PC2-1-INV New iron-based superconductors with separate double FeAs Layers *Guang-Han Cao 1 , Zhi-Cheng Wang 1 , Si-Qi Wu 1 , Chao-Yang He 1 Department of Physics, Zhejiang University, Hangzhou 310027, China 1 In the view of crystal structure, cuprate and iron-based superconductors contain quasi-two- dimensional CuO2 planes and FeAs layers, respectively, which play the decisive role for high- temperature superconductivity. In iron-based superconductors, the FeAs/Se layers are either separated each other to form a single-layer compound (e.g., LaFeAsO), or connected by monatomic layer to form an “infinite-layer” material (e.g., BaFe2As2), or simply stacked together with van de Waals force (e.g., FeSe). Are there any analogs of double-CuO2-layer or multi-CuO2-layer materials in iron-based systems? In a paper published four years ago [1], we proposed an intergrowth structure which contains separate double-FeAs layers. Recently, motivated by the discovery of “1144-type” iorn-based superconductors [2], we succeeded in synthesizing the target compound (KCa2Fe4As4F2) for the first time [3]. The double FeAs layers are connected by K+, which are separated by the insulating Ca2F2 layers. The material itself is almost optimally hole doped, making it superconducting (at 33 K) without extrinsic chemical doping. We also tried to expand this 12442-type family of iron-based superconductors. We find that the lattice match is crucial for the intergrowth structure. The tolerance of lattice mismatch is about 2% for the synthesis under ambient pressure. So far we have obtained 18 new superconductors in the 12442 family whose Tc values span from 28 to 37 K [4]. The possible structural correlations with Tc will be discussed in the talk. References [1] H. Jiang et al., Chin. Phys. B 22, 087410 (2013). [2] A. Iyo et al., J. Am. Chem. Soc. 138, 3410 (2016). [3] Z.-C. Wang et al., J. Am. Chem. Soc. 138, 7856 (2016). [4] Z.-C. Wang et al., Sci. Chin. Mater. 60, 83 (2017); Z.-C. Wang et al., J. Phys.: Condens. Matter 29, 11LT01 (2017); Z.-C. Wang et al., Chem. Mater. 29, 1805 (2017); S.-Q. Wu et al., Phys. Rev. Mater., in press. Keywords: Iron-based superconductors, Lattice match, Crystal structure