Microstructural properties of K_0.8 Fe_1.6 S ₂, K_0.8 Fe_1.75 Se_2-y S_y (0 ≤ y ≤ 2) and K_0.8 Fe_1.5+x S₂ (0 < x ≤ 0.5) single crystals

The structural features of the antiferromagnetic K_0.8Fe _1.6S₂ have been studied in the temperature range from 300 K up to 700 K by means of in situ transmission electron microscopy (TEM). The superstructure with a wave vector originating from a Fe-vacancy order has been clearly observed; moreover, the structural analysis shows that K _0.8Fe_1.6S₂ undergoes a transition from the Fe-vacancy order to disorder at about 585 K. The S substitution effect on the phase separation and superconductivity in the K_0.8Fe _1.75Se_2-yS_y 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 K_0.8Fe_1.75Se₂ superconductor. A phase-separated state formed by the coexistence of two Fe-vacancy orders with wave vectors and in K_0.8Fe_1.5+xS₂ (0 < x < 0.1) has been briefly discussed.