Intra-layer atomic ordering and semi-conductivity in CsAMS2 (A = Li, Ag; M = Co, Fe)

Three layered compounds CsLiCoS2, CsAgCoS2 and CsLiFeS2 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 CsLiCoS2 is isostructural to ThCr2Si2-type CsCo2S2 with random mixture of Li+ and Co2+. The latter two compounds exhibit √2 ​× ​√2 ​× ​1 superstructure of ThCr2Si2-type because the monovalent Ag+/Li+ and divalent Co2+/Fe2+ independently occupy inequivalent sites in [AMS2]- layer. Through comprehensive investigation of the crystallographic lattice in CsAMS2, 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.