Microstructural properties of K0.8 Fe1.6 S2, K0.8 Fe1.75 Se2-y Sy(0 ≤ y ≤ 2) and K0.8 Fe1.5+x S2(0 < x ≤ 0.5) single crystals

The structural features of the antiferromagnetic K0.8Fe1.6S2 have been studied in the temperature range from 300K up to 700K by means of in situ transmission electron microscopy (TEM). The superstructure with a wave vector q(1) = 1/5(3a* + b*) originating from a Fe-vacancy order has been clearly observed; moreover, the structural analysis shows that K0.8Fe1.6S2 undergoes a transition from the Fe-vacancy order to disorder at about 585K. The S substitution effect on the phase separation and superconductivity in the K0.8Fe1.75Se2-ySy materials has been systematically investigated by SEM and TEM structural analyses, as well as by electrical resistivity measurements. Our experimental results reveal that the S element adopts a homogeneous distribution in all investigated materials, and the essential phase-separation nature is very similar to what was observed in the K0.8Fe1.75Se2 superconductor. A phase-separated state formed by the coexistence of two Fe-vacancy orders with wave vectors q(1) = 1/5(3a*+b*) and q(3) = 1/4(3a*+b*) in K0.8Fe1.5+xS2 (0 < x < 0.1) has been briefly discussed. Copyright (C) EPLA, 2013