iron and copper chlorides were mixed in different ratios together with sulphur and diethanolamine, and sonicated to achieve efficient mixing; already at this stage, the first FeS precipitate was formed. TEM investigation of Cu-doped mackinawite-like FeS showed enhanced crystallinity accompanied with expansion of the unit cell along the c-axis, propor- tional to the amount of Cu adsorbed between the (001) layers of the mackinawite structure. The subsequent solvothermal treatment and sulphurization of undoped FeS resulted in formation of pyrite, at low doping Cu- rich mackinawite and cubic (Fe, Cu)S with a sphal- erite-type structure were formed, while at higher Cu concentrations the end products were chalcopyrite and bornite, corresponding to the initial amount of Cu in the reagents.