  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.

Superconductivity