Phase Separation and Suppression of the Structural and Magnetic Transitions in Superconducting Doped Iron Tellurides, Fe_1+xTe_1−yS_y

Single crystal and powder samples of the series of iron chalcogenide superconductors with nominal composition, Fe_1.15Te_1−yS_y, are found to form for 0 ≤ y ≤ 0.15. They crystallize in the tetragonal anti-PbO structure, which is composed of layers of edge-shared Fe(Te, S)₄ tetrahedra. For y = 0, Fe_1+xTe (x ≈ 0.12(1)) is nonsuperconducting and undergoes a tetragonal (P4/nmm) to monoclinic (P2₁/m) structural transition at ∼65 K, associated with the onset of commensurate antiferromagnetic order at q = (0.5 0 0.5). We show that on sulfur substitution, Fe_1+xTe_1−yS_y becomes orthorhombic (Pmmn) at low temperature for 0 ≤ y ≤ 0.015, where the greatly suppressed magnetic scattering is now incommensurate at q = (0.5-δ 0 0.5) and possesses short ranged magnetic correlations that are well fitted with a two-dimensional Warren peak shape. At much higher concentrations of S (y ≥ 0.075), there is suppression of both the structural and magnetic transitions and a superconducting transition at 9 K is observed. Between these two composition regimes, there exists a region of phase separation (0.025 ≤ y ≤ 0.05), where the low temperature neutron diffraction data is best refined with a model containing both the tetragonal and orthorhombic phases. The increase in the amount of sulfur is found to be associated with a reduction in interstitial iron, x. Microprobe analysis of a single crystal of composition Fe_1.123(5)Te_0.948(4)S_0.052(4) confirms the presence of compositional variation within the crystals, rationalizing the observed phase separation.