Superconductivity on the verge of Mott localization in ternary iron   sulfide
  We report the results of electrical and magnetic properties on two new compounds, K0.8Fe1.7S2 and K0.8Fe1.7SeS, both having similar structures to newly discovered superconducting K0.8Fe1.7Se2. K0.8Fe1.7S2 exhibits a semiconductor-like electrical property and undergoes an anti-ferromagnetic transition at about 260 K. Upon replacing half of S with Se, K0.8Fe1.7SSe becomes a superconductor at 25 K, implying the superconductivity evolves from a Mott AFM state in Fe-Se based superconductors. 
Jiangang Guo, Xiaolong Chen, Gang Wang, Tingting Zhou, Xiaofang Lai,   Shifeng Jin, Shunchong Wang, Kaixing Zhu
cond-mat.supr-con cond-mat.str-el