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Three layered compounds CsLiCoS 2 , CsAgCoS 2 and CsLiFeS 2 have been synthesized by conventional solid-state reaction method. All of three compounds exhibit semiconducting behavior in electrical transport with small thermal activation energy gap. The crystal structure of CsLiCoS 2 is isostructural to ThCr 2 Si 2 -type CsCo 2 S 2 with random mixture of Li+ and Co2+. The latter two compounds exhibit √2 ​× ​√2 ​× ​1 superstructure of ThCr 2 Si 2 -type because the monovalent Ag+/Li+ and divalent Co2+/Fe2+ independently occupy inequivalent sites in [AMS 2 ]- layer. Through comprehensive investigation of the crystallographic lattice in CsAMS 2 , a phase boundary between disorder and ordered phases may be identified, which highlights the importance of the atomic size and lattice mismatch in determining the crystal structure. Image 1 • Three layered sulfides CsAMS 2 (A ​= ​Li, Ag; M ​= ​Co, Fe) are synthesized by solid state reaction method. • Introducing monovalent Li+/Ag+ could significantly change the structure and property of Co-based sulfides. • A phase boundary between random and ordered occupation in the [AMS 2 ]- layers is determined by the atomic size difference. [ABSTRACT FROM AUTHOR]