Superconductivity and magnetism in Iron Sulfides Intercalated by Metal Hydroxides

Inspired by naturally occurring sulfide minerals, we present a new family of iron-based superconductors. A metastable form of FeS known as the mineral mackinawite forms two-dimensional sheets that can be readily intercalated by various cationic guest species. Under hydrothermal conditions using alkali metal hydroxides, we prepare three different cation and metal hydroxide-intercalated FeS phases including (Li$_{1-x}$Fe$_x$OH)FeS, [(Na$_{1-x}$Fe$_x$)(OH)$_2$]FeS, and K$_x$Fe$_{2-y}$S$_2$. Upon successful intercalation of the FeS layer, the superconducting critical temperature $T_c$ of mackinawite is enhanced from 5 K to 8 K for the (Li$_{1-x}$Fe$_x$OH)$^{\delta+}$ intercalate. Layered heterostructures of [(Na$_{1-x}$Fe$_x$)(OH)$_2$]FeS resemble the natural mineral tochilinite, which contains an iron square lattice interleaved with a hexagonal hydroxide lattice. Whilst heterostructured [(Na$_{1-x}$Fe$_x$)(OH)$_2$]FeS displays long-range magnetic ordering near 15 K, K$_x$Fe$_{2-y}$S$_2$ displays short range antiferromagnetism.