Superconductivity and magnetism in iron sulfides intercalated by metal hydroxides

Abstract

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−xFe_xOH)FeS, [(Na_1−xFe_x)(OH)₂]FeS, and K_xFe_2−yS₂. 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−xFe_xOH)^δ+ intercalate. Layered heterostructures of [(Na_1−xFe_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−xFe_x)(OH)₂]FeS displays long-range magnetic ordering near 15 K, K_xFe_2−yS₂ displays short range antiferromagnetism.