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The binary Fe chalcogenides FeSe and FeS are two superconductors which were discovered in recent years and have attracted a lot of attention in physical, chemical and materials research fields. In this work, we have synthesized FeS1−xSex solid solution crystals with the whole x range (0 ≤x≤ 1), where the S-rich part (0.1 ≤x ＜ 0.5) is reported for the first time so far. A hydrothermal route was carried out by de-intercalating K from the K0.8Fe1.6(S1−xSex)2 precursor and FeS1−xSex crystal sheets were thus obtained. Powder X-ray diffraction shows that the tetragonal lattice is maintained from x = 0 to 1 in this solid solution, and the selected area electron diffraction indicates the S/Se mixed occupation without any superstructure. As the Se content x increases, the lattice parameters expand due to the larger radius of the Se anion. Magnetic susceptibility demonstrates that with x≤ 0.3, FeS1−xSex crystals superconduct with the critical temperature basically unchanged (∼4 K). On the other hand, the Se-rich part of FeS1−xSex crystals (x≥ 0.6) exhibits an antiferromagnetic transition below 50 K, which could be attributed to the oxygen incorporation during the synthesis. Resistance measurement shows that the nematic order transition which was reported for FeSe in previous studies is absent in FeS1−xSex (x≤ 0.3) superconductors. This suggests that the superconducting mechanism in FeS-based chalcogenides may differ from that of FeSe. [ABSTRACT FROM AUTHOR]

Abstract Investigation on the magnetic interaction in Fe-based ThCr 2 Si 2 -type compounds is very important not only for understanding the structure-property relationship in this family but also for exploring the superconducting mechanism in related materials. In this work, we report the synthesis and physical properties of two new Fe-based chalcogenides KFeAgSe 2 and KFeAgS 2. These two compounds adopt ThCr 2 Si 2 -type structure with I 4/ mmm space group and contain the Fe/Ag mixed occupation in the centre of edge-sharing Fe/AgCh 4 tetrahedron (Ch = Se, Te). The lattice parameters are refined to be a = 4.1866(2) Å, c = 13.7731(4) Å and a = 3.9845(3) Å, c = 13.3610(4) Å for KFeAgSe 2 and KFeAgS 2 respectively. Both two compounds show semiconducting behavior with electrical resistivity increased as the temperature decreased, while the resistivity of KFeAgS 2 is much larger (~61 Ω cm at room temperature) than that of KFeAgSe 2 (~2.5 Ω cm at room temperature). Most interestingly, despite the similar electronic structure of these two compounds, KFeAgSe 2 features a long-range antiferromagnetic order below 26 K while KFeAgS 2 shows a spin-glass state below 19 K. It is indicated that the long-range magnetic ordering can be suppressed in these ThCr 2 Si 2 -type transition metal chalcogenides via compressing the unit cell, that is to say, by chemical pressure. Our work not only gives a new sight to understand the structure-magnetism relationship in ThCr 2 Si 2 -type transition metal chalcogenides, but also provides clue to explore the mechanism of superconductivity in Fe-based family. Graphical abstract The layered chalcogenide KFeAgSe 2 adopting ThCr 2 Si 2 -type structure features a long-range antiferromagnetic order (AFM) below 26 K, while the magnetic interaction turns to be spin-glass state (SG) in isostructural KFeAgS 2 below 19 K. fx1 Highlights • Two new layered selenides KFeAgSe 2 and KFeAgS 2 were synthesized. • They show ThCr 2 Si 2 -type structure with edge-sharing Fe(Ag)Se 4 and Fe(Ag)S 4 tetrahedron. • KFeAgSe 2 shows antiferromagnetism while KFeAgS 2 shows spin-glass state in low temperature range. • This transition on magnetism is suggested to be drove by a aixis of the lattice. [ABSTRACT FROM AUTHOR]

Previous study suggested that Se-site doping with Te or S in tetragonal FeSe adjusts the superconductivity only by expanding or compressing the lattice parameters. However, three chalcogens X ( X = S, Se, Te) show different electronegativity, which could directly change the interaction between Fe & X , and their electronic structures. In order to understand this underlying mechanism, the samples with different anion compositions but the same lattice parameters are required. In this work, two superconductors: FeSe 0.95 Te 0.03 S 0.02 and FeSe 0.9 Te 0.057 S 0.043 have been synthesized by solid state reaction. The thus-obtained samples have the same lattice parameters in tetragonal single phase as that of undoped FeSe. It is found that despite the unchanged lattice, S/Te co-doping on Se-site still increased the density of the electronic states (DOS) near the Fermi level, thereby enhanced the superconductivity. Our results suggest that the influence of Se-site doping with the same valence chalcogen in superconducting FeSe is not simply in the view of the lattice size, effects on electronic structures originated from different electronegativity during Se-site doping should also be considered. [ABSTRACT FROM AUTHOR]