Structure and property correlations in FeS

For iron-sulfide (FeS), we investigate the correlation between the structural details, including its dimensionality and composition, with its magnetic and superconducting properties. We compare, theoretically and experimentally, the two-dimensional (2D) layered tetragonal (“t-FeS”) phase with the 3D hexagonal (“h-FeS”) phase. X-ray diffraction reveals iron-deficient chemical compositions of t-Fe_0.93(1)S and h-Fe_0.84(1)S that show no low-temperature structural transitions. First-principles calculations reveal a high sensitivity of the 2D structure to the electronic and magnetic properties, predicting marginal antiferromagnetic instability for our compound (sulfur height of z_S = 0.252) with an ordering energy of about 11 meV/Fe, while the 3D phase is magnetically stable. Experimentally, h-Fe_0.84S orders magnetically well above room temperature, while t-Fe_0.93S shows coexistence of antiferromagnetism at T_N = 116 and filamentary superconductivity below T_c = 4 K. Low temperature neutron diffraction data reveals antiferromagnetic commensurate ordering with wave vector k_m = (0.25,0.25,0) and 0.46(2) µ_B/Fe. Additionally, neutron scattering measurements were used to find the particle size and iron vacancy arrangement of t-FeS and h-FeS. The structure of iron sulfide has a delicate relationship with the superconducting transition; while our sample with a = 3.6772(7) Å is a filamentary superconductor coexisting with an antiferromagnetic phase, previously reported samples with a > 3.68 Å are bulk superconductors with no magnetism, and those with a ≈ 3.674 Å show magnetic properties.