Superconductivity in KCa$_2$Fe$_4$As$_4$F$_2$ with Separate Double   Fe$_2$As$_2$ Layers
  We report synthesis, crystal structure and physical properties of a quinary iron-arsenide fluoride KCa$_2$Fe$_4$As$_4$F$_2$. The new compound crystallizes in a body-centered tetragonal lattice (with space group $I4/mmm$, $a$ = 3.8684(2) {\AA}, c = 31.007(1) {\AA}, and $Z$ = 2), which contains double Fe$_2$As$_2$ conducting layers separated by insulating Ca$_2$F$_2$ layers. Our measurements of electrical resistivity, dc magnetic susceptibility and heat capacity demonstrate bulk superconductivity at 33 K in KCa$_2$Fe$_4$As$_4$F$_2$. 
Zhi-Cheng Wang, Chao-Yang He, Si-Qi Wu, Zhang-Tu Tang, Yi Liu,   Abduweli Ablimit, Chun-Mu Feng and Guang-Han Cao
cond-mat.supr-con cond-mat.str-el