The Chemistry of Tetragonal FeS

Research into tetragonal FeSm, the synthetic equivalent of the mineral mackinawite, is currently at the frontiers of theoretical and applied chemistry. FeSm is stoichiometric and crystallizes with a structure dominated by Fe-Fe layers. The familiar black, nanoparticulate precipitate develops from aqueous FeS clusters and displays varying initial compositions. Particle growth and crystallization are through oriented attachment of FeS nanoplates. Conflicting magnetic properties of FeSm result from itinerant Fe d-electrons in the ground state displaying some localization experimentally. It is highly sensitive to the method of synthesis and this has led to widespread irreproducible, and often conflicting, results. At the same time this sensitivity offers the opportunity to synthesize FeSm varieties with technologically valuable properties. FeSm displays unconventional superconductivity (Tc ∼ 5K) derived from spatial anisotropy of electron pairs. Exotic compounds can be inserted in the vdW gap between the FeS layers giving rise to a spectrum of interlayered compounds. FeSm can be highly efficient in sequestering a large array of environmentally deleterious inorganic and organic compounds including halogenated hydrocarbons. However, FeSm nanoparticles are genotoxic and this needs to be further investigated before they are widely distributed in the environment or used for medical purposes.