The Chemistry of Tetragonal FeS

Research into tetragonal FeS_m, the synthetic equivalent of the mineral mackinawite, is currently at the frontiers of theoretical and applied chemistry. FeS_m 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 FeS_m 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 FeS_m varieties with technologically valuable properties. FeS_m displays unconventional superconductivity (T_c ∼ 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. FeS_m can be highly efficient in sequestering a large array of environmentally deleterious inorganic and organic compounds including halogenated hydrocarbons. However, FeS_m nanoparticles are genotoxic and this needs to be further investigated before they are widely distributed in the environment or used for medical purposes.