Microstructural properties of K <sub>0.8</sub> Fe <sub>1.6</sub> S <sub>2</sub> , K <sub>0.8</sub> Fe <sub>1.75</sub> Se <sub>2-y</sub> S <sub>y</sub> (0 ≤ y ≤ 2) and K <sub>0.8</sub> Fe <sub>1.5+x</sub> S <sub>2</sub> (0 < x ≤ 0.5) single crystals