Two spatially separated phases in semiconducting Rb0.8Fe1.5 S2

We report neutron scattering and transport measurements on semiconducting Rb0.8Fe1.5S2, a compound isostructural and isoelectronic to the well-studied A0.8FeySe2(A=K,Rb,Cs,Tl/K) superconducting systems. Both resistivity and dc susceptibility measurements reveal a magnetic phase transition at T=275K. Neutron diffraction studies show that the 275 K transition originates from a phase with rhombic iron vacancy order which exhibits an in-plane stripe antiferromagnetic ordering below 275 K. In addition, the stripe antiferromagnetic phase interdigitates mesoscopically with an ubiquitous phase with 5×5 iron vacancy order. This phase has a magnetic transition at TN=425K and an iron vacancy order-disorder transition at TS=600K. These two different structural phases are closely similar to those observed in the isomorphous Se materials. Based on the close similarities of the in-plane antiferromagnetic structures, moments sizes, and ordering temperatures in semiconducting Rb0.8Fe1.5S2 and K0.81Fe1.58Se2, we argue that the in-plane antiferromagnetic order arises from strong coupling between local moments. Superconductivity, previously observed in the A0.8FeySe2-zSz system, is absent in Rb0.8Fe1.5S2, which has a semiconducting ground state. The implied relationship between stripe and block antiferromagnetism and superconductivity in these materials as well as a strategy for further investigation is discussed in this paper.